Weatherford Sand Control Manual A4 R1

GENERAL INFORMATION

ON

WeatherfordWeatherfordWeatherfordWeatherfordSand ControlSand ControlSand ControlSand Control

SystemsSystemsSystemsSystems

External Manual Rev 1.0External Manual Rev 1.0External Manual Rev 1.0External Manual Rev 1.0June 2001June 2001June 2001June 2001

For further information, contact your nearest Weatherford Completion Systems location or the addresses below.A list of locations can be found on the world wide web at http://www.weatherford.com/locations/index.asp. Thehome page is located at http://www.weatherford.com.

Americas

Weatherford Completion Systems515 Post Oak Blvd.Suite 600Houston Texas 77027Tel: 713 693 4000Fax: 800 257 3826

Asia/Pacific

Weatherford Completion SystemsRohas Perkasa12th Floor, West Wing8 Jalan Perak50450 Kuala LumpurMalaysiaTel: 603 2168 6000Fax: 603 2162 2000

Europe/Africa

Weatherford Completion SystemsWeatherford HouseLawson Drive, DyceAberdeen AB21 0DRTel: +44 (0)1224 762 800Fax: +44 (0)1224 771 309

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TABLE OF CONTENTS1. Introduction.................................................................................................................................. 1

Scope .......................................................................................................................................... 1Weatherford Completion Systems Capabilities & Contact Information....................................... 1Sand Control ............................................................................................................................... 1Acknowledgements ..................................................................................................................... 1

2. WCS Sand Control Systems Overview....................................................................................... 2Background ................................................................................................................................. 2Slotted Liners .............................................................................................................................. 3Wire-Wrapped Screens............................................................................................................... 4Pre-Packed Screens ................................................................................................................... 5Premium Screens........................................................................................................................ 6Gravel & Fracture Packs ............................................................................................................. 7Expandable Systems .................................................................................................................. 9

3. Sand Control Issues.................................................................................................................. 10Sand Grains .............................................................................................................................. 10Particle Size Distributions ......................................................................................................... 13Media Sizing Rules.................................................................................................................... 15Open Area to Flow .................................................................................................................... 19Draw-Down................................................................................................................................ 22Borehole Support ...................................................................................................................... 23Mud and Drill-in Fluid Conditioning Recommendations ............................................................ 26Centralisers ............................................................................................................................... 34Inflow Profile along Horizontal Wells......................................................................................... 35Erosion Resistance ................................................................................................................... 37Corrosion Resistance................................................................................................................ 45Screen Specifications................................................................................................................ 46

4. Sand Control System Selection ................................................................................................ 47Selection Process...................................................................................................................... 49Risk Management ..................................................................................................................... 56Weatherford System Advantages ............................................................................................. 57

5. Wire-Wrap Screen Solutions..................................................................................................... 58Wire-Wrap Screens................................................................................................................... 60Houston-Weld® Screens............................................................................................................ 61Dura-Grip® Screens................................................................................................................... 61Free-Flow™ Screens ................................................................................................................ 65Pre-Pack Screens ..................................................................................................................... 66Zonal Isolation ........................................................................................................................... 71Pre-pack & Wire-wrap Selection Criteria for Internal Gravel Packs.......................................... 72Wire-Wrap Screens Safe Application Limits ............................................................................. 73

6. Stratapac® Screens ................................................................................................................... 74Stratapac Construction.............................................................................................................. 75Screen Pore Size Distribution Technology................................................................................ 83Screen & Media Selection Guidelines....................................................................................... 85Comparison Tests ..................................................................................................................... 87

7. Gravel Pack Issues ................................................................................................................... 95Horizontal Open-Hole Gravel Packing ...................................................................................... 95Cased Hole Gravel Packing ...................................................................................................... 97Gravel Sizing and Screen Selection.......................................................................................... 99

8. Expandable Sand Screens...................................................................................................... 105Introduction.............................................................................................................................. 105ESS Construction .................................................................................................................. 107Expandable Isolation Sleeve (For Information)....................................................................... 110Expansion Systems................................................................................................................. 110ESS Mechanical Properties .................................................................................................... 112ESS Installation Procedures ................................................................................................... 113

Use of ESS in Combination with Other EST Products............................................................ 119Accessory Equipment ............................................................................................................. 119Technical Issues...................................................................................................................... 119ESS Media Testing.................................................................................................................. 121Borehole Support .................................................................................................................... 123

9. Support Services ..................................................................................................................... 133Technical & Operational Support ............................................................................................ 133Sand Prediction & Production Technology Support................................................................ 133Engineering, Manufacturing and Service Support .................................................................. 133

LIST OF FIGURESFigure 1: Slotted liner ............................................................................................................................. 3Figure 2: Dura-Grip wire-wrap screen .................................................................................................... 4Figure 3: Effect of wrap wire shape........................................................................................................ 4Figure 4: Uniform sand grains on wire-wrap (cross-section and plan view) .......................................... 5Figure 5: Pre-packed screen cross-section............................................................................................ 5Figure 6: Stratapac PMM screen............................................................................................................ 6Figure 7: Internal gravel pack with CIV .................................................................................................. 7Figure 8: CIV operation with IGP............................................................................................................ 8Figure 9: EGP & ESS IPR comparison .................................................................................................. 9Figure 10: Particle population, weight and volume ratios..................................................................... 12Figure 11: Retention of uniform and non-uniform sands...................................................................... 13Figure 12: Presentation of particle size data ........................................................................................ 13Figure 13: Sand distributions illustrating different sorting .................................................................... 14Figure 14: Sand plotted cumulatively to illustrate sorting..................................................................... 15Figure 15: Gravel pore throats and sand particles ............................................................................... 16Figure 16: Mesh pore throats and sand particles ................................................................................. 16Figure 17: Field PSD data showing heterogeneous sands (plotted non-standard) ............................. 17Figure 18: Field PSD data showing a very homogenous reservoir (plotted non-standard) ................. 17Figure 19: Effective inflow areas of wire-wrap, pre-pack and multi-layer screens ............................... 20Figure 20: Effects of fibre diameter on porosity (top). Effects of sand control media design on porosity

(bottom) ......................................................................................................................................... 21Figure 21: Unloading of non-fixed pore medium .................................................................................. 22Figure 22: Media migration................................................................................................................... 22Figure 23: Stress analysis plot ............................................................................................................. 24Figure 24: Equipment set-up for laboratory mud qualification test....................................................... 33Figure 25: EGP horizontal inflow.......................................................................................................... 35Figure 26: ESS horizontal inflow .......................................................................................................... 36Figure 27: Specific erosion for various types of screen including Stratapac screens. Data on

competitors’ screens interpolated as required from SWRI data.................................................... 39Figure 28: Effect of erosion on the size of the screen openings. A low open area medium is a lot

more sensitive to slot erosion than a high open area medium...................................................... 40Figure 29: Cross-sections of two outer protective cage designs.......................................................... 41Figure 30: Gas erosion test .................................................................................................................. 41Figure 31: Erosion resistance of Stratapac PMM screen as a function of gas velocity and particle size

distribution. .................................................................................................................................... 42Figure 32: Effect of cage design on erosion resistance ....................................................................... 43Figure 33: Representation of the chisel effect showing the effect of impingement angle on the ease of

penetration into the material. ......................................................................................................... 43Figure 34: Effect of impingement angle on erosion rates of stainless steel......................................... 43Figure 35: Schematic representation of the effect of a louvered cage on the impinging gas flow angle.

....................................................................................................................................................... 44Figure 36: Screen Specification Schematic.......................................................................................... 46Figure 37: Sand Control Decision Flow 1............................................................................................. 51Figure 38: Sand Control Decision Flow 2............................................................................................. 52Figure 39: Sand Control Decision Flow 3............................................................................................. 53Figure 40: Sand Control Decision Flow 4............................................................................................. 54

Figure 41: Wire wrap jacket manufacturing process ............................................................................ 58Figure 42: Shrink fit Dura-Grip screen.................................................................................................. 58Figure 43: Wrap Wire Shapes .............................................................................................................. 58Figure 44: Quality control of wire forming process............................................................................... 59Figure 45: Wire forming and inventory ................................................................................................. 59Figure 46: 88 Spindle drilling machine (44ft jts) ................................................................................... 60Figure 47: Wire-wrap screen types ...................................................................................................... 60Figure 48: Wrap-wire profile ................................................................................................................. 60Figure 49: Tensile testing of wire-wrap screens................................................................................... 63Figure 50: Poisson effect of base pipe expansion................................................................................ 64Figure 51: Free Flow Screen................................................................................................................ 65Figure 52: Free flow screen section ..................................................................................................... 65Figure 53: Pre-pack configurations ...................................................................................................... 66Figure 54: Resin coated gravel............................................................................................................. 67Figure 55: Resin coated proppant (Carbolite) ...................................................................................... 67Figure 56: Comparison between cured and uncured proppant permeability ....................................... 68Figure 57: Pre-pack tower .................................................................................................................... 68Figure 58: Steam curing process ......................................................................................................... 69Figure 59: Isolation Sleeve .................................................................................................................. 71Figure 60: Screened isolation sleeve ................................................................................................... 71Figure 61: Stratapac screen (Range III) with solid rotating centralisers installed ................................ 74Figure 62: SEM of Stratapac PMM medium......................................................................................... 74Figure 63: SEM of Stratapac PMF-II media ......................................................................................... 75Figure 64: Crush samples (left) and apparatus (right) ......................................................................... 76Figure 65: Welded vs non-welded seals .............................................................................................. 78Figure 66: Welded end-ring seal .......................................................................................................... 78Figure 67: Longitudinal media welds.................................................................................................... 78Figure 68: Construction and porosity of non-woven media.................................................................. 84Figure 69: Comparison of PMF and pre-pack pore size distributions .................................................. 84Figure 70: Stratapac media selection chart.......................................................................................... 86Figure 71: Fine screens - plugging and retention................................................................................. 87Figure 72: Plugging potential - 20/40 ................................................................................................... 88Figure 73: Sand retention efficiencies .................................................................................................. 89Figure 74: Challenge sand sieve analysis............................................................................................ 90Figure 75: Media selection chart with challenge sand plotted.............................................................. 91Figure 76: Coarse sand screen plugging resistance test ..................................................................... 92Figure 77: Coarse sand - sand retention efficiency.............................................................................. 92Figure 78: Coarse sand - effluent particle size distribution .................................................................. 93Figure 79: PMF & DTW plugging resistance - normalised for porosity ................................................ 94Figure 80: Openhole gravel pack ......................................................................................................... 95Figure 81: Angle of repose (dry sand).................................................................................................. 95Figure 82: Partial pack of long horizontal well...................................................................................... 96Figure 83: IGP cross-section ............................................................................................................... 97Figure 84: Typical perforation tunnel.................................................................................................... 98Figure 85: Perforation packing ............................................................................................................. 98Figure 86: Effective of formation damage ............................................................................................ 99Figure 87: Example formation sieve data........................................................................................... 100Figure 88: Limitations of Saucier's rule to gravel sizing ..................................................................... 101Figure 89: 3D Gravel pack structure generated by 3D modelling ...................................................... 101Figure 90: Gravel pack Pore Throat Distribution (PTD) ..................................................................... 102Figure 91: Curve match of the gravel pack PTD with formation sand PSD ....................................... 102Figure 92: Schematic of interfacing bridging ...................................................................................... 103Figure 93: Bridging mechanism with example data............................................................................ 103Figure 94: Productivity comparison for a various ESS & EGP completion options in a 6" heavy oil

producer....................................................................................................................................... 105Figure 95: Flow contribution along well bore of various ESS & EGP completions ............................ 106Figure 96: ESS Construction .............................................................................................................. 107Figure 97: ESS Assembly................................................................................................................... 108Figure 98: Stabbing in ESS pin connection........................................................................................ 109

Figure 99: ESS Connections .............................................................................................................. 109Figure 100: Expanded ESS................................................................................................................ 110Figure 101: Expansion Cone.............................................................................................................. 110Figure 102: Expansion mandrel.......................................................................................................... 111Figure 103: First 4” Compliant Rotary Expansion System (CRES) tool as used on Shell Brigantine 111Figure 104: Non Compliant Rotary Expansion Subassembly ............................................................ 111Figure 105: Compliant Rotary Expansion System ............................................................................. 112Figure 106: Two trip installation: Trip 1 - Set Hanger......................................................................... 116Figure 107: Two trip installation: Trip 1 - Expand screen................................................................... 117Figure 108: Photographs following mud filter cake removal testing................................................... 120Figure 109: Comparative screen plugging tests................................................................................. 121Figure 110: Erosion testing................................................................................................................. 122Figure 111: Sand exclusion testing .................................................................................................... 122Figure 112: ESS connector strength testing....................................................................................... 122Figure 113: ESS mechanical testing .................................................................................................. 123Figure 114: Surplus expansion........................................................................................................... 124Figure 115: Change of EST Radius with external pressure ............................................................... 125Figure 116: Castlegate TWC testing with & without EST support...................................................... 127Figure 117: Increase of Castlegate TWC strength with increasing internal pressure. ....................... 129Figure 118: Effect of internal pressure on increase in TWC of loose sand and Castlegate sandstone

..................................................................................................................................................... 130Figure 119: UBI log data illustrating borehole support ....................................................................... 132

LIST OF TABLESTable 1: Classification of sediments..................................................................................................... 10Table 2: Gravel and slot size data........................................................................................................ 18Table 3: Mesh size conversion table .................................................................................................... 26Table 4: Wire-wrap metallurgy options................................................................................................. 45Table 5: Stratapac metallurgy options.................................................................................................. 45Table 6: ESS metallurgy options .......................................................................................................... 45Table 7: Applications & risks for sand screen systems ........................................................................ 56Table 8: Gravel pack risks.................................................................................................................... 56Table 9: Weatherford screen features matrix ....................................................................................... 57Table 10: Dura-Grip & Dura-Grip Plus standard dimensions ............................................................... 62Table 11: WW Screen diameter change with temperature .................................................................. 63Table 12: Micro-Pak standard dimensions ........................................................................................... 70Table 13: Typical isolation sleeve dimensions ..................................................................................... 71Table 14: Wire-wrap safe application limits.......................................................................................... 73Table 15: Screen crush test - total suspended solids .......................................................................... 77Table 16: Standard Stratapac screen dimensions ............................................................................... 79Table 17: Standard Stratacoil screen dimensions................................................................................ 79Table 181: Standard Stratacoil screen safe application limits .............................................................. 80Table 191: Standard Stratapac screen safe application limits.............................................................. 80Table 201: Hi-Flow Stratapac screen safe application limits ................................................................. 81Table 21: Recommended Stratapac base-pipe safe build angle application guidelines...................... 82Table 22: Safe build angle application limits for Stratapac screen jackets .......................................... 83Table 23: Standard Stratapac media ratings........................................................................................ 85Table 24: Key to screen types.............................................................................................................. 90Table 25: Media porosity comparison .................................................................................................. 93Table 26: GP productivity comparisons................................................................................................ 99Table 27: ESS application limits......................................................................................................... 112Table 28: ESS dimensional data (mm & inches)................................................................................ 113Table 29: Operation time estimate ..................................................................................................... 115Table 30: Surplus expansion (i).......................................................................................................... 123Table 31: Surplus expansion (ii) ......................................................................................................... 123Table 32: BHC testing ........................................................................................................................ 127

General Sand Control Information Manual External Revision 1.0 - 1

1. Introduction

ScopeThe purpose of this document is to provide background technical information on Weatherfordsand control systems for customers and application engineers, and to provide assistance inthe selection of the most appropriate sand control system for any particular client application.The scope of this document is therefore limited to the areas of sand control wereWeatherford equipment and services can make a positive difference to the client in terms ofexpenditure and well productivity. It will therefore not cover the history, geology, productionand reservoir engineering aspects of sand control in detail except where Weatherford sandcontrol systems have an impact.

Weatherford Completion Systems Capabilities & Contact InformationA current overview of Weatherford is provided at the http://www.weatherford.com website.Weatherford consists of three major divisions: Drilling and Intervention Services (DIS),Completion Systems and Artificial Lift Systems (ALS). There is much interaction betweendivisions, especially during thru-tubing interventions with DIS and upper completions (eg gaslift) with ALS. WCS itself is sub-divided into distinct operating areas; North & South America,Far/Middle East and Europe/Africa/CIS. Local support is available through the nearestWeatherford location and strong centralised sand control support is therefore located inHouston, Aberdeen and Singapore/Kuala Lumpur. To contact the nearest Weatherfordlocation, please visit the above website or contact the Weatherford offices listed on thecover.

Sand ControlIn the ideal world, sand control would be about preventing any solid particle from moving intothe well stream and certainly from reaching the surface production facilities, while at thesame time not introducing and additional skin or pressure drop to the completion. Weattempt to do this generally by supporting the well-bore and filtering the sands downhole.However in the real world, every filter will plug in time.And so sand control involves getting the balance just right between producing small sandparticles, which do not cause a problem at surface, and not plugging the filter system down-hole. By directly supporting the sand face, we can reduce the number of large, load bearingsand grains from being produced.Therefore for effective sand control, we need to support, control and manage the sand faceat the producing formations.This manual describes the technology currently available from Weatherford to perform thiscomplex task.

AcknowledgementsThis manual was first developed to be used internally by Weatherford employees and isdesigned to be included in an intensive sand control training course. It has beensubsequently adapted for use by external client and alliance personnel.Weatherford acknowledges the contribution in the form of much information from manysources including Shell International, BP, Chevron, Exxon/Mobil, Norsk Hydro, Statoil, Mobil,Pall Corporation, Schlumberger, Baker, Halliburton and USF Johnson.


2. WCS Sand Control Systems Overview

BackgroundThe causes of sand production and the reasons for sand control are covered in the generalsand control course and other publications. An introduction to sand control can be found inan SPE series on special topics – “Sand Control” by W L Penberthy and C M Shaugnessy(ISBN 1-55563-041-3) which provides an excellent overview of conventional sand controltechnology prior to horizontal gravel packing, premium and expandable screens.Sand screens have been in use for over one hundred years in very many industrialapplications, not limited to the oil industry. A wealth of information has been developed overthis period and certain standard industry practices have emerged, especially with regard togravel packing and wire-wrap type sand screens. When it comes to the newer metal meshtype and expandable screens, the old rules are called into question. Indeed, with the adventof horizontal wells, even the standard gravel packing and screen selection practices arereceiving increased scrutiny. Conventional technology in difficult horizontal wells has arelatively poor record to date in terms of gravel pack placement and effectiveness.Weatherford Completion Systems with its unique sand control technology is forging a brandnew path away from conventional capital intensive pumping technology and is focussed onproviding customers with a safe and efficient means of achieving sand control while allowingmaximum well & field productivity. It is however recognised that gravel packing (andespecially fracture packing) still has an important place in the totality of sand controlapplications and these areas will be highlighted and addressed. Weatherford manufacturesand supplies the sand control screens & equipment for standalone, gravel packing, fracturepacking and expandable applications, and in conjunction with other service companies canalso provide the full range of gravel and fracture packing technology.There are a number of companies involved in screen manufacture and the provision of sandcontrol systems and services. The main competitors in terms of screen manufacture are:

• USF Johnson, a well established manufacturer of wire-wrap screens owned by Vivendi aFrench water and entertainment company. Johnson (not to be confused with FlopetrolJohnston, an old Schlumberger company) have acquired several smaller wire-wrappingcompanies including Effimax and Wesco. They have manufacturing locations in severallocations round the world and are a big supplier to the water well industry. Johnson haverecently introduced a premium screen.

• Baker Oil Tools, a large oilfield service company provides several variants of slip-on wire-wrap, pre-pack and premium screens, notably the Excluder, to complement its gravelpacking services. BOT has a significant presence outside of the US.

• Halliburton, through its Howard Smith acquisition and its Purolator (a US based filtercompany) alliance provides a similar product & service offering to Baker Oil Tools.

• Other smaller manufacturers of screen only products (mostly wire-wrap) include Conslot,Nagoaka, Pippimas, Slotco/Citra, Tigaiken, Reslink and Cook. Secure Oil Toolsmanufacturing the Mesh-rite product have recently been acquired by Schlumberger.Many of these smaller companies are focussed primarily on the water industry (eg Cook)or are very regional in nature (eg Reslink).


Companies providing gravel packing and pumping capabilities to the oilfield are listed below:

• Baker Oil Tools, as mentioned above, have recently established a track record for theprovision of horizontal gravel packs. The track record is worth examining in detailhowever. They promote their high-rate water pack technology as a viable method ofgravel placement in long horizontal wells.

• Schlumberger have adopted a similar approach and also promote their All-Pak shunt-tubes (licensed from Exxon-Mobil) as their way of getting a good gravel pack. AlternativePath Technology, All-Pak & All-Frac are registered trade marks of Schlumberger.Weatherford Stratapac screens have been supplied for All-Pak type applications.

• Halliburton provide a similar package and are working on alternative systems to theSchlumberger All-Pak Screens.

• BJ Services provide gravel packing services and systems, but do not manufacturescreens.

• OSCA provides also gravel packing services and promotes its Advance Pack systemwhich packs the screens inside and outside with gravel downhole and hence by-passesany high leak-off zones and bridges in the annulus. Weatherford manufactures thesystem and screen for OSCA.

The sand control market is hence diverse and very competitive. The big service companieshave much capital invested in pumping gravel packs and are therefore very aggressive inpromoting this technology. Of the above companies, Weatherford supplies screens andmechanical accessories to Schlumberger, BJ and OSCA.

Slotted LinersThe simplest form of sand control involvescutting slots in standard oilfield casing,tubing or liners. The slots are generally 1.5”to 2.5” long and their width typically variesfrom 0.012” to 0.250”. The smaller slotwidths are cut with smaller circular sawblades and tend to be 1.5” long. Becausethe axle of the saw cannot pass below thetube surface, the end of the slots are undercut. Most typical slot patterns are staggeredslots, but other variations are possible toshorten the manufacturing process. Theopen area of a slotted pipe ranges from1.5% to 6% over the slotted length.

Figure 1: Slotted liner

There are two types of slot available, keystone and straight (see Figure 1). The keystoneslots are considered better than straight slots for their self flushing or cleaning ability.Keystone slots are generally twice as expensive as straight cuts. Slotted liners are only veryslightly cheaper to manufacture than wire-wrap and are often more expensive. They areconsidered to be tougher but have much less open area.


Wire-Wrapped ScreensWeatherford manufactures wire-wrap screens in Houston, Singapore and Indonesia. Thereare three main types of wire-wrap screen; rod-based screens (Houston Free Flow™), pipe-based slip-on (Houston Weld®) & pipe-based direct build (Dura-Grip®) screen. Thesescreens are used in a variety of applications including the water well industry which is a verysignificant market on a global scale, disposal wells, environmental applications (eg land-fillsites) and of course the oil industry.

Figure 2: Dura-Grip wire-wrap screen

Wire wrapped screens are normally made from triangularshaped wrap wire, see Figure 3. The gap between theedges of the wrap wire is sufficient to allow quite largesand grains to pass through. If rectangular wires wereused, there is a possibility that odd shaped grains could bejammed or wedges into the groove and stopping othersand grains from passing. The triangular shape of thewrap wire reduces the chance of these large or oddshaped sand grains from getting trapped in the depth ofthe groove and hence plugging the screen.

Figure 3: Effect of wrap wire shape

These triangular wrap wires are also referred to as wedge or keystone shaped. In the waterwell industry, these screens are generally referred to as “wedge-wire” screens.Wire-wrap & pre-pack screens are discussed in detail in Section 5.

Perforated API base-pipe

Rib wires

Wrap wire

End-ring or weld

GapGapGapGap


Figure 4 illustrates how wire-wrap screens control sand production by enabling the formationof sand arches or bridges. A sand arch is formed normally by two or three sand grains.

Bridges formed with more sand grains are relativelyunstable. As the sand bridges form, they restrict thepassage of the other grains and these grains in turnbridge off over the initial arch forming a more stablestructure. The formation of this stable structure orsand filter cake is key to controlling the passage ofsand in through the screen and into the well stream.Generally, the fine and very small particles (fines)are able to pass through, whereas the largeparticles are stopped. The large particles cause themost damage in terms of erosion and clogging ofthe surface production facilities.The sand filter cake should be permeable. If thefines were stopped also, they would likely plug thesmaller pores in the sand filter cake and hencereduce the overall permeability of the system. Theopen area under the sand filter cake addssignificantly to the screen’s ability to allow fluid flowby eliminating dead space. If the size of the sandgrains varies considerably, the filter cake formedwould have a reduced permeability. Therefore, theability of wire-wrap screens to control poorly sortedsand sands is limited. In general terms, wire-wrapscreens are used behind gravel packs, which ineffect have large, well sorted grains and can bridgeoff over the wire-wrap screen effectively.

Figure 4: Uniform sand grains on wire-wrap(cross-section and plan view)

Pre-Packed ScreensWeatherford manufactures a variety of pre-packed screens including; Perma-Bond®, Micro-Pak® and Exact-Pak™ pipe-based screens and Muni-Pak™ rod based screen. Perma-Bondand Micro-Pak are generally used in oilfield applications and Muni-Pak and Exact-Pak aregenerally used in water well applications. Essentially the Exact-Pak and Perma-Bondscreens are based on a Dura-Grip inner screen and a wire-wrap outer screen, with gravel orproppant sandwiched between. The pre-pack gravel (orceramic proppant beads) is the main filtration medium and thewire-wrap jackets are designed to hold only the proppant inplace. Perma-Bond screens are used in horizontal wells ormarginal wells were the use of a gravel pack would beuneconomic. Micro-Pak screens are a slimmer design of thePerma-Bond screen and are primarily used behind fracturepacks or high rate water packs in horizontal wells. The pre-packed screens provide insurance against voids in the gravelpack. The rod based Muni-Pak screen has been very effectivein municipal water well projects as an economic and effectivesand control device in shallower water wells.

Figure 5: Pre-packed screencross-section

Cross-Section

Plan View


Premium ScreensPremium screens are typically an all-metal design, with a metal mesh filtration media and aprotective outer metal shroud. The metal mesh can be either a metal weave, typically adutch twill, or metal fibres or powder particles embedded within a square metal mesh. Theapertures (called pore throats) generally very from 60 micron to 300 micron. The concept isfor the mesh to prevent the larger sand particles from travelling through and allow theformation fines to pass. The larger particles form a permeable sand filter cake on the screensurface. Premium screens are typically run in long horizontals, often behind gravel packsand have similar sand control properties to pre-pack screens. The main improvements arethe generally improved plugging resistance and, in most cases, the ability to flow back drillingmuds through the screens.Weatherford Stratapac® PMM® 60micronscreens were introduced in 1995 for highrate gas wells in the Gulf of Mexico. PMMis Porous Metal Media and is a sinteredmetal powder screen. Stratapac PMMscreens generally are not used in drillingmuds due to their small pore throat size.Stratapac PMF-II® (Porous Metal Fiber)screens can as they are rated at 120microns or 200 microns, and hence allowthe mud particulates to pass. Premiumscreens are discussed in more detail inSection 6. PMM and PMF-II meshes aretrade marked by Pall Corporation and aremanufactured for Weatherford.

Figure 6: Stratapac PMM screen

Stratapac has the added benefit of being the most diameter efficient, non-expandable buthighly porous premium screens on the market today.


Gravel & Fracture PacksGravel packs (GP) and fracture packs are very useful techniques for completing sand pronereservoirs in a wide variety of sand types and completions. GP systems have been incommon use for many years, and service companies and operators have built up a wealth ofexperience and knowledge. Gravel packing in open-hole (EGP – External Gravel Pack) isuseful for preventing annular flow and controlling sand in heterogeneous formations. Gravelpacking in cased-holes (IGP – Internal Gravel Pack) is useful for protecting the sand screensfrom erosional flow. After the liner is run and perforated, the sand face completion is run.The perforations may then be washed and the GP packer set. Figure 7 below illustrates thesequence of events involved. There are many tricks of the trade involved in getting a goodpack and it is beyond the scope of this document to cover this in any great detail.

Figure 7: Internal gravel pack with CIV

Reverse circulateexcess gravel fromwell

Run sandface completionand set gravel pack packer.Squeeze gravel intoperforations

Circulate gravelpack, takingreturns throughshifting tool atbottom of innerstring

Flow is via open x-over tool

Returns via open by pass area

Ball lifted off seat

X-over toolabove packer


Pick up inner string andlocate shifting tool in CIV.Pull ball closed.

Shear shifting tool andpull back. Close gravelpack sleeve. Pressuretest and circulate tocompletion fluids

When the GP operation is completed, it is necessary to remove the wash string and servicetools prior to installing the upper completion. Often, some sort of loss control device is usedin the sand face completion and the selection of such a device can greatly improve operationflexibility and efficiency. Figure 8 shows the use of a Weatherford Completion Isolation Valve(CIV) which can be opened and closed hydraulically. The other commonly used loss controldevices are various types of flapper valves. The use of LCM pills in gravel packs and wellscreens is to be avoided and it is very difficult to return the pack and screen to fullpermeability along the entire well length.

Figure 8: CIV operation with IGP

Gravel packing becomes increasingly difficult and complex with hole angle and length. Also,in open hole situations, high permeability streaks (leak off zones) and washouts can interferewith the uniform gravel placement. Low net to gross pays with shales/clays can also intermixwith the gravel during pumping and impair the gravel pack permeability and henceproductivity. To counteract these problems, the industry has responded by developingspecialised techniques and equipment, eg All-Pak® from Schlumberger (in co-operation withJohnson and Exxon-Mobil). This system uses shunt tubes on the exterior of the screens andallows the gravel to by-pass blockages in the well-bore annulus. Systems exist from otherservice companies, which aim to achieve the same result. For example, OSCA havedeveloped an Advance-Pak® system (in co-operation with Weatherford), and Halliburtonhave their CAPS system.

GP sliding sleeve open

GP sliding sleeve closed


Expandable SystemsThe Weatherford ESS® system is currently revolutionising sand control operations.Developed in conjunction with Shell International, the system combines the ease of use andfiltration capabilities of premium screens with the borehole support and flow conformancecapabilities of gravel packs. The elimination of the annulus allows zonal isolation andremedial options as well as better production logging and reduces the risk of screen erosion.The large ID allows increased productivity and more uniform inflow over the length of theinstalled screen plus more flexibility in completion/remedial operations and ESP deployment.The mechanical deployment system eliminates most if not all of the fluid compatibilityproblems normally associated with gravel packing.

Well Slimming

Figure 9: EGP & ESS IPR comparison

The benefit of increasing the screen ID is a marked improvement in well productivity asshown in Figure 9. This is a comparison for a 2500ft long 6” hole producing 21API oilbetween an ESS completion (5.5” ID) and a shunt-tube completion (2.44” ID). The solutionpoints for the inflow performance and vertical lift performance curves indicate a significant4000 stb/day difference in productivity.The increased productivity of an ESS installation in a 6” hole is comparable to that of a 6.5/8”standard screen in an 8.5” hole and so by using the ESS, a 6” hole is possible instead of an8.5”. This will afford the operator tremendous savings in terms of casings and completionjewellery and also the possibility to use smaller and lighter drilling rigs. This concept isreferred to as Well Slimming. The large bore of ESS allows more flexibility for ESPdeployment. The use of ESS and other expandable technologies (liner hangers for example)should allow operators to realise significant cost savings and improve marginal projecteconomics.

IPR vs VLP6" Horizontal 21 API Oil Producer 2,500 ft

0

500

1000

1500

2000

2500

3000

0 5000 10000 15000 20000Flow Rate STB/D

VLESS IPR (5.5"SHUNT GP IPR (2.44"

Solution6" ESS - 16,7002.78" Shunt GP - 12,330


3. Sand Control IssuesOperators generally have a number of concerns when it comes to selecting a sand facecompletion strategy. Primarily, revenue (expected production rates) and expenditure (bothinitial installation and also operational) considerations provide an envelope of possiblesolutions. Installation costs would include the totality of well costs as the type of sand facecompletion impacts on the hole size and drilling fluids used. Operating costs would includethe cost of waste disposal and interventions required (eg ESP replacement or remedial sandcontrol).To make decisions of the correct completion type to select it is important to be aware of themany sand control issues and the relative strengths and weaknesses of the products &systems available to the operator. A suggested method of narrowing down the choice ofsand control system is presented in later in Section 4.

Sand GrainsClastic rocks, meaning rocks made up of grains, are classified according to the particle sizeof the grains. This is a fairly straightforward classification, for sedimentologists, the termssand, silt and clay are defined in terms of the particle size. Sandstones are composed ofsand-sized grains, usually quartz, and may be cemented together with another material.Clay minerals and other fine particles will be present in the pore spaces in varying amountsdepending on the rock. Shales or mudrocks are made up of silt and clay sized grains. Table1 gives a simplified classification. One micron (µm) = 0.001 millimeters (mm).

SIZE (mm) CLASSIFICATIONMedium

Fine

v. fine

Pebbles GRAVEL

v. coarse

coarse

medium

fine

v.fine

Sand SAND

v. coarse

coarse

medium

fine

v. fine

Silt

8

4

2

1

0.500

0.250

0.125

0.062

0.031

0.016

0.008

0.004

0.002

Clay

MUD

Table 1: Classification of sediments


The nature of a sedimentary rock depends on its depositional environment; sands may bedeposited for example as beaches, river channels or wind-blown desert. The nature of thedepositional environment will affect the size, shape and sorting of the grains. Later changesduring burial will affect the sandstone’s cementation characteristics and strength. The laterchanges sometimes caused by volcanic activity, mountain building or tectonic movementscan fault and change the direction of the beds.To decide on the most appropriate filtration or sand control system, it is very important tounderstand the size and types of sand grains that may impact the screen.

Grain size definitions

• Sand - strictly particles between 62 µm and 2mm

• Silt - strictly particles between 2 µm and 62 µm

• Clay - strictly particles less than 2 µmThese strict definitions are not usually followed by engineers, you will find fine sands referredto as silts and fine silts as clays.Mudrocks and shales - engineers usually refer to all mudrocks as shales or shalysandstones, mudrocks are composed of silt and clay. Fines can mean different sizes todifferent people and this should be clarified in any discussion. Silts and clays are understoodto be ‘fines’, ie less than 62 µm. However, fines are sometimes defined as less than 44 µm.

Selecting Rock Samples for Particle Size AnalysisThis is not always easy! Often, core samples are available from exploration or appraisalwells. These samples when analysed cross many individual sand zones or beds (facies).The rock of most interest is the one most likely to produce hydrocarbons. If the reservoir isheavily faulted this might not be obvious. The zones most prone to failure (according to therock mechanics data and models) need to be looked at closely. The core samples to beused for particle size analysis should be selected by visual examination and compressivestrength measurements.

Particle Size MeasurementParticle size can be measured a number of ways:

• Sieving (most common)

• Laser diffraction using a Malvern or Coulter counter

• Sedimentation – a gravity settling technique, really only used by sedimentologistsand is not covered here.

For any particle size analysis the rock sample has to be disaggregated by hand intoindividual grains with a mortar and pestle. Mechanical methods such as milling are notsuitable because the actual grains will be pulverised.

SievingSieving consists of assembling a stack of sieves so that the largest mesh opening is at thetop with the mesh opening decreasing in successive sieves with the smallest at the bottom.At the very bottom is a solid container (called the pan) which collects the finest particles. Thesand sample is put on the top sieve and the stack of sieves is shaken either by hand ormechanically, so that the sand particles fall down through the sieve stack until they arecaught by a sieve whose opening is too small for the sand to fall through. At the end of the


Particle Ratio Radius Ratio

1:11:2

Volume Ratio 1:8Weight Ratio 1:8

agitation period the sand fraction in each sieve is weighed. Around 10-20g sand is theminimum sample required for acceptable results

Laser DiffractionThis method measures down to fractions of a micron and the amount of sand required ismuch less than is required for sieving (around 1g or less depending on the particle size, forthese instruments the number of particles is important not mass). The sand sample ismeasured in water, and is agitated ultrasonically to aid dispersion. These systems arecomputer controlled, so that the machine indicates when the correct amount of sample hasbeen added, etc. This method is quick and fully automated (apart from the adding of thesamples!) and the data output can be varied, and plotted in terms of volume of fractions, ornumbers of particles etc. The size ranges can also be varied. The drawback of this methodis that it is limited in the maximum size of particle it can measure up to (usually around 1mm),also these instruments are extremely expensive.

Complications in Particle Size MeasurementMeasuring particle size is not as straightforward as it first appears, for the same sand sampledifferent measurement methods can give different distributions. This is due to the shape ofthe grains. Dry sieving throws the sand grains around until they go through the sieve by thesmallest axis, whereas, laser diffraction tends to take an average from randomly orientedgrains. So if the sand grains are elongated or flat different distributions will result from thedifferent methods of measurement. Another problem, which can happen with dry sieving, isthat very fine particles can stick to the coarser grains. Sieving also generally only measuresdown to 45 microns, laser diffraction will measure down to fractions of a micron.Both sieving and laser diffraction give particle size as the equivalent spherical diameter,sieving gives the smallest equivalent spherical diameter and laser diffraction gives theaverage. Neither method gives any indication of grain shape and grain shape may beimportant in the sand filter cake permeability. Results from lab tests on screen pluggingsuggest that grain shape can affect the results.Sieve analyses report particle distribution in weight percent. A larger particle will make alarger contribution to the analysis than a smaller particle. This means that there are far moresmaller particles in a given sand than the sieve analysis would indicate, and these particlesmay lead to screen plugging if the wrong pore size is chosen. Figure 10 illustrates therelative contributions to weight and volume a 1:1 distribution of particles make which differ bya factor of 2 in radius.Relative uniformity of the sample is animportant parameter to consider in selecting ascreen pore size. A screen may perform verydifferently with two sands with the sameaverage size (D50) but very differentuniformity coefficients. Figure 11 illustratesthe results that may be obtained using atypical sizing rule (D10), where the screen‘pore’ size is chosen at the 10th percentile ofthe sand particle size. Bridging efficiency(and permeability) is maximised by uniformsands, and minimised by non-uniform sands.The non-uniform sand is on the left.

Figure 10: Particle population, weight andvolume ratios


Figure 11: Retention of uniform and non-uniform sands

Particle Size DistributionsThe results of a sieve analysis or laser diffraction measurement of a sand sample arepresented as a particle size distribution (PSD).Particle size data is generally plotted as cumulative weight–percent as a function of particlesize on a log/linear scale. Conventionally, the data is plotted so that the accumulatedweights start with the largest particle size (the conventional sizing rules are based on theseplots) as in Figure 12. However, the data may also be plotted the other way around so thatthe cumulative weights start with the smallest size – this tends to happen with laserdiffraction data.

Figure 12: Presentationof particle size data

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particle size (microns)

sand Asand A

D10 = D50 = D90 =

Poor vs. Uniform Sand with Respect to Pore Size

Pore Size = D10

d10 = 380 microns

d90 = 145 microns

d50 = 246 microns

Cumulative ‘Weight’ Curve

% ‘weight’ curve


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sand Bsand C

Another complication with these different ways of presenting particle size data is that particlesize data is often referred to in terms of the diameter at certain percentiles. For example, thed10 is the particle diameter at the 10% point on the cumulative curve. The d50 is the particlediameter at the 50% point on the cumulative curve, this is also the median grain size. Wherethe data is plotted cumulatively from different start points (largest or smallest), this willobviously affect the size of these percentile points (apart from the d50). Common parametersare based on cumulative plots where the data is plotted cumulatively from largest to smallest.So that the d10 point refers to the largest 10%.It should also be noted that these particle size plots are based on mass (normally andincorrectly referred to as weight) which will emphasise the largest particles, because thesewill be more massive than the smaller grains. If size data is plotted in terms of numbers ofparticles in each size range, then the plot would look completely different with much highervalues at the lower sizes. See Figure 10.

InterpretationThe slope of the cumulative particle size curve expresses the sand’s uniformity or sorting.This is basically a measure of the spread in particle size of the grains making up the sand.The steeper the curve (the more vertical) the more uniform or better sorted the sand. Wellsorted and uniform sands are composed of particles with a small spread in size. The flatterthe curve the more non-uniform the sand, meaning that there is a large range in the particlesize of the grains making up the sand. Examples of uniform and non-uniform sands areshown in Figure 13.The uniformity of the sand can be expressed as a ratio called the uniformity coefficient, this isalso a measure of the sorting (though the actual sorting ratio used by sedimentologists ismore complicated than this). The uniformity coefficient (Cu) is defined as the d40/d90. Sandswith Cu below 3 are considered uniform, for values between 3-5, the sand is considered to benon-uniform, and sand with Cu’s >5 are highly non-uniform. This uniformity can give anindication of the sand’s propensity to plug a screen, the more non-uniform the moretroublesome the sand. Note the Cu is always greater than 1.0.

A well sorted (uniform)sand (B) and a morepoorly sorted (non-uniform sand (C) asdifferential distributions

Figure 13: Sand distributionsillustrating different sorting


The same sands in now plotted cumulatively. Sorting coefficient Cu = ds40/ds90.

B Sand Cu = 1.6(well sorted)

C Sand Cu = 3.0(non-uniform)

Figure 14: Sand plottedcumulatively to illustratesorting

The normal sand sieve analysis is plotted cumulatively with the particle size decreasing leftto right and the weight sample increasing upwards. The typical curve is hence ‘S’-shaped.Figure 14 illustrates a curve plotted backwards from normal. A normal sand sieve analysisstarts with the larger sieves and the grains fall through towards the smaller sieves, thus theweights are added from large to small. Thus, the d10 is always larger than the d90.Weatherford standard is hence to plot particle size on a logarithmic scale decreasing left toright and the weight increasing upwards, giving a ‘S’-shape. Caution is required when somecompanies or individuals plot the cumulative weight starting from the pan and adding fromsmall to large sieves. They then plot the particle size increasing from left to right. This alsogives an ‘S’-shape and can lead to confusion.

Media Sizing RulesThis section briefly describes the existing work performed on screen sizing and is intendedas background information.There are no definitive sizing rules on which to base screen slot or pore throat size selection.Work done in the past can give some guidance, but no exhaustive work has been performedon plugging and retention. Often, a size of gravel or slot has been developed for a particularfield or area. Generally screen selection will be a trade off between retention efficiency andpressure drop, the better the retention the higher the pressure drop across the screen. Thissection will give a short summary of some of the main work and current thinking.Pioneering work on screen slot size selection was performed by Coberly in the ‘30’s. Heconcluded that spherical particles could generally be retained when the slot width was 2.5times the particle diameter or less. Also with a mixture of sands of different sizes, the sandcontrol properties of a screen mainly depends on the largest particles in the mixture. Fromthis work he came up with his criteria of slot width being between 1 to 2 times the d10, herethe d10 is the largest 10%. A criteria of 1 x d10 is commonly used in the Gulf of Mexico.For gravel packs, the previous criteria of 10 x d50 recommended by Coberly had led tofailures in the Gulf of Mexico which led to a general reduction in the gravel sizing. Saucierdid some tests with gravel packs in the ‘70’s. From this work, he came up with the 6 x d50rule for gravel selection. This has formed the basis of much of the sizing of gravel packssince.

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particle size( i )

sand Bsand C


More recently, work done by Amoco has tried to refine the Saucier criteria to concentrate therole of sub-44 micron particles in the plugging of gravel packs. In this work, sub-44 micronparticles are referred to as fines and correspond to particles which can pass through a US325 shaker mesh. Tests were performed on different sands with gravel sizes selectedaccording to Saucier’s criteria. The sands used had high proportions of sub 44 micronparticles (>10%). By using fuller descriptions of size distributions, ie the d10/d95, the d40/d90ratios and the amount of sub-44 micron particles, guidelines were produced to assist in theapplication of sizing rules. These guidelines can be used to identify sand distributions wherethe application of the standard Saucier rule could lead to plugging.Per Markestad et al performed a series of tests on wire wrap screens and decided that afuller description of the sand particle size distribution would give better indications as to themost appropriate slot width. In general, he found that larger slot widths than those predictedby Coberly gave the best results regarding plugging and retention. From this tests heproduced a commercial computer programme ‘SANDS’, which gives recommendations onscreen slot width.Most of this work has been centred aroundfinding a suitable gravel size for gravel packingand then selecting a suitable mesh or slot sizeto retain the gravel. There is relatively littleexperimental evidence to support mesh sizingfor stand-alone type applications. For thisreason, it is currently recommended thatscreen sizing selection should be based ongravel sizing and confirmed by laboratorytesting. However, most tests and theoriesindicate a stand-alone media pore throat to bewithin the d10 to d50 range depending onuniformity and fines content.

Figure 15: Gravel pore throats and sandparticles

Figure 16: Mesh pore throats and sandparticles

Selection of Test SandsAfter having measured a range of samples from a reservoir, some sort of sand distributionhas to be chosen on which to base either laboratory testing or the application of theoreticalscreen sizing rules. This can be a difficult decision if there is often a very wide variation insand sizes and distributions. Although there is no laid down procedure to assist in thisprocess, selection of a suitable sand is a matter of reviewing the available information andfield experience. Figure 17 and Figure 18 show some typical particle size distributions whichillustrate the difficulty in applying simple rules to mesh & media selection.

D = 630 microns

d = 90 microns

D = 6.5 x d

D = 630 microns

d = 260 microns

D = 2.4 x d

D

d

Sand screenmesh sizingprotocol similarto gravel pack

weft wirewarp wire

aperture orpore throatsand particle


Figure 17: Field PSD data showing heterogeneous sands (plotted non-standard)

Each line in the figure represents an individual sample sand taken at different point along awell or field or zone.Consideration would be given to the weakest sands, ie most prone to sanding and also to thesands exhibiting most permeability. If the data is from a vertical exploration/appraisal well,there may be sands included which would not be targeted in the development wells.

Figure 18: Field PSD data showing a very homogenous reservoir (plotted non-standard)

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.)

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Traditional Gravel Size SelectionGravels used in sand control are defined by their sieve sizes. For example, a 20/40 gravel issieved between a 20 and a 40 US Mesh, ie particles range in size from 840 microns from420 microns, with a median grain size of 584 microns. See Table 3: Mesh size conversiontable.Saucier’s rule multiplies the formation df50 grain size by 6 to obtain the gravel grain size:

dg50 = 6.df50

For example, if a formation sand has a median grain size of 100 microns, the median gravelsizing (according to Saucier) should be 600 microns, and a 20/40 gravel would be selected.

GravelMin Grain

(µm)

MaxGrain(µm)

MedianGrain*(µm)

GravelPore

Throat(µm)

GravelPermeability*

(Darcy)

ScreenSlot Size(inches)

12/20 840 1680 1260 200 450.020

(500µm)

20/40 420 840 630 75-90 121.012

(300µm)

40/60 250 420 335 45-50 45.008

(200µm)

Table 2: Gravel and slot size data

* = approximate value

A pore throat is the size of the smallest restrictions in a porous medium (rock or gravel pack)and so the pore throats associated with the gravel correlate with the median formation grainsize when the Saucier criteria is used.Other rules or criteria include Schwartz, Stein and Hill but these are not commonly used.More details on gravel and slot sizing can be found in Section 7.

Wire-Wrap Slot & Mesh SizingWire wraps have been sized according to Coberly criteria, ie 1 to 2 times d10, and somecompanies base their slot sizing on d15. Meshes with their larger open area can be sizedmuch smaller without plugging, somewhere equivalent to the d50 is often found to be theoptimum size in tests. This means that much better sand retention is gained by the smalleraperture size, ie less small particles and ‘fines’ can pass through.

Sand Filter-cakeExperimental evidence indicates that once a sand layer (filter-cake) has built up on thescreen surface, no more sand is produced. This sand layer prevents fine particles fromgetting to and through the screen and protects the screen from further plugging. It is verydifficult to provide an accurate estimate of the sand cake permeability. A rough estimate ofunconfined natural sand ‘pack’ permeability is provided below:

K = 760 . (df50/1000)2 . e(-2.8355 x LOG10(Cu))

where K is permeability in Darcies


This value needs to be modified for packing, ie the pack structure, and for example requiresto be reduced by about 30% for gravel packs. Other correlations exist to estimate the sandpermeability, but more work is required to relate this to sand filter cake permeability. Theoverall filter cake/screen permeability will be improved by the gradual formation over thescreen surface so that the filter cake is packed lightly and by increasing the porosity of thescreen. For this reason, sand control completions should be brought onto full productiongradually over as long a period as is practical (1 day to 6 months) and a bean-up procedureshould be developed and discussed with operations/production.

Sand SamplesTo select the optimal screen and slot size, particle size analysis data (ideally from corematerial) from the relevant zones are required. Alternatively, actual sand samples would besatisfactory so that a measure the particle size distribution can be performed in-house.Listed below are the types of samples that might be available, in order of preference.

Core Material (from full capture core barrels)The particle size analyses should have been performed on actual core material from thesame formation that they intend to drill through. The distributions of most interest are thosefrom the weak zones which are anticipated to fail during production. An indication of thespread in sand sizes that the screen will need to control is also necessary for correct slot sizeselection.

Sidewall cores (from a wireline run core sample taker)These can be crushed and mud contamination can cause problems. Therefore distributionsfrom these samples may not be representative, tending to be finer than the formation fromwhich they were sampled. Rotary side-wall cores are an improvement over standardpercussion types.

Bailed samples (taken from the bottom of the well)These are not ideal and will only represent the coarse end of the produced sand, ie thatwhich has not made it up the well-bore during production. These samples will be coarserthan the parent sand and the distribution will give no indication of the sorting of the parentsand because the finer fraction will be missing, so both the size and amount of the finermaterial will not be known.

Separator samples (taken from downstream surface separatorAs with bailed samples, these will not give a representative distribution of the parent sand. Inthis instance mostly the finer material will have made it into the separator, the coarse fractionwill be missing. Additionally, the very fine particulates may have passed further down theprocess and so the remaining sample is in all probability not representative of the sandhitting the screen.

Open Area to FlowAll filters will plug with time if they are doing their job. The bigger the open area/porosity, themore the resistance to plugging as there are more pore throats available and open to theflow path. Also the higher the open area/porosity, the lower the velocity of fluid flow throughthe media and the lower the pressure drop – less erosion and more production.


High Inflow AreaA sand control screen needs to have highly porous media, but it is also important that the fullsurface of the media be exposed to the well flow. Figure 19 illustrates the typical open areaassociated with wire-wrap, pre-pack and mesh type screens.

Figure 19: Effective inflow areas of wire-wrap, pre-pack and multi-layer screens

The flow of fluids through the screen (the screen face fluid velocity) is also important toconsider. At low flow rates, this flow is likely to be laminar and at higher rates the flowbecomes turbulent. The flow rate has an impact on which parts of the screen are exposed toflow and hence erosion. Generally, at high rates the flow does not change direction andfinds the shortest path through the screen.Typical wire-wrap screens (WWS) have between 6-12% open area (with smaller wrap wires,this may be increased). The flow is straight through with a small diversion to the base-pipeperforation. The open area of wire-wrap screens can be improved by using very thin wrapwires, although this has the draw-back of making them more sensitive to damage anderosion.A pre-pack screen usually consists of either an outer wire wrap screen or a perforated outershroud which has less than 20% open area. The pre-pack gravel (or proppant) has aporosity of 25-35%. At high rates, the flow is funnelled or concentrated over the base pipeperforations. Due to the random and three-dimensional nature of the pre-pack, it is no longerpossible to measure the open area and hence porosity values are quoted. The porosity (andhence permeability) of the pre-pack can be improved by using very uniform and sphericalgrains or beads. Micro-Pak and Perma-Bond screens hence are typically made with aceramic proppant which is more porous and permeable than the gravels commonly used byother manufacturers.Mesh screens have a higher media porosity behind a protective shroud. Some screens havedrainage layers included to distribute the flow over the base-pipe perforations. Stratapacscreens also have an outer drainage layer to distribute flow over the media.Most conventional screens have roughly similar outer diameters and screen lengths and sothe screen surface area is similar. Certain screen types are slimmer than others and so alarger base pipe and hence outer diameter can be used. This is true of the Dura-Grip, Micro-Pak and Stratapac products. The area open to flow is hence this screen surface area timesthe open area % (or the porosity % assuming all the media is exposed to the flow).

Prepack Screens:

30% Open Area of Gravel under 6-12% Open Area of WWS or Perf. Shroud

Multilayer Screens:

50-70% Open Area of Media Under Flow Distribution Layer

Wire Wrap Screens: 6-12% Open Area


A major advantage of the ESS product is the very large outer diameter, mesh media and nodead space at the couplings allowing it to provide the highest inflow area of all.

Maximising Media PorosityAnother way of increasing the open area to flow is by increasing the media porosity ininstances where increasing the diameter is a difficult option. Open area is key to minimisingplugging, reducing local velocities, and providing low pressure drop across a screen. Figure20 illustrates different types of media and how their construction relates to porosity.

Figure 20: Effects of fibre diameter on porosity (top). Effects of sand control media designon porosity (bottom)

The top illustration in Figure 20 shows how reducing fibre diameter increases the amount ofopen area in a given media. For well screen applications, mesh and pre-pack screens areoften used in stand-alone situations or behind gravel packs as insurance. They are thereforeexposed directly to the formation sands and so should be capable of controlling it. Typicallythree different types of media are employed; gravel (pre-pack screens), woven media (Dutchtwill weave (DTW) and/or square weave mesh screens), and non-woven fibrous media.Gravel typically has a porosity of 25-35%, reflecting the large bulk of the gravel making upthe matrix. Woven wire technology can increase the porosity to approximately 45-55%. Wiremeshes can be made with larger open areas, but to do this would require either making amesh with far too large a pore size for sand control or making a mesh with very thin wireswhich would have very little tensile strength. The design of weave has an impact on sandretention through the resulting aperture or “pore-throat” size, and also in fluid pressure drop.Weatherford uses woven wire technology in ESS and can also in Stratapac. Additionally inESS, the weave has to be designed to be flexible to withstand the expansion process.The non-woven media used in Stratapac is made from a composite of powdered metal in awire square mesh substrate (Porous Metal Membrane or PMM as shown in Figure 62) orsandwiching thin fibres in between square mesh substrates (Porous Metal Fibre or PMF-II(second generation) as shown in Figure 63). The woven mesh substrate has a large poresize, but high tensile strength. The non-woven fibres or powder have very high porosity andcontrolled, engineered pore sizes. This gives the PMF and PMM metal media the followingimportant characteristics:

Gravel

Porosity = 30%

Woven Media

Porosity = 45%

Non Woven Media

Porosity = 70%


• High porosity range (50-70%)• Excellent sand control capabilities (βx filtration factor) for a wide variety of sand

types and sizes.The PMF pore throat distribution closely matches that of gravel and generally correlates withthe particle size distribution of the formation sands. This promotes interface bridging and theformation of permeable sand layers over the screen. See Section 7 on gravel pack sizing.

Fixed Media ConstructionFor Stratapac screens, the filtration media is sintered to prevent ‘unloading’ and ‘mediamigration’. Sintering involves heating the woven and non-woven media to very hightemperatures at which the individual fibres bond together and fuse to the restraining mesh.This means that the fibres are not free to move and hence the ‘pore-throats’ are fixed.Similarly for wire-wrap screens, the slot size isfixed. In pre-pack screens, the proppant isnormally resin coated and cured at hightemperatures to fix the proppant grains in place.‘Unloading’ (see Figure 21) occurs when aparticle is forced through a non-fixed poremedium under applied differential pressure. Afixed media will not give way to sand particlesunder normal applied differential pressures.

Figure 21: Unloading of non-fixed pore medium

Media migration (Figure 22) occurs in non-fixed pore media when the media gives way underapplied pressure or with high flow-rates and forms part of the flow stream. This occurs withsome non-sintered fibre type (wire-wool) screens and leads eventually to premature screenfailure.

In ESS, the woven mesh is unsintered as some flexibility isrequired while the screen is expanded and the mesh needsto move over the basepipe. Note that the mesh itself is notexpanded and retains its pore-throat sizing. The ESS hasan extremely high open area to flow and so its performanceshould at least match and is expected to exceed that ofunsintered premium screens.

Figure 22: Media migration

Draw-DownThe pressure developed across a sand screen is often of concern to operators. In actualfact, a properly installed screen with good plugging resistance will develop very little pressuredrop as the filtration media has a very high permeability and is relatively thin. The formationof a sand filter cake on the screen surface is much more significant and this is improved by aincreasing the drainage area or pore throat density beneath the cake, ie increased mediaporosity and open area.Draw-down over the screen is mainly important in short length of screen with highproduction. High draw-down is also associated with erosional risks due the higher particlevelocities expected. For horizontal completions, pressure drop associated with the screen isgenerally neglected.


Borehole SupportIn gravel packs and ESS deployments, the well bore is supported to some extent. Thismeans that flow in the well-bore and screen annulus is restricted or eliminated, and that thehole is prevented from collapsing. This has several advantages over stand-alone screensystems:

• Production is forced to flow directly into the screen allowing flow conformance

• Flow conformance means that hot spot erosion channelling is unlikely (ie annularflow channelled into the screen at a point of well bore collapse).

• Zonal isolation and treatment is possible

• Load bearing sand grains are kept in place, reducing total sand production

• Any collapsed well sections are prevented from producing increased sandquantities

• Production logging and reservoir management is more effectiveA certain amount of borehole support is provided by an ESS system, but there may also be adanger that the in-situ rock stress could cause collapse of the ESS. For horizontal and highangle open hole applications, an assessment of the rock stresses during depletion isrecommended to ensure that these will not exceed the strength of the ESS.Weatherford has developed an analytical model for the generalised prediction of rock failurewithin a well-bore. The same model can be used to predict the point at which an ExpandableSand Screen will begin to elastically and plastically deform. Weatherford uses the model asa means of screening for ESS candidate well selection. Because of the spread of theanalytical data points used to construct the model, caution should be used in interpreting theresults to all rock types and potential stress regimes.

Basis of model designThe rock mechanical input value used in this model is the Thick Walled Cylinder (TWC) testvalue of the rock. This is accepted as the best form of measurement of a particular rock’sreaction to the triaxial stresses imparted on it under relevant downhole conditions.This program corrects (reduces) the TWC value for hole size (TWC① ) and corrects(increases) TWC① to compensate for the effect of borehole support, TWC② .

Borehole size correction - TWC①①①①

Larger well bores allow formation rocks to fail at lower applied stresses than those observedin the TWC test. TWC① is calculated from a linear regression plot of experiments carried outin 9mm, 115mm and 216mm holes using Castlegate sandstone with a typical TWC value ofapproximately 275 Bar. The correction has not been benchmarked for other sandstone typeswith different strengths. Work performed by van der Hoek of Shell International suggests therelationship between TWC and hole size remains approximately constant, irrespective ofTWC.

Borehole support correction - TWC②②②②

TWC① is then corrected (increased) to reflect the support offered by the ESS. Under localfailure conditions, failed particles of rock can be trapped in place by the ESS leading to aredistribution of stresses away from the ‘well-bore’. The ESS will not see load until this newsteady state condition, TWC② is again exceeded, at which point the ESS will start to seeelastic deformation.


Results of tests with ESS support in 4-1/2” hole have been used to determine the stressesrequired to initiate ESS deformation. In all tests, stresses much higher than the rock’s TWC①

were required to elastically deform the ESS. These tests also showed that a stiffer ESSwould increase TWC② failure point more than a weak ESS. Further, for a given ESS type,significantly higher TWC② values are obtained with increasing base TWC. This program hasbeen calibrated on tests performed in loose sand and Castlegate sandstone. Again, a linearregression plot is used to infer TWC② for rocks with strengths between the data points andbeyond. The linear regression plot is considered conservative in its estimation of TWC② withESS support.

Calculation of effective stressesThe model assumes an isotropic, homogeneous reservoir. The applied vertical stress iscalculated from the overburden gradient. The effective vertical stress is calculated from theapplied stress and the prevailing reservoir pressure, modified by Biot’s constant (orporoelastic constant). Effective horizontal stresses are considered equal and derived fromthe vertical stress using the rock’s Poisson ratio.The maximum combined stress acting on a well-bore of a given angle is calculated from aforce vector resolution of the effective vertical and horizontal stresses.

Results presentationThe results of the modelling run are presented graphically as a plot of effective stress versusreservoir depletion. Once the resolved effective principle stress acting perpendicular to theborehole exceeds TWC② the ESS begins to elastically deform.

Figure 23: Stress analysis plot

RESERVOIR STRESS ANALYSIS PLOT

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

210023002500270029003100

RESERVOIR PRESSURE (PSI)

STR

ESS

/ STR

ENG

TH (P

SI)

TWC HOLE DIAMETER CORRECTED TWC ESS SUPPORT CORRECTED TWC ESS YIELD THRESHOLD EFFECTIVE STRESS ON HOLE (PSI)


Once this stress exceeds TWC② plus the ESS collapse pressure, permanent plastic failure ofthe ESS can occur. This line is represented as the “ESS yield Threshold”.This run predicts failure of the reservoir as soon as it is drilled since the effective stressesexceed the borehole support corrected TWC. Further, it predicts the ESS being loadedelastically throughout the reservoir depletion, but not plastically. The ESS could be usedsafely in this scenario with a 400 psi stress safety margin.

The derivation of this model is discussed in more detail in Section 8.


Mud and Drill-in Fluid Conditioning RecommendationsDrilling and Completion FluidsWeatherford Completion Systems does not supply drilling and completion fluids; however, itis recognised that the success of a sand control completion will be affected by the choice ofthese fluids. Communications between the fluids supplier and Weatherford CompletionSystems is important to optimise the drilling mud and the well clean-up procedures to preventformation damage and plugging of the screens.Normal sand control standard practice includes recommendations to minimise plugging ofthe screens at all the stages of the installation including handling of the screens on the pipedeck, making up the assemblies, usage of pipe dope, solids composition of drilling mud,removal of mud filter cake after screen installation.

Mesh & Microns Conversion TableUS Standard

Mesh Tyler Mesh Microns Mm inches4 4760 4.76 0.1875

6 6 3360 3.36 0.13238 8 2380 2.38 0.093712 10 1680 1.68 0.066114 12 1410 1.41 0.055516 14 1190 1.19 0.046818 16 1000 1.00 0.033320 20 840 0.84 0.033125 24 710 0.71 0.027930 28 590 0.59 0.023235 32 500 0.50 0.019740 35 420 0.42 0.016545 42 350 0.35 0.013750 48 297 0.297 0.011760 60 250 0.25 0.009870 65 210 0.21 0.008280 80 177 0.177 0.0069

100 100 149 0.149 0.0059120 115 125 0.125 0.0049140 150 105 0.105 0.0041170 170 88 0.088 0.0035200 200 74 0.074 0.0029230 250 62 0.062 0.0025270 270 53 0.053 0.0021325 325 44 0.044 0.0017400 400 37 0.037

477 31 0.031565 26 0.026673 22 0.022800 18.5 0.018

Table 3: Mesh size conversion table


Open Hole Completions OverviewThe standard practice for open hole sand control completions is to drill the reservoir with aconditioned, screen compatible, non invasive, non damaging drill-in fluid. Screen compatiblemeans that the solids composition is designed so that the majority of particles flow throughthe filter media. After the reservoir has been drilled, the mud is conditioned to maintain theparticle size within the desired range. The open hole can then be displaced to a low solidcontent mud (base polymer in the case of water based mud) taking care not to disturb themud filter cake. When the work-string is pulled out of the hole, the riser is then displaced to afiltered completion fluid. The screens are made-up at the rotary table and the filtered fluiddisplaces the air inside the screens without plugging the filter media. Having run in the holewith the screen assembly, it is a practice to displace the drilling mud to a clear fluid, oftenincorporating a breaker system, prior to bringing the well on production. However, thistechnique cannot always be practised for the following reasons:

• Displacement of long horizontal sections from synthetic mud to brine can beineffective and result in damaging emulsions

• Displacement of the open hole to clear fluids could result in large losses, with thepotential for severe formation damage and a well control incident

• For very heavy muds, the sand screen (eg pre-packs) cannot be selected to allowmud solids to pass through

• The combination of formation and filter cake does not restrict hydrocarbon flow intothe well-bore.

An alternative clean-up strategy employed in some applications consists of conditioning themud over fine shaker screens, running the sand control screens to bottom, and then backflowing mud through the screens without displacing to clear fluid or attempting to remove themud filter cake. Nevertheless, experience using this technique highlights the following:

• The need to carry out formation damage testing on representative core samplesprior to drilling the reservoir section

• Laboratory mud may give different results to field mud• Careful control of excess emulsifier levels is required when the mud is run in the

field• Back production of mud through screens can be a viable technique, but carries an

increased risk of screen damage and should be verified in the laboratory with fieldmuds

• The key to implementing this strategy is very careful rig-site treatment of the mud tocontrol the solids size distribution, including the use of screen plugging apparatus

• Pre-packed screen are more prone to plugging while single membrane screensallow drilling fluid solids to pass more easily

• Mechanical skins of around zero can be achieved with the right combination of filtermedia and optimised drilling fluid

• Good quality control is required in the mechanical integrity of the screens.• Rotation of Pipe/Pump Rate - during pumping operations it is critical that the pipe is

rotated as this will draw the clean-up chemicals into the low side of the hole. It isalso important that any clean-up pills are pumped in turbulent flow to aid holecleaning.

The final recommendations on well clean-up procedures should be the result of differenttests taking into consideration the challenges and limitations of a particular development. Itis recommended that laboratory (ie return permeability, mud cake clean up) and flow looptests to evaluate screen plugging and formation damage potential are performed. Flowing


mud once conditioned over fine mesh shaker screens, requires careful quality control of themud properties on the rig site prior to running the screens to ensure the screens are notplugged either while running in or during initial hydrocarbon production. It is alsorecommended that steps be taken to ensure unconditioned mud cannot be introduced intothe mud system for example through unconditioned mud lines or reserve pits.

Mud Cake RemovalOpen hole completion present a special challenge to screen because the mud filter cakemust be removed or reduced after the screen is installed. The residual filter cake andinsoluble drill solids collected in the cake can damage screens if not removed or treatedproperly. The best approach is to have as little insoluble solids in the fluid as possible andmaintain a small particle size. This can be achieved by running the drill-in fluid through anefficient solids control system while drilling and monitoring the insoluble solids quantity andsize. A suggested procedure to monitor mud quality is the mud plugging index test (MPI)described in the section on Lab & Well-Site Mud Conditioning Tests below.Each mud company will have recommendations regarding the pump rates required toachieve successful clean up of the mud filter cake. These are often hole deviationdependent, varying between 130 ft/min and 300 ft/min for horizontals.It is important to have as thin a cake as is safe and practical. The various muds havedifferent mud cake properties. Oil base muds can have relatively thin cakes and some othermud types can have thicker or ‘fluffy’ cakes if not scoured properly. It is recommended toverify that such mud cakes can lift off and flow back through the screen without impairment.Removal of the filter cake can be achieved either by chemical dissolution or back flowing thecake through the screen. Successful back-flow of mud filter cake requires that the solids inthe cake do not bridge or plug the media. For the mesh media this requires that the solids betypically less than 75 microns. Most drill-in fluids have particles well below this size. Largerparticles, drilling debris and agglomerates can be minimised while drilling by conditioning thefluid system through shaker screens. To achieve efficient mud cake flow back, it is desirableto have a sufficient draw-down over the borehole wall along the length of the completion. Forthis reason, the cake lift-off draw-down pressure is very important. A low value is preferableto ensure the cake is back flushed entirely.For the Stratapac PMM and pre-pack type screens, back flowing the mud filter cake is notrecommended unless a very fine (<30 micron) drill-in fluid can be used. All residual drill-influid in the hole must be displaced to a solids-free fluid before running the screens and thedrill-in fluid must be conditioned while drilling through <53-micron (sub 270 mesh) shakerscreens.The mud is sized to control losses into the formation and also to control the well. For heavymuds, the particle size may need to be quite large. Pore throat sizes of the formation can beestimated using the Kozeny correlation,

r = ( 8 x ku / Ø )0.5

where r = pore throat size in µmku = permeability in µm2 [to convert mD to µm2, multiply by 9.8717.10-4]Ø = porosity fraction

For example, a uniform sandstone has a permeability of 480 mD and a porosity of 17%. Thepore throat size can be estimated at

( 8 x 480 x 9.8717 x 10-4 ÷ 0.17 )0.5 = 4.72 µm


Another more common approximation is to use the square root of the permeability inmillidarcies, ie

r = (k)0.5 = ( 480 )0.5 = 21 µmFrom stable arch theory, it is assumed that particles down to 1/3rd of the pore throat size canbe effective in bridging and plugging off the formation. Therefore the optimum particle size inthe drill-in fluid would be between approximately 21 µm and 2 µm. These particles wouldeasily pass through most mesh sand screens sized 100 µm and above.Mud particulates should be kept to at least less than a 1/3rd of the aperture size of the mesh.For example, an ESS 230 screen mesh should be used with a mud shaker mesh size of 77microns or less. Referring to Table 3, the appropriate shaker screen would be a 200 USMesh. For an ESS 150 screen, the particles should be less than 50 microns and hence a270 or 325 US Mesh. Please note that in all cases, these are only guidelines and theyshould be confirmed by laboratory testing, field experience and in co-operation with theselected mud company.To summarise, the following procedures are recommended for optimum productivity of themesh screens (Stratapac and ESS) in open-hole wells.

Water Based Drill-in FluidsESS & Stratapac PMF

! Shaker screens should be sized by the 1/3rd rule. Condition drill-in fluid throughmesh shale shakers while drilling to minimise size and amount of insoluble drillsolids

! Monitor mud quality while drilling to maintain good flow through screens! Displace residual drill-in fluid with “virgin” system containing only base brine,

polymers and starches, leaving only a thin filter cake of drill-in fluid solids andinsoluble drill solids

! Remove cake by back flowing through screen or dissolving in appropriate breakersystems.

Stratapac PMM! Condition drill-in fluid through 325 mesh shale shakers while drilling to minimise

size and amount of insoluble drill solids.! Monitor mud quality while drilling to maintain good flow through screens.! Displace residual drill-in fluid with “virgin” system containing only base brine,

polymers and starches, leaving only a thin filter cake of drill-in fluid solids andinsoluble drill solids.

! Dissolve cake with appropriate breaker systems. Back flowing cake through PMMwill not provide optimum productivity.

Oil/Synthetic Based Drill-in FluidsESS & Stratapac PMF

! Shaker screens should be sized by the 1/3rd rule. Condition drill-in fluid through200 mesh shale shakers while drilling to minimise size and amount of insolubledrill solids.

! Monitor mud quality while drilling using the MPI test to maintain good flow throughscreens.

! If possible, displace residual drill-in fluid with “virgin” system containing only basefluid with no bridging solids, leaving only a thin filter cake of drill-in fluid solids andinsoluble drill solids.

! Remove cake by back flowing through screen! An acid wash recipe can be formulated as a contingency wash procedure.


Stratapac PMMPMM is best used in a sized salt system and is not recommended for oil-based or synthetic-based fluids. However, if necessary, the following procedures will minimise plugging.

! Condition drill-in fluid through 325 mesh shale shakers while drilling to minimisesize and amount of insoluble drill solids.

! Monitor mud quality while drilling with MPI test.! Displace residual drill-in fluid with “virgin” system containing only base fluid with

no bridging solids, leaving only a thin filter cake of drill-in fluid solids and insolubledrill solids. PMM cannot be run into solids-laden fluids.

! Back flowing cake through PMM! An acid wash recipe can be formulated as a contingency wash procedure.

Cased Hole CompletionsOptimum performance of the screens can be obtained if all drilling debris and mudcontaminants are removed from the well bore prior to running the screen. The proceduresinvolved for preventing screen damage to fine pre-pack screens may involve runningscrapers, viscosified pills, solvents, and circulating completion fluid filtered to 2 microns andpreferably 0.5 microns (β>5000) until the NTU of the returns are below 25 – an onerousprocedure and this may be relaxed with coarser mesh screens. Because of the variety ofcompletions and fluids no single recommended clean out procedure can be provided,however some general guidelines are given below1.

Fluid Cleanliness CriteriaBefore running the screen the return fluid cleanliness should be below 100 mg/L (100 ppm).Field experience has shown that the particle size distribution in filtered completion brine with100 mg/L or less usually has 50 percent of particles smaller than 2 microns.However, clear fluid returns do not insure that the well has been displaced of all solids. Forthis reason displacement techniques should be incorporated which include mechanical andchemical clean up methods.A clean displacement fluid is required for the following reasons:

• To prevent damage to the formation and screen from particulate matter• To prevent solids interfering with the operation of down-hole equipment such as

packers, hangers, etc.• To allow corrosion inhibitors to function by coating the screen, casing and down-

hole equipment.To effectively displace and clean-up the well it is very important that the following two criteriaare met:

Conditioning the mudA properly conditioned fluid will be least damaging to the screens, therefore it is veryimportant to circulate and condition the mud in the pits as well as in the hole. Conditioningshould heat the mud, reduce the viscosity and break down any progressive gel strengths, butnot to a level where it loses its ability to suspend weight material.

1 Information provided by Well Flow Technologies, Inc.


Rotation of Pipe/Pump RateDuring pumping operations it is critical that the pipe is rotated as this will draw the clean-upchemicals into the low side of the hole. It is also important that clean-up pills are pumped inturbulent flow to aid hole cleaning.The most effective displacements use a multiphase spacer system. The system iscomprised of the following:

Oil Based Mud• Base Oil• Weighted viscosified spacer• unweighted viscosified spacer• Solvent/surfactant wash• Unweighted viscosified spacer.

The weighted and unweighted viscosified spacers should contain a water wetting surfactantwith oil mud particle suspension action. The solvent/surfactant wash should include a blendof solvents and surfactants that will remove any remaining OBM residue including pipe dope,sand, barite and other solids on the metal surfaces.

Synthetic Based Mud• Unweighted viscosified spacer• Sea water• Solvent wash• Solvent/surfactant wash• Unweighted viscosified spacer.

The unweighted viscosified spacers should contain a water wetting surfactant with oil mudparticle suspension action. The solvent wash should contain a blend of solvents specificallydesigned to remove pipe dope and oil based muds. The solvent/surfactant wash shouldinclude a blend of solvents and surfactants that will remove any remaining OBM residueincluding pipe dope, sand, barite and other solids on the metal surfaces

Water Based Mud• Viscosified spacer• Sea water or brine• Solvent wash• Sea water or brine• Completion fluid.

The viscosified spacers should contain a water wetting surfactant with oil mud particlesuspension action. The solvent wash should contain a blend of solvents specificallydesigned to remove pipe dope and oil based muds.

Lab & Well-Site Mud Conditioning TestsPrevious information highlighted the critical need to control mud quality to minimise screenimpairment. Conditioning criteria must be determined by preliminary laboratory testing usingthe screen medium selected and the actual DIF to be used on the job (preferably a fieldsample). These tests will determine the shaker screen mesh size required to provide


adequate protection against screen impairment and quantify the mud plugging index (MPI) tobe used on the rig to control mud quality prior to running the screen in the hole.

Example Mud Conditioning Qualification TestTo select the shaker screen size that will remove mud solids to prevent screen plugging.The mud contained in the annulus between the screen and well-bore wall must be producedthrough the screen. Assuming a 1’’ annular space, this corresponds to a mud volume of2.7L/ft² of screen (assuming a 5 ½ in screen inside an 8 in hole). Scaling it down to thescreen sample size (2.5 in disk), this corresponds to 94 cc of mud. Using a safety factor of10 (to account for possible damage during the running of the screen), the mud volume thatmust flow through the screen sample must be equal to 940 cc. A simple criterion to use is topass 3 ‘lab barrel’, ie 1050 ml of mud through the screen2. The equipment required wouldconsist of:

• HPHT cell with 2.5’’ screen media disks and drainage meshes.• Sieves of various size (200, 230, 270 & 325 mesh).• Constant air pressure source (100 psi).• Graduated cylinder.

See Figure 24 for set-up.

Procedure1. Install the screen samples at the bottom of the cell with the proper drainage mesh

underneath and close the valve downstream of the cell.2. Fill the container with unconditioned mud (approximately 500 ml) and pressurised the

reservoir to 100 psi.3. Flow 350 ml of fluid through the screen and relieve the pressure inside the cell.4. Repeat step 2-3 a total of three times (to get a total mud throughput of 1050 ml) or record

the total volume of mud throughput;5. Change the screen sample.6. Repeat step 1-5 with mud conditioned through 200 mesh sieve, mud conditioned through

230 mesh sieve, etc. down to 325 mesh sieve if necessary, until the required volume ofmud (1050 ml) passes through the screen sample without plugging it.

2 This rule of thumb is used by BP in the North Sea (Clive Bennett, personal communication).


Screen Sample (2 1/2" disks) and Drainage Mesh

HPHT Cell (500 mL)

Pressure Source (Air or Nitrogen)

100 psi

Pressure Relief Valve

Graduated Cylinder (500 mL)

Figure 24: Equipment set-up for laboratory mud qualification test

Example MPI Index for Use with Well-Site Mud Condition TestingTo qualify a field test procedure that will be used to monitor mud quality and conditioning onthe rig, prior to running the screen. This test is very similar to a Marsh funnel test but isdesigned to monitor plugging instead of fluid rheology.When plugging occurs, the leak off rate through the screen medium decreases. Bycomparing the leak off time, T1, of a first batch of mud (1 lab barrel), with that of a secondtime, T2, through the same screen sample, one can evaluate the plugging tendency of thescreen and monitor mud quality. This test is designed to require very little equipment andexpertise so that it can easily be run on the rig.The equipment required consists of:

• HPHT cell with 2.5’’ screen media disks and drainage meshes.• Graduated cylinder.• Stopwatch.

Procedure1. Close the valve underneath the screen sample and fill the mud cell with 350 ml of

unconditioned mud.2. Open the valve and measure the time taken by the mud volume to flow through the

medium, T1.3. Close the valve again and fill the mud cell with another lab barrel.4. Open the valve and measure the time taken by the second lab barrel to go through the

same screen sample, T2.5. Calculate the MPI: MPI = T2/T1. Note: When no plugging takes place, MPI = 1. As

plugging increases, MPI increases.6. Change the screen sample and repeat the test with mud conditioned through the sieve

selected from the Mud Conditioning Qualification test. Note: in this test, no pressure isapplied. The mud flows by gravity only.


These two MPI indices obtained with the unconditioned and conditioned mud provides arange that can be used as baseline for the subsequent mud monitoring on the rig.Circulation through the shakers should be continued until the MPI is typically within 20% ofthe value obtained in the lab3.Note: the issue of using a new mud system versus the old one seems to be open todiscussion amongst operators. Some prefer to use a new mud, free of drill solids; othersprefer to continue using the old mud as it is better dispersed by the repeated circulation inthe well.

CentralisersThe centralisation of sand screens is highly recommended. This is thought to protect thescreens during installation and to improve well bore clean out. For certain extended reach orunusual trajectory wells, some sort of centralisation is normally required to reduce frictionand improve deployment. Weatherford has the most complete range of screen centralisationoptions available and has developed in-house software to aid in centraliser placement(Centro-Pro Plus) and also has Torque & Drag software for more detailed well planningrequirements.For conventional type screens, Weatherford centralisers can be provided to customerspecification at an additional cost. Typical centralisers are welded fins, spring bow, LoDRAGand LoTORQ roller systems and Spira-glider rotating centralisers. They can be placed:

• Top (Box) – if placed at the box, only one stop collar is required. Unless hinged,they must be installed prior to bucking-on the coupling.

• Mid-span – two stop collars are required for centralisers placed between screenjackets. Unless hinged, they need to be installed during manufacture.

• Bottom (Pin) – two stop collars are required for centralisers placed at the pin end.Can be installed at rig site.

It is recommended for stop collars to be placed between the centraliser and the screen jacketend ring or weld. This is because the end ring or weld is tapered. If the centraliser hits anobstruction and is forced against the tapered weld, it can split and cause serious deploymentproblems. For this reason, it is recommended to install the centraliser at the box end so thatit butts up against the coupling and not a screen jacket. It is recommended but not essentialthat Range 3 (40ft) pipe should be centralised in the middle of the joint (mid-span).Welded fin centralisers can be installed in various metallurgies and are normally used only incased-hole applications. Special curved fins are available to prevent casing damage.Spring bow centralisers can be used in open-hole situations and are normally used in straightor partially deviated holes.Spira-glider centralisers offer certain advantages over normal zinc or cast solid rotatingtypes, including the fact that they are crushable in the event of hole problems or irregularities,and they are lighter and malleable.Please refer to http://intranet.weatherford.com/EncycWeb/main.htm for details on the completeWeatherford centralisation systems available.

3 Other tests with mud samples conditioned with coarser meshes are also recommended to get a feeling of thesensitivity of the test with respect to particle size cut-off. Depending on the mud (mud type, solids loading, etc..), the MPI mayvary and the recommended field MPI criteria may need to be modified.


Inflow Profile along Horizontal WellsHorizontal wells are increasingly popular especially in offshore areas where the number ofwells to drain a reservoir is required to be limited. The trajectory of the wells and theirintersections with the reservoir hydrocarbon accumulations is the subject of much study andcomputer modelling. To efficiently drain a reservoir with the minimum number of wells, it isadvantageous to be able to produce along the entire length of the completion.It is often commented that horizontal wells only produce from the first 400ft (ie the heel).While this may an exaggeration, it is important to take steps to improve the productivity fromthe toe of the horizontal well.Apart from ensuring the formation damage is minimised and that the well is cleaned outproperly to control completion skin, the most effective way to ensure uniform well inflow is toreduce the frictional pressure drop associated with horizontal completion. This can beperformed in two ways, either reduce the number of perforations in the well screen orincrease the screen internal diameter.In conventional screens, reducing the number of base-pipe perforations is feasible only withvery low inflow rates associated with long production intervals. This approach has beenadopted in the Norwegian sector. Even in this case, the inflow tends to be non-uniform andmainly from the heel. In such cases, inflow control devices have been employed to introducechoke at the heel to encourage production from the toe. Weatherford is developing a self-regulating device to perform this function, the Completion Inflow Regulator (CIR). The use ofthe CIR requires careful evaluation, because it in effect obtains production from the toe at theexpense of restricting the maximum production rate of the well.

Figure 25: EGP horizontal inflow

Problems associated with the non-uniform draw-down along the screen length are illustratedin Figure 25. These include coning, inefficient reservoir sweep and drainage, poor mudclean-up at the toe, increased sand production and increased erosional risk at the heel.Another approach is the use of ESS; by increasing the screen ID, the frictional pressure dropassociated with the base pipe perforations and inflow is greatly reduced. This has the effectof evening out the production inflow along the well, see Figure 26. Also illustrated is anexpandable liner hanger greatly simplifying the well completion.

GravelPackGravelPack


Figure 26: ESS horizontal inflow

The use of ESS in horizontal wells has the following major benefits:

• Improved draw-down, clean-up and hence productivity at the toe

• Overall increase in well productivity (less or smaller wells required)

• Improved reservoir sweep efficiency (less wells required)

• Possible well slimming cost savings (rig rates and completion equipment)

• Less difference in draw-down along the well length (reduced coning tendency forwater and gas)

• Less fluid velocity in the reservoir (less erosional and sanding risk)

ESSCompletionESSCompletion


Erosion ResistanceSand screens may well be subject to various erosion mechanisms. Where erosion isexpected to be a significant risk for the completion, the use of ESS, gravel packing and highflow screens should be considered. The high flow screens have increased base pipeperforation density. These applications should be discussed in detail with Weatherford.Erosion issues are always a consideration in every screen installation. In reservoirs prone tosand production, the sand grains are carried with the produced fluids and, if this rate issufficient, can abrade and erode the metal structure of the sand control screen. The rate atwhich a screen is eroded is dependent on many factors:

• screen design• screen metallurgy• particle velocity• angle of impingement• number of particles• particle hardness and shape.

Erosion caused by actual gases or fluids is not normally a significant factor. From the abovefactors, the only effective ways in which an operator can mitigate against erosional risks is bycontrolling the particle velocity and screen design/metallurgy.Screen metallurgy is normally limited to stainless steel or an alloy material, such as Incoloy825 or Carpenter 20Cb3, and this metallurgy is associated with the wire material used inwire-wrap based screens or the metallic mesh media used in the premium and the ESS®

screens. In unusual circumstances, Weatherford does have alternative metallurgy anddesign options specifically for the premium and wire-wrap type screens. Please consult withWeatherford directly to discuss the applicability of these options.

Fluid & Particle VelocitiesParticle velocity is probably the most significant controllable factor and this can be reducedby either limiting production or by increasing the surface area of the screen exposed to fluidflow. Limiting production is not generally desirable and so increasing the surface areaavailable to flow (open flow area) is the key area. There are several ways this can beachieved in open-hole situations:

• by using of deviated or horizontal wells can increase the overall screen length and area

• by the use of a high porosity/open area mesh (Stratapac) or thin wrap wires

• by selecting the optimum mesh/slot-size for mud compatibility and plugging resistance

• by minimising dead space at connections on the screen (ESS)

• by eliminating the annulus and ensuring flow directly into screen (EGP & ESS)

• by increasing the OD of the screen (ESS).Additionally, in cased hole (IGP) situations, the fluid velocity is determined by

• the shot density

• the number (%) perforations open

• perforation hole size.


In practice, although erosional issues should be considered in open-hole situations, the areaof screen open to flow is normally sufficient to limit the downhole fluid velocities to withinreasonable limits if the completion and deployment is performed correctly.The highest fluid velocities and hence greatest risk of screen erosion are normally associatedwith cased hole perforated liners.

Cased Hole Screen CompletionsThree main options are available:

• Stand-alone screen (insert & thru-tubing) – Stratapac, Micro-Pak & Perma-Bond

• Some form of internal gravel pack

• ESS.There are often many factors which determine the optimum completion including zonalisolation considerations and, for the purposes of this report, the stand-alone screen and ESSoptions shall be considered here. This represents our current position and further work isunderway to provide more definitive guidelines.The first step in evaluating the erosional risks associated with the screen completion is tocalculate the downhole fluid volumetric flow rates. This is normally available from theoperator or can be calculated from the surface production rates (gas, oil & water), fluidproperties and reservoir conditions.Next, the velocity of the fluid exiting the perforations needs to be calculated. This will be thevolumetric flowrate divided by the total area of open perforations. This area is determined bythe perforation hole diameter multiplied by the number of perforations. This area should bediscussed with the operator as the % of open perforations may be reduced with time and thepossibility of re-perforating should be discussed. The use of high shot densities, doubleperforating and big-hole charges can be considered.Once the exit velocity has been calculated (with a 100% safety factor to allow for pluggedperforations) this should be compared to the various screen designs.Typically, a limit of 0.1 ft/sec is applied to stand-alone screen completions. Above this limit,an internal gravel pack or ESS completion should be considered.For ESS, the limit is 0.27 ft/sec based on field experience.

ESS in Cased Hole CompletionsIn unconsolidated formations, the perforation tunnel is likely to collapse and this can limitproduction. High shot densities and big-hole charges may not be sufficient. Orientedperforating should be considered to reduce sand production tendency. IGP in thesesituations can also be very problematical due to mixing the proppant with formation sands inthe perforation tunnel. The collapse of the tunnel and its producibility would be related to theformation rock strength and sand characteristics, and it may be possible to predict tunnelcollapse and well productivity from this information.In more competent formations, the tunnel may stay open and also in certain situations, thepossibility of cavities behind the casing may exist. In such cases, the produced sand/fineswill be largely retained at the ESS surface assuming the mesh is sized correctly. Theformation of the filter cake of sand particles at the screen surface will protect the screen fromerosion by the larger particles. It may be possible to pre-pack the perforation tunnels, prior toexpansion of the ESS. Erosion from fines is a much slower process as the erosion rate isalso dependent on particle mass. It is therefore advisable to bean the well up slowly to allowthe cake to form prior to full production. The bean up period can be over several weeks and


any prior sand production history would be useful information in deciding on the requiredbean up period.As the flowing perforations become filled with sand, the flow velocity in the remaining openones increases. When calculating the erosion rate to decide on the bean up periods, asafety factor should be applied to the number of open perforations and account should bemade of the typical field sand production rate in non-sand controlled wells.

Stratapac Liquid Erosion TestsTesting by Southwest Research Institute, conducted for the Erosion Joint Industry Project co-ordinated by major operators in the Gulf of Mexico, showed that under the extremely highflow-rate conditions (24kbpd/ft2) used for the testing, the erosion of the Stratapac screenmedium (as measured by a parameter called Specific Erosion and defined as gram of metalremoved per gram of sand ingressed) was relatively high compared to wire-wrap screens. Itshould be noted that in its standard configuration, the standard Stratapac screen is notdesigned for the extremely high and turbulent flow regimes experienced under the testconditions. The standard Stratapac outer and inner drainage layers are designed for normallaminar screen inflow conditions. The drainage area should be reconfigured to allow betterdrainage flow distribution if high turbulent flow rates are expected. This was achieved byusing coarser meshes to slightly increase the space between the base-pipe and the filtermedium while maintaining the screen structural integrity. The number of base-pipeperforations is also increased from 2” pitch to a 1.2” pitch.An alternative method of improving drainage by creating a gap or a void underneath the filtermedium (as in other ‘multi-layer’screen designs) was notperceived as a viable option, asthis would have reduced thedamage tolerance of the totalassembly by allowingmovement and flexing of thefilter medium eventually leadingto metal fatigue and/orpuncturing. Erosion tests of theHigh Flow Stratapac screendesign are shown below inFigure 27.

Figure 27: Specific erosion for various types of screen including Stratapac screens.Data on competitors’ screens interpolated as required from SWRI data.

Wire-Wrap Erosion DesignsIt is interesting to note that Figure 27 shows that wire wrap screen has the lowest specificerosion of all tested screens. Unfortunately, these results have not been correlated withvariations in sand retention after erosion as this was deemed beyond the scope of thetesting. A relatively simple model can be used to illustrate the importance of these variationsin opening size as a function of weight loss for a standard wire wrap screen (90 gauge wirewith 8 gauge slots (200 µm)) and a simple square weave mesh made of fine 200 µm round

0.00E+00

5.00E-07

1.00E-06

1.50E-06

Standar

d Stratap

ac

Competi

tor #

1

Dual Scre

en Pre-

pack

Competi

tor #

3

High Flow

Stratap

ac

Wire

Wra

p Screen

Spec

ific

Ero

sion

(gm

/gm

) Volumetric Flux = 23,800 bpd/ft²


wire woven to make 200 µm square opening. Figure 28 shows the effect of erosion on theopening size, assuming certain erosion pattern for both wire shapes4.

150

200

250

300

350

400

0 1 2 3 4 5 6 7 8 9 10

% Weight Loss

Equi

vale

nt S

lot S

ize

(mic

rons

)

Wire Wrap Screen (90 gage wire - 8 gage slot)

Square Weave Mesh (200 micron wire - 200micron opening)

Erosion pattern

Figure 28: Effect of erosion on the size of the screen openings. A low open area medium is a lot moresensitive to slot erosion than a high open area medium.

This simple model shows that despite its apparent high erosion resistance in terms of weightloss, weight loss erosion on wire wrap screen is much more significant than on multi-layerscreens.In gravel and fracture packs, the erosional risks may be associated with the top most screensin only during the gravel pumping, ie for a short period of time. Weatherford has thereforedeveloped special coatings and wire-shapes, which can mitigate against erosional problemsin pre-pack and wire-wrap screens. Such design changes can only really delay erosionalfailures for a matter of hours or days, and as such they are expected to be used mainly onscreens employed in gravel and fracture packs where the erosional risks are short lived.

Gas/Direct Impingement ErosionGas erosion5 is different from liquid erosion in that the abrasive sand particles are generallymore free to directly impact the metal surfaces of the screen as the gas can exert lesseffective contact pressure on the sand particle to counteract its inertia. Hence where the gasfluid path is tortuous, the sand particles impact the channel (screen) surfaces generally moreso than in liquid flow streams. This gives prominence to the so-called ‘sand blasting’ or‘scouring’ effect. This mechanism6 is dependent on the quantity, shape, relative hardness ofthe sand particles, their direction and speed and also on the metallurgy/geometry of thesubstrate. In comparing the abilities of different screens to resist gas particle erosion, themain parameters which can be controlled or otherwise affected by screen design is theparticle mass, speed and direction and these would be primarily controlled by the outerprotective cage design. 4 While the shape of the wire influences the erosion of the slots, the most important factor remains the open area of themedium, ie screens characterised by a low open area like a wire wrap screen are much more sensitive to slot changes than ahigh open area medium like the square weave mesh used in this example.

5 A more complete description of the testing performed by Scientific Laboratory Services is contained in Report SLS#6338“Accelerated Erosion Tests on Stratapac and Baker Hughes Inteq Excluder Sand Control Screens”

6 The mechanism is more fully described in “Solid Particle Erosion and Erosion Corrosion of Materials”, Alan Levy 1995 (Libraryof Congress 94-73647).


Premium Screen Shroud DesignThere are currently two major types of effective screen cages available as shown in Figure 29.

Figure 29: Cross-sections of two outer protective cage designs

Comparison Testing of Screen Designs in Gas FlowAn in-house test apparatus was developed to test and evaluate the erosion resistance ofsand screens under various gas flow conditions. Parameters such as gas velocity and sandparticle size were optimised to provide sensitive detection of the erosion resistance ofvarious screen designs. A comparison of the two cage designs using this test methoddemonstrated that the protection offered by baffles plates in a louvered cage design was notoptimal and that erosion under the conditions tested was controlled by the removal of erodedmaterial rather than by the sand kinetic energy.Also, a comparison between two Stratapac screens media (PMM and PMF2040 media) andBaker’s Excluder screen showed the superiority of the standard Stratapac screen design.

Erosion Test MethodAn erosion tunnel was built to generate gas velocities (100-300 ft/sec) such as could beexpected down-hole exiting from open perforations and to monitor screen erosion over a longtime period. Screen samples were prepared by cutting flat 3.5" discs of screen material.Each screen sample was challenged with sand introduced into a high velocity air streamunder various test conditions. Varying the line pressure controlled gas velocities, and arotating disk assembly was used to determine the actual particle velocity at the point ofimpact. See Figure 30.

Figure 30: Gas erosion test

Louvered Cage

Perforated Cage

Outer Cage

Media/Drainage Layers

Base-Pipe

Outer Cage

Media/Drainage Layers

Base-Pipe

air supply

sand inlet

to vacuum

blower

voltmeter

erosion sensor

screencomposite


Effect of Gas Velocity and Sand Particle Size DistributionA series of tests were undertaken at two different gas velocities and using two differentsands to determine the relative importance of these two parameters.Figure 31 below summarises the test results. It can be seen that the laws controllingmaterial erosion are qualitatively respected; ie the heavier the particles and the higher thegas velocity, the faster the erosion. Not surprisingly, gas velocity seems to be the parameteraffecting the most the screen erosion resistance. While sand particle size does not affecterosion as much, it is interesting to point out that even a relatively fine sand (d50 = 98 µm) ishighly abrasive to the screen.

46.3

35.3

13.18.5

0

10

20

30

40

50

60

Tim

e to

Fai

lure

(hou

rs)

Brazos Sand330 ft/sec

AFS60 Sand330 ft/sec

AFS60 Sand520 ft/sec

Brazos Sand520 ft/sec

Figure 31: Erosion resistance of Stratapac PMM screen as a function of gas velocity and particlesize distribution.

Erosion Resistance of Various Screen DesignsIn order to protect equipment against erosion, a known method is to force the abrasiveparticles to follow a tortuous path to reduce their kinetic energy and therefore decrease theirabrasive power. This concept was tested using a louvered cage as a baffle to redirect theimpinging gas flow. Figure 32 summarises the tests using the Stratapac PMM screen with itssimple perforated cage as benchmark, and an alternate design in which the perforated cageis replaced by a louvered cage. It can be seen from the test results, that the louvered cageactually speeds up screen erosion instead of slowing it down as would be expected from thekinetic theory point of view.


35.3

20.7

15.1

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40

Tim

e to

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lure

(hou

rs)

AFS60 Sand Gas Veloci ty = 330 ft/sec

Stratapac with Standard Cage

Excluder 110 with Louvered

Cage

Stratapac with Louvered

Cage

Figure 32: Effect of cage design on erosion resistance

The difference in erosional rate is also affected by the particle impingement angle; ie theangle at which sand carried by the gas impacts the screen surface. The chisel effect of sandimpinging a surface is illustrated in Figure 33, and the effects of angle of incidence onerosion rate is shown in Figure 34.

1

2

3

Figure 33: Representation of the chisel effect showing the effect of impingement angle onthe ease of penetration into the material.

0 30 60 90

Incidence Angle (deg)

0

0.01

0.02

0.03

Eros

ive

Wea

r (c

m3/

kg)

1

2

3

Figure 34: Effect of impingement angle on erosion rates of stainless steel.


At a normal or 90° angle of incidence to the surface, the erosion rate is at a low (Point 1 inFigure 34). At this high angle, the surface impacted by particles is compressed and work-hardened, but there is no active mechanism to remove material from the surface. As thechisel or sand impingement angle decreases from 90 degrees, other effects begin to appear.For example, when the chisel is at a 40-degree angle to the surface, the erosion rate beginsto increase (Point 2 in Figure 34). The erosion rate increases because material on thesurface of the screen can be more easily removed or "kicked off". This is known as achipping effect. This chipping or ‘chisel’ effect becomes worse at low angles of incidence.When the chisel or sand comes in at about 25 degrees, the erosion rate is at a maximum(Point 3 in Figure 34). Here the particles are gouging into the surface at angles which causelarge amounts of material to be chipped away from the surface. At angles lower than 25°,the particles loose their energy by bouncing off the surface instead of chipping away at it,and the erosion rate goes back to a minimum value.Applying this theory to screen design appears to be correct under the conditions tested.These conditions were selected to be as realistic of direct down-hole gas impingement onsand screens as possible and suggest that the louvered cage might actually take particlesthat would otherwise hit the screen at 90 degrees, one of the least damaging angles, andredirect them into the screen at a more damaging low incidence angle, thus increasing theerosion rate (Figure 35).

Figure 35: Schematic representation of the effect of a louvered cage on the impinging gasflow angle.

Erosional Issues Summary• Erosion of ESS and sand screens is the subject of further study during 2001.

• Where high flow rates are expected and there is an erosional concern, the use of ESS,gravel packs and high low screens should be considered.

• The fluid velocities through stand-alone screens (typically Perma-bond, Micro-Pak orStratapac) should be limited to 0.1 ft/sec, above which gravel packs or ESS installationsare required.

• For short exposure periods (during gravel or fracture packing), pre-packed or Stratapacscreens can be used at the top of the screens where the erosion risk is highest.

• In cased hole situations, the state of the perforation tunnels and performance needs to beinvestigated.

• Fluid velocities through ESS screens should be limited to 0.27 ft/sec.


Corrosion ResistanceWeatherford provides screens in a range of metallurgies to survive the various downholeenvironments encountered down-hole.Dura-Grip, Houston Weld, Free-Flow, Micro-Pak and Perma-Bond screens can be providedin the following configurations:

Base PipeCarbon Steels (N/L80, P110, etc)Chrome Steels (13Cr)Duplex & super- duplex

WireGalvanised Steel304316LIncoloy 825

Table 4: Wire-wrap metallurgy options

Stratapac Screens are provided in the following configurations:

Base PipeCarbon Steels (N/L80, etc)Chrome Steels (13Cr)Duplex & super- duplex

Mesh 316LAlloy 20 (Carpenter 20Cb3)

Shroud 316L(Alloy 20 if required)

Table 5: Stratapac metallurgy options

ESS screens are provided in the following configurations:

Base Pipe 316LSuper Duplex 25Cr

Mesh 316LIncoloy 825

Shroud 316LSuper Duplex 25Cr

Connectors Super Duplex 25Cr

Table 6: ESS metallurgy options

Most screens can therefore be provided in a configuration most well conditions and tocombat weight loss and stress cracking corrosion. When concentrations and partialpressures of O2, CO2, H2S are present and especially in the presence of water and chlorides,it is advisable to consult a metallurgist to select the optimum metallurgy screen. In certaininstances, it is also advisable to conduct corrosion tests.Screen construction and welding are qualified to the appropriate standards.Alloy 20 and Incoloy 825 are suitable by NACE standards and qualified for H2Senvironments.


Screen SpecificationsTo obtain pricing & delivery information from the manufacturing locations, the customerservices representatives would require detailed information on the equipment. An examplecustomer specification schematic is included below in Figure 36. The intention would be todiscuss the application in detail with the client and develop a screen specification which theclient can sign off on. Customer services will provide a quotation based on this specification.

Figure 36: Screen Specification Schematic

For standard screens, the tong and slip spacings and the location of any centralisers is oftenomitted from any discussion or purchase order. These dimensions are crucial in ensuring asmooth and trouble free deployment.If the application requires more technical support, various dataforms are available to assistWeatherford and client personnel to acquire the relevant data and to aid in the selection ofthe most appropriate sand control technology. These would include special forms for torque& drag analysis and in-situ stress calculations, as well as metallurgical recommendations.See Section 4 below for more details on the required information.

STRATAPAC CUSTOMER SPECIFICATION SCHEMATICOVERALL LENGTH

TONGSPACING

MID SPANSPACING

SLIPSSPACING

JACKET 1LENGTH

JACKET 2LENGTH

THREAD TYPE

PIPESPECIFICATION PIPE OD

MAXIMUMSCREEN

OD

CENTRALIZERTYPE ANDLOCATION

PARAMETER VALUE DEFAULT OPTIONSPIPE OVERALL LENGTH FEET R1, R2, R3TOLERANCE ON LENGTH INCHES +/- 12 INCHESPIPE OD INCHESPIPE WTDRIFT ID INCHES APIPIPE GRADE N80, 13CR-80THREAD TYPE API, PREMIUM (SPECIFY)MAXIMUM SCREEN OD INCHESNUMBER OF JOINTS REQ’D

CUSTOMER NAMECUSTOMER LOCATIONCUSTOMER REPWELL NAMEFIELD NAMEDRAWING DATESALES ENGINEERDWG REF #SUPERCEDES DWG REF #

P ARAM ETER VALUE DEFAULT OPTIONSMEDIA TYPE PMF1220/PMF2040/PMMMESH/MEDIA METALLURGY 316L OR ALLOY20NUMBER OF MEDIA LAYERS PMF (2), PMM (4)DRAINAGE MESH INNER (1), OUTER (1)OUTER CAGE STD O R HI PERFCAGE METALLURGY 304LNUMBER OF JACKETS 2JACKET 1 LENGTH FEET 13FT OR 15FTJACKET 2 LENGTH FEET 13FT OR 15FTTONG SPACING INCHES 18 INCHES (MIN R2)SLIPS SPACING INCHES 14 INCHES (FIXED R2)MID SPAN SPACING INCHES 6 INCHE S (FIXED)BURST TYPE STD, HIGHFLOW TYPE STD OR HI FLOW

CENTRALIZER OD INCHESHOLE/CA SING ID INCHESMIN HOLE RESTRICTION INCHESPIN CENTRALIZER SOLID, FIN OR BOWMID SPAN CENTRALIZER SOLID, FIN OR BOWBOX CENTRALIZER SOLID, FIN OR BOWATTACH CENTRALIZER SCHEMATIC(S) A S REQUIRED

COMMENTS:

CUSTOMER SPECIFICATION APPROVALNAMESIGNATUREDATE

ABERDEEN MARKETING, VERSION 1.2, DATE 29 MAR 2000


4. Sand Control System SelectionTo make the best decision on what type of sand control system should be employed, anoperator needs to consider a large range of options. The decision processes involvedchange around the world. In the Gulf of Mexico, service companies and rigs are geared upto react quickly and the techniques employed are generally well tested and proven in thatarea. In offshore West Africa for example, where the supply chain is longer andcommunications a bit more difficult, projects require more long term planning. New fields,especially North Sea or deepwater, generally take months or even years in the planningphase and each option is considered in detail.Weatherford therefore has to cater for the market demands and make its proposals andrecommendations in a timely manner. These proposals are normally made in response to aspecific customer request, but some times the enquiry is more general. It is in Weatherford’sbest long term interest to ensure its equipment is used and applied correctly. In each case,Weatherford should make its best effort to examine and discuss each request in order toensure the equipment supplied and installed best meets the expectations of the end user.To make a recommendation to the client, Weatherford would require to gather somebackground information on the proposed application. Using this information, a number ofpossible screen and deployment options are likely to be identified. This information is fedback to the end user and an assessment of each option would be made. This assessmentmay take the form of a detailed net benefit analysis involving production modelling and rigdesign, or it could be a straight forward comparison of price quotes as the completion typehas already been decided upon.It should be pointed out that the advent of expandable screen solutions require that evenpreviously straightforward client sand control decisions be re-evaluated.

Background Information RequiredWhen preparing to select a suitable sand control system for a field of well, there is a numberof important questions to be addressed and an amount of back-ground information isrequired. To assist in the gathering of the required information, a Sand Control Dataform isavailable from Weatherford. This is provided as an attachment. If this is not available, belowis a summary of the information requested.

Company & Contact Information• Details on the operator and its partners is often useful to improve the sharing of

information.

• Contact information is also mandatory.

• A delivery date required (if applicable) for any equipment together with the expectedinstallation date or well programme.

Well Profile Information• Field name

• Well name

• Location (onshore/offshore/swamp)

• Rig type (jack-up / semi / platform, etc)

• Well type (Production/Injection/Gas Storage)


• Completion - (cased/open/thru-tubing)

• ESS expansion diameter required

• Minimum hole restriction

• Expected setting depth (measured & vertical: top and bottom)

• Well inclination at setting depth

• Casing (previous casing if open-hole) - size, weight, grade

• Shoe depth (measured/vertical)

Please state the units used.

Equipment Requirements• Metallurgy requirements

• ESS expansion diameter required

• Filtration requirements (nominally 60 – 300µ) (supply sieve data if available)

• Is borehole support or conformance required?


Formation Data at Setting Depth• Formation type (ie sandstone, sst/shale)

• Rock strength - TWC, UCS (specify)

• In-situ stresses - minimum & maximum

• Rock properties (if known - Poisson’s ratio, Young’s modulus)

• Overburden gradient

• Formation characteristics (permeability, porosity, shale content)

• Requirement for or possibility of zone isolation

• Pore pressure gradient

• Break down gradient

• Formation fluid type (Oil, Gas, Water)

• Fluid used to drill reservoir (type of fluid & weight)

• Reservoir completion fluid

• Expected downhole flowrate(s)

• Expected reservoir pressures/temperatures (Initial, Depleted)


Additional Comments & Information• Please add or attach any other information / drawings that may assist in the preparation

of your technical proposal.

• Well plan or final directional survey including: Measured depth, TVD, Deviation, Azimuth,dog leg severity


• Additional information required to correct depth measured from drilling rig with depthmeasured from work-over rig

• Perforation details if any

• Production history if any

• Provide produced/injected fluid properties (eg Heavy oil, wax, asphaltenes, H2S, CO2, Cl-)

• Well schematic (including completion) with dimensions, depths, casing specifications,cement top

• Sieve analysis

• Mud/well logs

• Summary of any problems experienced (and their reasons) when drilling and completingthe well, ie well control problems, fluid losses, differentially sticking, well-bore instability,cavings, bad cement job, fishing, side-track(s).

• If there were problems with fluid losses, specify the type of LCM used and its efficiency

Previous ExperienceTo determine the correct system and media sizing, it is extremely useful to know if theoperator has used other methods of sand control, eg what type and what happened.

Sand ProductionAny information on the amount of sand production that can be tolerated, for instance the useof ESP’s etc will restrict the amount of sand that can be produced

Injection/Production RatesAgain, this is important when considering erosional and screen system sizing.

Fluid Properties (Produced and Injected)The presence corrosive chemicals will affect the screen metallurgy. Heavy oils andasphaltenes may also affect media and screen type.

Selection ProcessA method of selecting the most appropriate open-hole sand control system is depicted in theflow charts overleaf. These charts are for guidance only and are designed around a newhole well. The selection process would also be broadly applicable to cased hole and work-over wells.

Chart 1 – New Well Decision TreeThis covers gathering the background information available in order to make an initialassessment as to the requirement for sand control. Obviously in some cases therequirement is well established, but for new fields this may not be so obvious. Especially atthe end of the field life when the reservoir is at depleted pressure and beginning to producehigher water cuts. In some cases, installing sand control systems later in the field life ispractical, but for deepwater or subsea wells this is unlikely.Legal options for the disposal of produced sand, sand monitoring and sand traps togetherwith sacrificial erosional sections in the production process equipment need to be evaluated.


If a screen based sand control is not installed initially and other steps are taken to reduce orcontrol sand production, for example sand consolidation (SCON), oriented perforation orproduction rate restriction, it is well worth considering what remedial actions are possible asa result of the decision. Even when sand screens have been selected, the remedial andwork-over options need to be considered.Once, a mechanical form of downhole sand control has been decided upon, the next bigquestion needs to be examined – whether borehole stability and well flow conformancecontrol is required. If it is thought that a collapsed well on a stand-alone screen may damageproductivity or that annular flow needs to be eliminated. If zonal isolation and control isrequired, then an ESS completion would be optimal.

Chart 2 – Remedial Options Decision TreeOnce the sand control method has been selected and implemented, the well production ismonitored for sand. If this is excessive, the well is choked back until and acceptable sandrate is achieved. If production is now too low, a number of remedial options need to beevaluated. These include

• Clean-up & ESS Screen installation. This will probably involve pulling the uppercompletion and installing the ESS. The advantage to this procedure is that a relativelylarge ID is left for production and for further remedial and zonal isolation options.

• Clean-up and install a thru-tubing screen (eg Stratacoil). This is quick and inexpensivebut results in a smaller ID and hence a reduced production rate. For smaller and olderwells, these has proven to be very effective. In the future, new deployment methods willallow installation of ESS in many thru-tubing applications. Certain ‘monobore’applications are possible with ESS now.

• Clean up and install an insert screen (eg Micro-Pak or Stratapac). Again, this will involvethe removal of the upper completion and the installation of an insert screen. The screenshould be as slim as possible to provide maximum inflow area with maximum internaldiameter for production.

• Side-track the well and install sand control. Possibly an economic option when the wellexpense is compared to increased production.

• Drill and complete a new well. The last resort, but again may be economical in certainsituations.


Figure 37: Sand Control Decision Flow 1







Chart 3 – Well Flow Conformance Decision TreeOften the option to gravel or fracture pack or even run ESS is forced upon the operator byerosional and flowrate concerns. In some instances, the rates are so high that even ESSand gravel packs are risky. In these cases, there is no option but to reduce the rates orredesign the well trajectory or completion.


ESS screens offer many and considerable advantages over gravel packs. However indeviated or horizontal wells, there is a possibility that the in-situ rock stresses may overcomethe support provided by ESS and fail the completion. In these instances, a gravel packwould be required. For this reason, a good knowledge of the rock strengths in the open holesections are required. Generally speaking, the overburden pressure is normally the largeststress and so other in-situ stresses are less important in horizontal and highly deviated wells.If the other advantages offered by ESS are also important, it may be worthwhile re-planningthe well trajectory or changing the hole size to accommodate the ESS.If an ESS cannot be run due to rock strength limitations, note that this will also affect thecomplexity and success rate of the gravel pack.If the well has a low net to gross ratio (ie shales, mudrocks and silts/clays are evident) or ifthe well has some high permeability streaks or if the well is highly deviated (>62 degrees andcertainly greater than 80 degrees), then the resulting gravel pack operation is likely to bevery complex. Specialised techniques and experienced personnel would be required toachieve an adequate gravel pack. The use of shunt tubes should be considered. In reality,even companies with experience in pumping long horizontals would not guarantee achievinga perfect pack and so would use premium screens behind the pack. Depending on thecircumstances, it may be possible to use a Micro-Pak screen also. With gravel packs, thescreens should be as slim as possible and hence Stratapac and Micro-Pak screens arecandidates in this instance.If a normal gravel or fracture pack is possible, then Dura-Grip, Houston-Weld, Micro-Pak orStratapac screens can be used. If a fracture pack is expected or there is a risk of erosionduring the pumping operations, the top screens should be more erosion resistant and so apre-pack or premium screen can be used. In extreme cases, specialised erosion resistantscreens are available.If ESS screens are suitable then the completion options are simpler. If either increasedproductivity, reservoir sweep efficiency, well slimming, or OBM DIF are needed then ESSwould be the preferred option. Also, if the well has any propensity to coning (either gas orwater) then again ESS is preferred.

Chart 4 – Stand Alone Screens Decision TreeOften an operator will install a stand-alone screen if the well is extremely long or if the onsetof sand production is not expected to occur until much later in the life of the well. If a stand-alone screen is suitable option, the first step is to select the most appropriate media tocontrol the expected sands. This will often narrow down the choice of screen to a verylimited number of possibilities. Coarser well sorted sands can be controlled perfectly well bywire-wrap screens. If an operator is concerned about deployment damage or quality, adouble wire-wrap or pre-pack can be considered. For poorer, less well sorted sands andlonger completion lengths, premium and pre-pack screens are the main options.If high erosional flowrates are expected, alternative screen designs can considered beforeutilising a gravel pack or ESS option.And finally, the mud system should be selected to be compatible with the screen and media.For example, OBM systems which give improved hole stability and shape can be used withESS, PMF Stratapac and Dura-Grip, whereas completion brines can be used with all screentypes. Generally, unless in very heavy muds, it is possible to modify the drill-in fluidproperties to pass through the screen.


Risk ManagementThe above process should have allowed the operator to decide upon a short list oftechnically suitable candidate systems. The next step is to examine these in more detail andquantify the risks associated with each system and their overall costs. Most clients will ofcourse introduce their own priorities, contracting arrangements, experience and subjectivityinto this process. Table 7 below illustrates the applications and the relative risks involvedwith each stand-alone screen type from the Weatherford perspective. The risks would needto be evaluated for each application against the required production, projected life of the welland the cost of any remedial operation. Note that some risks can be designed out ormitigated by changing the completion procedure, auxiliary equipment or by modifying thestandard screen design. For example, it is possible to include a protective shroud on theDura-Grip which would mitigate against deployment risks.

Table 7: Applications & risks for sand screen systems

Table 8: Gravel pack risks


Table 7 shows that each screen system has its own risks, which need to be considered.Installing the screens behind a gravel pack will mitigate against some risks (namely erosionand sand plugging), but will introduce other risks associated with the gravel pack itself.Table 8 compares the screen risks associated with gravel and fracture packs. Note this tabledoes not include the risks of the actual pumping operations themselves, eg high permeabilitystreaks, bridges, etc.

Weatherford System AdvantagesTable 9 compares Weatherford screen systems with their equivalent competitor screens andlists their positive advantages. Although, in Table 9 the actual productivity differencesbetween the various sand screen products (like-for-like) is relatively little, the productivitydifferences between various completion types (eg IGP, EGP, stand-alone and ESS) can beconsiderable.

Table 9: Weatherford screen features matrix

Feature Effect Advantage

No rib stand-off Uniform rotational wrap deformation, returns when base-pipe untorqued Rotational limit increased

No rib stand-off Jacket 'push off' strength increased Local tension/compression limit increased and more damage tolerant during deployment

No rib stand-off Wrap tensioned with base pipe expansion (Poisson effect) counters metal expansion with temperature Sand control maintained at high temperatures

Wire-shape Promotes stable filter cake formation Improved filter cake permeability*

Superior sphericity on grains gives less friction during pre-packing Elimination of pre-pack voids Mitigation of erosional hot-spots from screen pre-packing

Steam resin curing More even and controlled curing process Mitigation of erosional hot-spots from screen deployment

Superior sphericity on grains gives improved pore volume Improved permeability and plugging resistance Longer screen life and higher productivity

Highest media pore volume Improved plugging resistance and dirt holding capacity Longer screen life and higher productivity

Highest media pore volume PMF can be used in a wide variety of sand types Suitable for long heterogenous intervals

PMM 60 micron filtration PMM provides finest filtration capability Suitable for very fine sands (gas wells)

Multi-layer construction Multiple barriers to sand Longer screen life

All-welded and offset construction No elastomers, folds or crimps No direct flow through paths in the event of failure

Borehole conformance Gravel and specialised fluid programmes are not required. Fluid communication up screen/wellbore annulus curtailed.

No fluid compatablity problems, no specialized pressure pumping units and personnel, less rig time, simple completion, simple logistics and less safety hazards => improved well economics. Zonal isolation and remedial treatments are effective.

Highest completion ID Less frictional pressure drop along well length, can drill a slimmer well for the same production ID, can run larger remedial equipment

Higher well production capacity & improved welleconomics => well slimming

High(est) Inflow Area More mesh apertures open to flow, longer to plug. Low fluid velocity through screen and therefore less erosion.

Longer completion life leading to improved welleconomics

Lowest frictional pressure losses More uniform drawdown along drain length, better filter cake removal Reduced gas & water coning tendency, improved reservoir sweep efficiency* => improved field economics

Borehole support Formation remains intact as load bearing sand grains are supported, less fines plugging

Any fines that flow are produced, improving completion skin with time* => longer completion life

Micro-Pac compared to Standard Oilfield Slim Style Pre-Pack

Dura-Grip compared to Standard Oilfield Wire-Wrap

Stratapac compared to other Premium Screen Types

ESS compared to Gravel Packs


5. Wire-Wrap Screen SolutionsThe manufacturing process involvesforming the steel wires themselvesand then wrapping the formedtriangular wires over the straight ribwires in a lathe like machine. As themachine rotates, it electric weldseach rib to the wrap wire. The wiredimensions, gauge, number of ribs,metallurgy, jacket OD, jacket lengthand weld strength determines thetime taken to manufacture one jacket.

Figure 41: Wire wrap jacket manufacturingprocess

For pipe-based screens, the base-pipe is perforated by a drilling machine and deburredready for installation of the screen jacket assembly. The base-pipe is normally threaded andcoupled and can be carbon steel (typically J55 to P110) up to super-duplex. The customerwould normally specify the type and threading depending on his application. Once the jackethas been made, it is cut tolength and “slipped on” to theperforated base-pipe. In thecase of Dura-Grip, thescreen jacket is wrappeddirectly on to the perforatedbase-pipe in effect ‘shrink-fitting’ the ribs to the base-pipe and forming a verystrong product, ie muchstronger than a slip-onequivalent as shown inFigure 42.

Figure 42: Shrink fit Dura-Grip screen

The screen jacket, which is thecompleted wire-wrap and ribassembly, is then fillet weldeddirectly to the base-pipe or -depending on metallurgies andapplications - can be welded to anend-ring which is in turn welded orpoxyed to the base-pipe.

Figure 43: Wrap Wire Shapes

The wrap wires are generally triangular or ‘keystone’ in cross-section and have varyingdimensions and metallurgies. Typically, they are 0.090” and 0.105” deep and either 304L,316L or Incoloy 825. The gap between the wrap wires is called the screen gauge is typically

SLIP-ONSCREEN

DURA-GRIP

‘Triangular’x-section

‘House’x-section


measured in thousandths of an inch. Thus a “12 ga” screen is called 12 gauge and has a0.012” (approximately 300 microns) air gap. The open area of such a screen jacket wouldhence be (0.012)/(0.090 +0.012) or 12%. The smaller the gauge, generally the smaller theopen area unless the wrap wire width is reduced. The wrap wire can also be trapezoidal(house-shaped) in cross-section and some operators believe that this adds significantly tothe screen’s resistance to erosion. See Figure 43.Unlike most other screen manufacturers, Weatherford makes and treats its own wire andtherefore maintains a large stock of raw material, see Figure 45. Weatherford can hencerespond rapidly to non-standard specifications and quality controls the wrap-wires using on-line laser measurement asshown in Figure 44.The rib wires (sometimescalled rod wires) can becircular or triangular in cross-section. For rod basedscreens (ie with no base-pipe), the ribs are of coursethicker and more numerousto provide the requiredmechanical strengths.

Figure 44: Quality control of wireforming process

Wire-wrap screens are typically used to control coarse and very well-sorted sand grains.They are sometimes used in “stand-alone” applications, for example in Norway where thesands are unusually large and well sorted in some fields. Generally, they are used behind agravel pack and are sized/selected to keep the gravel in place. The gravel is of course much

larger and much better sorted than formation sandgrains. The gravel, packed into the annulus betweenthe screen and the formation, actually controls theformation sands and the wire-wrap screen simply keepsthe gravel in place.

Figure 45: Wire forming and inventory


Figure 46: 88 Spindle drilling machine (44ft jts)

Wire-Wrap Screens

Figure 47: Wire-wrap screen types

There are three basic types of wire-wrap screens available from Weatherford illustratedabove in Figure 47. All use the wire-wrap jacket with triangular or house shaped wrap wires.As Weatherford stocks raw material and forms its own wire shapes, a very wide range ofshapes and size wires can be used.The wrap wire can be house or triangular in profile. The height and width can be varied tosuit any particular application, in terms of open area and strength. The curvature at theedges is created by the tungsten carbide dye used to form the wire and can also be adjustedto suit a clients particular request. Some companies require very acute angles and some donot specify the curvature at all. The standard Weatherford curvature is selected to promotethe formation of a stable permeable sand filter cake.

Figure 48: Wrap-wire profile

FREEFLOW

HOUSTONWELD

SLIP-ON

DURAGRIP

Width:Typically 0.070” or 0.090”

Height:Typically 0.105” or 0.140”


The standard tolerance on the screen gauge is +0.001” and –0.002”, ie plus 1 thou andminus 2 thousandths of an inch.Erosional tests on house shaped wires and normal wedge shaped are frankly inconclusive.The wedge shape did exhibit more specific erosion, ie metal loss, but the affect on gauge incomparison with the house shaped wire was not significant. The choice of house or wedgeshaped wire is therefore a matter of client preference.The rib wires (strapped longitudinally along the base-pipe) also are available in differentsizes and shapes. Standard rib wires are round wires, circular in cross-section with adiameter of typically 0.105”. Rib wires are available of course in triangular cross-sectionalso. The number of rib wires is standardised but can be increased or decreased asrequired. Too many rib wires can decrease in-flow area and add unnecessarily to the cost ofmanufacture. Too few rib wires will make a fragile jacket.The number and size of rib wires is very important in Free-Flow type screens where theycontribute entirely to the tensile and compressional strength of the screen.The weld strength of a 0.090” x 0.140” wrap wire to a 0.105” rib wire is 448 lbf. The industrystandard is about 350 lbf. Larger rib wires and adjustments in weld voltage and feed-ratescan increase this weld strength still further.

Houston-Weld® ScreensThe screen jacket or slip-on screen is manufactured to exact specifications by diameter,length, gauge, and strength. Selecting the correct profile wire and support rods and feedingthem into a dedicated wrapping machine does this. On this electronically controlledequipment, the screen is produced by cylindrically wrapping while fusion welding the profilewire onto a number of support rods at each intersection. The synchronised feeding ofsupport rod with profile wire wrapping results in a uniform cylinder-like, continuous, andconstant gauge screen.The base-pipe is selected to API specifications by size, grade, and weight for eachapplication. The pipe is then perforated, leaving blank areas on each end for make-up andhandling. The screen jacket is then slipped over the perforated base pipe. The twocomponents are then joined together by end welds.Houston-Weld screens are essentially slip-on screens and are manufactured in a similarprocess to most other wire-wrap screens available. Consequently, their performance andratings are also similar.Slip-on screens offer certain advantages in terms of cost and delivery. Often, if a rush orderis received, the wire-wrap jackets can be manufactured before the base-pipe has beenprocured and perforated.For this reason, Weatherford has invested in fast perforating machines (Range III 44ft jointscan be perforated in a single chucking, see Figure 46) and maintains a substantial inventoryof base pipe and wire-stock.

Dura-Grip® ScreensDura-Grip (DG) screens are manufactured with the rib wire in place on the base-pipe duringjacket manufacture. This has the effect of shrink-wrapping the screen jacket to theperforated base-pipe - a process similar to winding a rope around your hand, the bindingforce keeps increasing with each wrap. Consequently, even the minimum specification Dura-Grip screens are among the very strongest wire-wrap screens available.A table presenting the standard dimensional data of DG screens is provided below.


Dura-Grip Screens

Dura GripOpen Area (In2/Ft)

Dura-Grip Plus ScreenOpen Area (In2/Ft)

Scre

en S

ize

(In)

Pipe

OD

(In

)

Pipe

ID

(In

)

Pipe

Wt/

Ft

Hol

es P

er F

oot

Hol

es S

ize

(In)

Ope

n Ar

ea O

f H

oles

(In2 /

Ft)

Appr

ox O

D D

ura-

Grip

.008"Slot

.012"Slot

.020"Slot

Appr

oxO

D D

ura-

Grip

Plu

s

.008"Slot

.012"Slot

.020"Slot

1" 1.315 1.049 1.7 36 3/8 3.98 1.715 5.56 8.00 12.32 1.815 5.59 8.05 12.44

1-1/4" 1.66 1.38 2.3 48 3/8 5.30 2.060 6.68 9.61 14.79 2.160 6.65 9.58 14.81

1-1/2" 1.900 1.610 2.75 60 3/8 6.63 2.300 7.46 10.73 16.52 2.400 7.39 10.64 16.45

2-1/16" 2.063 1.751 3.25 72 3/8 7.95 2.463 7.99 11.49 17.69 2.563 7.89 11.37 17.57

2-3/8" 2.375 1.995 4.6 84 3/8 9.28 2.775 9.00 12.94 19.93 2.875 8.85 12.75 19.71

2-7/8" 2.875 2.441 6.4 96 3/8 10.60 3.275 10.62 15.27 23.52 3.375 10.39 14.97 23.13

3-1/2" 3.500 2.992 9.2 108 3/8 11.93 3.900 12.65 18.19 28.01 4.000 12.31 17.74 27.42

4" 4.000 3.548 9.5 144 3/8 15.90 4.400 14.27 20.52 31.60 4.500 13.85 19.96 30.84

4-1/2" 4.500 4.000 11.6 168 3/8 18.56 4.900 15.89 22.85 35.19 5.000 15.39 22.18 34.27

5" 5.000 4.408 15 180 3/8 19.88 5.400 17.51 25.18 38.78 5.500 16.93 24.39 37.70

5-1/2" 5.500 4.892 17 192 3/8 21.21 5.900 19.13 27.52 42.37 6.000 18.46 26.61 41.13

6-5/8" 6.625 5.921 24 204 3/8 22.53 7.025 22.78 32.76 50.45 7.125 21.93 31.60 48.84

7" 7.000 6.273 26 216 3/8 23.86 7.400 24.00 34.51 53.14 7.500 23.08 33.26 51.41

7-5/8" 7.625 6.969 26.4 228 3/8 25.18 8.025 26.02 37.43 57.63 8.125 25.00 36.04 55.69

8-5/8" 8.625 7.921 32 240 3/8 26.51 9.025 29.27 42.09 64.81 9.125 28.08 40.47 62.55

9-5/8" 9.625 8.681 47 252 3/8 27.83 10.03 32.51 46.75 71.99 10.13 31.16 44.91 69.40

Table 10: Dura-Grip & Dura-Grip Plus standard dimensions

Note that Dura-Grip Plus refers to a Dura-Grip screen made with house-shaped wrap wires.

Base-Pipe FrictionBecause the rib wires are in close contact with the base-pipe, a frictional force is evidentwhenever the base-pipe and rib wires are subject to differential stresses. For example, if thewire-wrap jacket is pushed along the pipe axis relative to the base-pipe, on a 4” screen aforce equivalent to 25,000 lbf per foot of screen is required to move the jacket.


Figure 49: Tensile testing of wire-wrap screens

This high frictional force helps the DG screen to maintain its gauge even after extremestresses (eg torsion, tension, compression) have been applied. In general terms, as long asthe stresses are within the elastic limit of the pipe & jacket assembly and as long as thescreen assembly is left in a relatively unstressed state, the screen gauge can be expected tohave returned to within tolerance. Testing indicates that DG screens maintain approximately90% of the unperforated base-pipe strength as shown in Figure 49.

Thermal ApplicationsAt elevated temperatures and especially in thermal cycling applications, a number ofpotential issues arise with wire-wrap screens. The end-weld of the jacket to the base-piperequires to be qualified for the application and also attention to the screen gauge is required.

Table 11: WW Screen diameter change with temperature

Screen Size Diameters Diametric Differenceat 700 F at 70 F in inches

2-7/8 OD (in.) 3.243 3.228 0.015ID (in.) 2.456 2.441 0.015

4 OD (in.) 4.404 4.361 0.043ID (in.) 3.591 3.548 0.043

5-1/2 OD (in.) 5.92 5.888ID (in.) 4.924 4.892

0.0320.032


The forces on the screen and base-pipe during thermal cycling can be considerable. Testsindicate that heat cycling concerns (to 700°F) are not applicable to DG end welds as thebase pipe frictional forces distribute the thermal loads along the base-pipe.If a slip-on screen is used in these cases, special floating end-rings will be required, specialweld procedures and also the screen gauge may be affected.Further tests indicate that there is no appreciable change in DG gauge over 70 to 675°F.However, it should be noted that the metals will undergo yield strength degradation at thesehigh temperatures (about 20% at 600°F for example) and their safe application limits must bereduced accordingly.The three factors contributing to gauge alteration at high temperatures are

• Base pipe axial elongation

• Wrap wire diameter thermal expansion

• Base pipe expansion

Figure 50: Poisson effect of base pipe expansion

At 700°F, a base pipe will expand 0.0005” (ie within tolerance) over the gauge. The diameterthermal expansion of the wrap-wire has a negligible effect on the screen gauge. Finally, theeffect of Poisson thinning of the wrap wire was found to be less than 0.0001” and is alsohence negligible.

Summary of Dura-Grip Features• Base-pipe is perforated in up to 44ft lengths in a single chucking

• Base-pipe is cleaned, deburred and prepared for wrapping on high speed lathes

• Wedge wire is formed from a large raw stock wire inventory to a wide range of shapesand sizes. Weatherford can therefore respond very quickly to customer requirements.

• Wire-wrap jackets are wrapped directly on to the base pipe. This results in a screenwhich is stronger and more robust than competitor products.

• Screen is available in a wide range of metallurgies and base-pipe strengths.


Free-Flow™ ScreensThe Houston Free-Flow™ screen provides the maximuminlet area of any wire-wrap based well screen design. Thisscreen is versatile and can be adapted to many industrialfunctions. The strength and durability of this type of screencomes from the shape and mass of the wrap wire and rodscombined with the efficiency of the welding techniquesused in manufacturing.The special trapezoidal shaped wire is drawn fromcarefully specified material, double-annealed and rollformed to precise desired dimensions. The wire is thenspiral wrapped around the longitudinal rods of the samematerial and resistance welded at each point of contact.

Figure 51: Free Flow Screen

Summary of Free-Flow Key Features

• Of like material to avoid electrolytic corrosion

• Cross-sectional shapes selected to provide maximumopen area with sufficient strength

• Welded at each junction with support rod

• Inwardly widening openings assure non-clogging, self-cleaning slot configuration

• Continuous slot construction provides maximum openarea which reduces entrance velocity and increasedhydraulic efficiency

• In-plant design and manufacturing of wire shapes ensurestolerances within rigid specifications.

• Fittings bevelled to ensure complete weld material fill forultimate strength

• All standard and custom made end fittings rigidly securedto screen

• End fittings can be made of different material than thescreen body.

Figure 52: Free flow screensection


Pre-Pack Screens

Figure 53: Pre-pack configurations

Perma-Bond®

Perma-Bond combines an internal Dura-Grip screen with an outer screen jacket. Theannular space between the screens is packed with appropriate size sand or proppant, whileindustrial vibration system insures proper compactness. The pack material can be eithercurable phenolic resin coated or non-resin coated. The Perma-Bond screen was originallydesigned as an alternative to gravel packs. It is used primarily as a stand alone sand controldevice. Perma-Bond screens have also been used effectively in wells with marginalproduction potential where gravel packing has been deemed uneconomical.

Micro-Pak®

Micro-Pak is manufactured in much the same manner as Perma-Bond. Its unique designmakes it one of the most versatile screens as it provides both a similar OD and a larger IDthan conventional pre-packed screens. Micro-Pak is an excellent choice in advanced sandcontrol techniques such as high-rate water packs and frac packs to provide insuranceagainst voids in the gravel pack. It also minimises the chances of erosion and leak-offproblems during the sand placement part of these techniques. Micro-Pak is a proven productin horizontal wells and thru-tubing applications world-wide.

Protecto-Pak®

Protecto-Pak consists of a perforated outer shroud and a Dura-Grip inner screenencapsulating a layer of pre-cured phenolic resin-coated sand or proppant. Protecto-Pakwas designed primarily for wells with damaged casing or where a casing window has beencut and placement of a wire wrapped screen could result in damage. This screen producthas also been used successfully in horizontal wells and thru-tubing applications.

PERMABOND

MICROPAK

MUNIPAK

EXACTPAK


Exact-Pak® & Muni-Pak™ ScreensLarge diameter water wells have been gravel packed as a technique to provide sand control.This technique remained largely unchanged until the introduction of the Exact-Pak and Muni-Pak screens. Eliminating the need for the conventional gravel packing, this thin pre-packdesign allows for closer contact between the inside of the screen and the aquifer, whichresults in better well efficiency. This unique design guarantees positive placement of gravelpack with no bridges or voids.

Pre-Pack Screen ProppantThe main concern regarding pre-packed screens is their susceptibility to plugging withcompletion, drill-in fluids and muds. To address this issue, Weatherford uses Carboliteproppant as the main pack media as opposed to resieved sand. Carbolite advantagesinclude larger pore throats and more precise sorting, which improve the global permeabilityof the system. Comparison tests show that resin coated Carbolite proppant allows 120%more fluid volume to pass through than resin coated sand. Also the sphericity of theCarbolite virtually eliminates the bridging concerns during the packing process of the screen.Bend tests showed Micro-Pak screens packed with the resin coated Carbolite to be able towithstand up to 40 degree per hundred bend radius with no damage to the pack or wire wrap.Since acid stimulation treatments are a standard in the Gulf of Mexico, a resin coatedproppant is recommended. Phenolic resin coating of the proppant will give best results if HFacid stimulation is required in the life of the well.Figure 54 & Figure 55 shows the difference in sphericity and uniformity between the naturalgravel and the man-made Carbolite proppant.

Figure 54: Resin coated gravel

Figure 55: Resin coated proppant(Carbolite)

Another phenomenon that operators are often concerned with is the difference betweencured and uncured resin screens and how this affects screen permeability. When sand isresin coated and cured, due to the less precise sorting and non-sphericity, the pack tendedto be tighter and less porous. The resin when cured was thought to blind off some of thepore throats and this would affect the screen permeability. For ceramic proppants, this is notan issue as, from inspection of Figure 56, the difference is not significant at flow rates thatthe screen is likely to see, ie less than 700 stb/ft. Note that for chemical compatibilityreasons, it is advisable to run resin coating on the proppant and if required a pre-cured resincoated proppant can be supplied. Micro-Pak screen can be packed with curable resincoated Carbolite, pre-cured resin coated Carbolite, or non-resin coated Carbolite.


Figure 56: Comparison between cured and uncured proppant permeability

Weatherford manufactures 40ft pre-packscreen jackets without a centre break. Theouter jacket is installed on the screen andraised to the upright position. The proppant isthen introduced into the screen annulus andallowed to settle under agitation from industrialsized vibrators. As the proppant is veryspherical, it gives much less friction and hencecan flow and pack properly over a long length.See Figure 57.

Once the proppant is in place, the screen is thewelded closed and prepared for the curingprocess. This process uses steam which ispumped inside the screen. Conventional curingis performed in convection ovens and becausethe heat is transferred by air, the process isless consistent and reliable. Steam conductsthe heat more efficiently and also flushes outany debris/loose fine grains. See Figure 58.

Figure 57: Pre-pack tower

Performance of 2 7/8" Micro-Pak w/ 20/40 Carbolite

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 200 400 600 800 1000 1200

BBL/day/ft

Pres

sure

dro

p/ft

(psi

/ft)

Cured Micro-Pak Uncured Micro-Pak


Figure 58: Steam curing process

Weatherford Pre-Packs – Key features• Patented Dura-Grip wrapping process to adhere the inner screen to the base pipe for

added strength

• Custom-designed sand packing system to centralise the annulus during the fill cycle

• Industrial vibrating system to prevent internal sand bridging and ensure an even,consistent fill and compaction

• Multiple wire gauge and filter media options including resieved and phenolic resin coatedmedia

• Steam thermal curing system to ensure a consistent bond for resin coated media for theentire joint

• Only company capable of producing custom lengths to 39’ of screen on a 40’ joint withouta centre break.


Micro-Pak Screens

Inner Screen OpenArea (In2/Ft)

Outer Screen OpenArea (In2/Ft)

Scre

en S

ize

(In)

Pipe

OD

(In

)

Pipe

ID

(In

)

Pipe

Wt/

Ft

Hol

es P

er F

oot

Hol

e Si

ze (

In)

Ope

n Ar

ea O

f H

oles

(In

2 /Ft

)

Appr

ox. S

cree

n O

D (

In)

Rad

ial P

ack

(In)

.008"Slot

.012"Slot

.020"Slot

.008"Slot

.012"Slot

.020"Slot

1" 1.315 1.049 1.7 36 3/8 3.98 2.011 0.228 6.90 9.77 14.66 7.78 11.09 16.85

1-1/4" 1.66 1.38 2.3 48 3/8 5.30 2.356 0.228 8.43 11.94 17.91 9.11 13.00 19.74

1-1/2" 1.900 1.610 2.75 60 3/8 6.63 2.596 0.228 9.49 13.45 20.17 10.04 14.32 21.75

2-1/16" 2.063 1.751 3.25 72 3/8 7.95 2.759 0.228 10.21 14.47 21.71 10.67 15.22 23.11

2-3/8" 2.375 1.995 4.6 84 3/8 9.28 3.071 0.228 11.60 16.43 24.65 11.87 16.94 25.73

2-7/8" 2.875 2.441 6.4 96 3/8 10.60 3.571 0.228 13.82 19.57 29.36 13.81 19.70 29.92

3-1/2" 3.500 2.992 9.2 108 3/8 11.93 4.196 0.228 16.59 23.50 35.25 16.22 23.15 35.15

4" 4.000 3.548 9.5 144 3/8 15.90 4.696 0.228 18.81 26.64 39.96 18.16 25.91 39.34

4-1/2" 4.500 4.000 11.6 168 3/8 18.56 5.196 0.228 21.02 29.78 44.67 20.09 28.67 43.53

5" 5.000 4.408 15 180 3/8 19.88 5.696 0.228 23.24 32.92 49.39 22.02 31.42 47.72

5-1/2" 5.500 4.892 17 192 3/8 21.21 6.196 0.228 25.46 36.07 54.10 23.96 34.18 51.91

6-5/8" 6.625 5.921 24 204 3/8 22.53 7.321 0.228 30.45 43.13 64.70 28.31 40.39 61.33

7" 7.000 6.273 26 216 3/8 23.86 7.696 0.228 32.11 45.49 68.24 29.76 42.46 64.47

7-5/8" 7.625 6.969 26.4 228 3/8 25.18 8.321 0.228 34.88 49.42 74.13 32.17 45.91 69.71

8-5/8" 8.625 7.921 32 240 3/8 26.51 9.321 0.228 39.32 55.70 83.55 36.04 51.42 78.09

9-5/8" 9.625 8.681 47 252 3/8 27.83 10.321 0.228 43.75 61.98 92.98 39.91 56.94 86.47

Table 12: Micro-Pak standard dimensions

Input numbers are nominal.


Zonal Isolation

Figure 59: Isolation Sleeve

Weatherford manufactures and provides wire-wrap screensslipped over and welded onto sliding sleeves. These are usedto select or shut-off production zones in a multiple zonecompletion. Typical assembly dimensions are provided inTable 13.

Figure 60: Screened isolationsleeve

Zone Isolation Screen Assemblies

Slip On ScreensSize Sliding Sleeve OD Screen ID Screen OD

2-1/16" 2.190" 2.20" 2.58"2-3/8" 2.75" 2.90" 3.30"2-7/8" 3.166" 3.18" 3.55"3-1/2" 4.10" 4.20" 4.576"

Micro-Pak ScreensSize Sliding Sleeve OD Screen ID Screen OD

2-1/16" 2.190" 2.20" 2.95"2-3/8" 2.75" 2.80" 3.58"2-7/8" 3.166" 3.18" 4.00"3-1/2" 4.10" 4.20" 5.00"

Table 13: Typical isolation sleeve dimensions

The ID and OD of screens are the same when not using sliding sleeves to allowcommunication between the screen ID and the outer non-perforated pipe.


Pre-pack & Wire-wrap Selection Criteria for Internal Gravel PacksThis guideline, developed for the Gulf of Mexico, addresses the requirement of conventionalslurry packs as well as high rate water packs and fracture pack type cased hole completions.Each presents certain challenges, with pressure pumping in particular exerting enormousforces on the screen and blank pipe assembly during the completion process. Dura-Grip hasfunctioned well in all of the above completion techniques when certain factors such as pumprate, return rate and the absence of formation fines are closely monitored. A lack of controlin any of the above noted areas can result in an erosional failure either during pumping or inthe early production life of the well. Because of these reasons, Micro-Pak screens areincreasingly used in these applications.The guidelines that follow deal primarily with cased hole completions. Openhole completionssuch as those required to complete horizontal wells and thru-tubing applications presentdifferent challenges and have been discussed already in the subsection on SelectionProcess on Page 49.

• Any gravel pack job pumped at a rate greater than 5 bpm should use a Micro-Pak screen.

• Micro-Pak pack media will match or be larger than the gravel.

• Slot size of the screen will correspond to the media being used to pack the well with.! 0.006 gauge with 50/70 or 40/60 gravel pack sand! 0.008 gauge with 40/60 gravel pack sand, 30/50 proppant, or 20/40 proppant! 0.012 gauge with 20/40 gravel pack sand or proppant also with 16/30 proppant! 0.020 gauge with 12/20 gravel pack sand

• Metallurgy of the wire wrap and base pipe will be determined by well conditions

• Standard base pipe will be 80kpsi grade with 110kpsi blank pipe. The blank pipe willhave a minimum collapse resistance of 10,000psi in all sizes

• Dura-Grip screen can be used on gravel pack jobs where the rate of pumping does notexceed 5 bpm

• Micro-Pak and Dura-Grip screens are available in standard keystone shaped or heavy-duty "house shaped" wrap wire

Screen OD Sizing Guidelines• 5" and 5 1/2" casing sizes - 2 3/8" Dura-Grip or Micro-Pak

• 7" casing - 3 1/2" or 4" Dura-Grip or Micro-Pak

• 7 5/8" casing - 4" or 4 1/2" Dura-Grip or Micro-Pak

• 9 5/8" casing - 5" or 5 1/2" Dura-Grip or Micro-Pak.A minimum annular radial clearance of 3/4" between the casing ID and the screen OD is theoptimum for obtaining a good gravel pack, although sometimes 1” radial clearance isrecommended depending on the length of the interval to be packed.Screen and blank used in cased hole completions are typically centralised every 15-20 feetdepending on joint length with 4 blade type weld-on centralisers. The OD of the centralisersis 1/8" less than the drift ID of the casing.


Wire-Wrap Screens Safe Application LimitsThe following table provides the safe application limits based on physical testing andinterpolate to different screen sizes. Note that a 20% safety factor has been applied to theyield values.Note that repetitive stresses & strains will reduce these limits. Note also that combinationloading will also reduce these limits.Although tensile ratings are relatively straightforward to test and calculate, compressionalratings are not. Most steels are stronger in compression than tension but because ofbuckling, it is not possible to provide compressional data without knowledge of thecompletion and supporting tubular dimensional data. Even then, any calculations are likelyto be highly approximate.

DURA-GRIP

Size(in)

Tensile(lbf)

Collapse(psi)

Burst (psi).006 slot

Burst (psi).010 slot

Pipe Wt(lbm/ft) Holes/ft

2-3/8 8,835 4.6 842-7/8 109,600 8,370 9,000+ 6.4 963-1/2 7,890 9.2 108

4 157,360 4,940 9.5 1444-1/2 4,915 11.6 168

5 4,890 15 1805-1/2 283,920 4,415 2,300 1,600 17 1926-5/8 4,160 24 204

7 26 216MICRO-PAK

2-3/8 8,835 4.6 842-7/8 109,600 8,370 9,000+ 7,400 6.4 963-1/2 7,890 9.2 108

4 157,360 4,940 9.5 1444-1/2 4,915 11.6 168

5 4,890 15 1805-1/2 283,920 4,415 5,400 5,000 17 1926-5/8 4,160 24 204

7 26 216Table 14: Wire-wrap safe application limits

The bend angle is normally limited to a maximum of 40°/100ft but varies with screen size andtype. Typically, the maximum torque is limited by the threaded couplings selected, but thisagain varies with coupling and screen size and type. Consult customer services to obtain theapplicable limits.


6. Stratapac® ScreensWeatherford Completion Systems Stratapac® Screens are currently installed inover 800 wells world-wide and in very many different applications ranging fromthru-tubing remedial to extended reach horizontal wells. An installation summary isavailable which details the fields, lengths, type of screen and application type.The first Stratapac screens were introduced to service the demands of Operatorsin the Gulf of Mexico for robust and rugged screens. The screen consists of anouter protective steel cage (sometimes called a shroud) which covers the internalfiltration media. The cage and filtration media – called the screen jacket – iswelded onto a machine perforated base-pipe via steel rings welded to each end ofthe screen jacket.Stratapac Screens represented a major departure from conventional screentechnology when they were first introduced in 1994 and again in 1996 with theStratapac PMF media and they received a number of awards for innovation andnew technology in the Oil & Gas industry.

Figure 61: Stratapac screen (Range III) withsolid rotating centralisers installed

PMM® (Porous Metal Media) is a fixed pore filtration medium with an engineeredpore throat size distribution. It provides the finest (60 µm) sand control available inmetal media. This is the original media introduced with Stratapac screens in 1994.Porosity is 52% by helium displacement. When introduced, it was designed toreplace 40/60 pre-pack screens and there is still no equivalent all-metal screencurrently available.

Figure 62: SEM ofStratapac PMM medium

PMF-IITM (Porous Metal Fibre)introduced in 1996 provides a much coarser medium and is available in 125 µm and 200µmsizes for sand control. PMF-II is an adaptation of Pall Corporation’s PMF media. The fibresare sintered within two layers of square weave mesh. PMF-II is hence a fixed pore filtrationmedium with an engineered pore throat size distribution. The measured porosity is 68 to


72% (depending on the media grade– PMF2040 is 68% and PMF1220 is72%). The Stratapac PMF screenswere designed to emulate pre-packscreens in filtration efficiency andthere remains no other fixed pore(sintered) media available with sucha high porosity to date. Theimportance of media porosity isshown and discussed in the variouscomparison tests with competitorscreens in the subsection onComparison Tests.

Figure 63: SEM of Stratapac PMF-II media

Stratapac ConstructionStratapac screen media are available in the three main filtration ratings and also in twomedia metallurgies. Other screen variations are largely determined by the base pipeselected (length, diameter, metallurgy, strength). Customers can request Stratapac screensto be manufactured with a Dutch twill weave media or with wire-wrap outer or inner cages.Stratapac down-hole screens are constructed from inside to outside of the followingindependent layers:

• A perforated base pipe (grade as required).• An inner layer of square weave woven wire mesh for support and drainage.• Two layers of PMF-II 1220 filter media or four layers of PMM filter media. Each

layer is welded independently and the weld seams are offset from one another by 120°.• An outer layer of square weave wire mesh for back pressure support and drainage

completes the screen media construction.• A perforated stainless steel outer cage for protection during handling and running in

the well bore. Various grades and thicknesses are available.Base-pipe metallurgy L/N80 with screen media and meshes in either 316L stainless steel orAlloy 20 (equivalent to Incoloy 825), depending on bore-hole conditions and whether thescreen must be acidised or not. The outer cage is normally of a slightly lower metallurgy(304L minimum) as its long term resistance to corrosion is not perceived to be critical since itis designed mainly to protect the screen medium during deployment. The base pipemetallurgy can be selected as required from J55 to super-duplex 28Cr, etc.

Pipe Length and SizesConnections, thread protectors, markings, documentation, packaging, handling clearances,joint lengths, screen length, base pipe OD and ID, screen OD details should be specified attime of order.


Acid Backwash and FlushingThe all-metal construction allows the Stratapac screens to be flushed and washed withrelatively high concentration acid breakers7. In addition, the Stratapac screen has excellentback pressure support and so can withstand cleaning and back flushing operations well.Please refer to the handling guidelines for reverse pressure limitations and cyclic service8.

Minimised Screen ThicknessThe standard Stratapac screen is about 1/3rd inch thick, and the increase in diameter fromthe base pipe is therefore only about 0.65 inches9. The use of Excluder10, wedge wire or pre-pack can increase screen diameter by as much as 1.5 inches. In many instances, this allowsthe Stratapac screen to use a larger (and also even stronger) base pipe. Use of a largerbase pipe can also result in higher flow-rates and hence increased production. An increasedID base pipe also provides a greater number of options to run service tools during coiledtubing and work-over operations. Alternatively, the Stratapac screen OD is smaller thanother comparable screens and thus will experience less deployment problems in short radiusturns, horizontal well sections or casing windows11 (see below).

Crush TestThe original Stratapac Screen equipped with PMM media had been subjected to anextensive series of mechanical strength tests, and the screen’s construction was determinedto meet or exceed all requirements for well service. The PMF-II media is constructed with adifferent substrate mesh than the PMM and two layers of PMF-II are used in the screen vs.four PMM layers so, additional tests were performed to validate the screen’s mechanicalstrength with the new media. To accomplish this a crush test was conducted on the screenand the sand retention was evaluated before and after crushing, see Figure 64 below.

Figure 64: Crush samples (left) and apparatus (right)

7 See “Material Selection and Recommendations for Protection of Stratapac Screen against Corrosion” (SLS Report # 5132)

8 See “General Handling and Running Guidelines”

9 See attached Stratapac and Stratacoil Brochure (Weatherford ‘00)

10 Trade mark of Baker Oil Tools

11 See “Bending Test of Stratapac and Wire-Wrapped screens for Shell Offshore, New Orleans, LA” (SLS Report # 5212)


Results Table 15 lists the total suspended solids obtained from the tests.

Sample Sand Retained on Mesh (g)Upstream 47.24Downstream, Intact Screen 0.24Downstream, Crushed Screen 0.26

Table 15: Screen crush test - total suspended solids

Damage ResistanceTesting showed that the Stratapac down-hole screen can be completely crushed at thewelded end-ring or in the screen centre and still maintain sand retention capabilities. Pre-packed and wire wrapped screen allow the passage of formation sand when deformed whicherodes and eventually damages completion equipment and surface facilities. Third partytesting to determine tensile strength showed elongation of the Stratapac down-hole screen totwo inches with no failure – this was the limitation of the test apparatus – whereas pre-packed screens failed at 0.39 inches of elongation. This indicates that the Stratapac down-hole screen will not fail prematurely when run in short radius, highly deviated or horizontalwells.Also, maintaining complete screen integrity will ensure an even flow distribution over theentire length of the Stratapac screen and hence minimise local screen erosion. If otherscreens are damaged, sand-loaded fluid flow is concentrated through the damaged areas (socalled “hot spots”) which greatly increases local erosion rates and hence causes prematurescreen failure12 as sand is eventually produced.

High Burst Pressure ApplicationsFor certain applications including fracture packs, operators sometimes require a screen witha higher burst rating. This would be necessary for example if the screen became pluggedwith LCM material from the inside during the completion and the differential pressure acrossthe screen could not be controlled within certain operational limits. In these situations anddepending on the overall differential pressure expected, variations to the standard Stratapacscreen design are available.The standard Stratapac screen burst rating is derived from the yield point of the outer cage.When the outer cage begins to yield (ie move) under the reverse pressure, the media wouldbe expected to start to stretch elastically at first. The media is more elastic than the cageand base-pipe, but due to the difficulty in predicting the exact failure pressure (ie thepressure at which the media is stretched plastically and this effect on sand retention issignificant), the yield point of the outer cage is selected as the burst pressure rating.Although, this rating incorporates a significant safety margin (50%), caution should be usedin case of pressure cycles.Field proven high burst pressure Stratapac screen designs are available. Please contactWeatherford Completion Systems for further details.

12 See “Accelerated Erosion Tests on Stratapac and Baker Hughes Inteq Excluder Sand Control Screens” (SLS Report #6338)


Bypass

No Bypass

Bypassing Seal

Welded Seal

Certain competitor screen designs involve crimping themedia rather than welding along the length of the sandscreen. This leads to the possibility of seal by-pass andpoor screen performance in reverse pressure situations.

Figure 65: Welded vs non-welded seals

Also, some screens have non-welded end-rings which fix the screen jacket to the base-pipe.The seal in this instance is sometimes provided by an elastomer. Again, with time this willbecome a potential failure point. Stratapac screens have welded end-rings applied understringent QC conditions to prevent this potential problem.

Figure 66: Welded end-ring seal

Stratapac screens are multi-layered and eachlayer is welded longitudinally along the length ofthe screen. The screen is then rotated through120 degrees and the next layer is welded. Eachweld is therefore offset from one another by 120degrees. In the event of a poor weld, theStratapac screen provides no direct fluid flowpath. In addition, each layer is independent asa perforated bronze “chill” strip inserted behindeach weld keeps the weld from penetrating tothe layer below. Some multi-layer screens haveonly one weld for all layers (Figure 67). In theevent of a weld defect, this could lead to theopening of a direct flow channel through thescreen and hence loss of sand control.

Figure 67: Longitudinal media welds


Standard Design Stratapac & Stratacoil DimensionsTypical dimensions for a standard design Stratapac screens built on API base pipe arepresented below in Table 16.

OD (in) Nom Wt1(lbm/ft)

Screen ID(in)

Screen OD2

[±0.06] (in)

TotalScreenWeight3(lbm/ft)

ScreenArea perJoint3 (ft2)

2.375 4.6 2.00 2.98 7.89 19.0

2.875 6.4 2.44 3.48 10.15 22.4

3.5 9.2 2.99 4.11 13.5 26.7

4.0 13.2 3.34 4.61 18.1 30.0

4.5 12.6 3.96 5.12 17.9 33.5

5.0 15.0 4.41 5.63 20.8 36.8

5.5 17.0 4.89 6.13 23.2 40.3

6.625 24.0 5.92 7.27 31.1 47.9

7 26.0 6.28 7.65 32.9 50.5

Table 16: Standard Stratapac screen dimensions1 API Base Pipe – typical values used. Customer can select alternative base pipe wts.2 Screen OD: Maximum outer diameter may change depending on the type of connection selected.3 Screen weight per foot and area per joint is dependent on overall pipe length. Values shown are for 31ft pipe.

Typical dimensions for standard design Stratacoil screens are presented below in Table 17.

OD (in) Nom Wt(lbm/ft)

Screen ID(in)

Screen OD1

[±0.03] (in)CouplingOD3 (in)

ScreenArea perJoint (ft2)

1.315 2.65 0.95 1.64 1.66 3.9

1.66 3.36 1.29 1.98 2.054 4.8

1.90 3.84 1.5 2.19 2.20 5.4

2.063 4.14 1.61 2.30 2.50 5.8

Table 17: Standard Stratacoil screen dimensions1 CAUTION: Maximum outer diameter may change depending on the type of connection selected. See Coupling.


Safe Application LimitsAs Stratapac can withstand minor impacts and dents, it can be handled in a similar fashion tocasing and tubing, and can be racked back in the derrick as required. Procedures for therunning and handling of the screens are reviewed prior to installation. Please note that thelimits noted in the published literature are our recommended application limits andincorporate a safety factor. This infers that the limits may be more conservative than thatrecommended by competitor manufacturers who often state failure limits. Note that theselimits are stated only for standard build Range II (31ft) screens. Special build screens can beprovided for example for increased burst ratings and high-flow or thermal cyclingapplications, etc.Table 18 lists the suggested safe application limits for Stratacoil Screen. Suggested safeapplication limits for the standard Stratapac Screens are given Table 19. Suggested safeapplication limits for Hi-Flow Stratapac Screen (base-pipe holes on 1.2" pitch centres) aregiven in Table 20.

NominalSize

MaximumTensile

Load (Lbf)2

MaximumTorsionalLoad (Ft-

Lbf)3

Weight(Lbm-Ft)4

MaximumCrush

Delta-P(psid)

MaximumBurst Delta-

P (psid)5

Base-pipeThickness

(in/#/ft)

1.315 9,380 287 2.65 1,800 1,360 .1251.66 11,950 461 3.36 1,500 1,100 .1251.9 13,604 584 3.84 1,200 990 .1252.06 14,450 668 4.14 1,000 910 .125

Table 181: Standard Stratacoil screen safe application limits

NominalSize

MaximumTensileLoad(Lbf)2


Lbf)3

Weight(Lbm-Ft)4

MaximumCrush

Delta-P(psid)

MaximumBurst

Delta-P(psid)5

Base-pipeThickness

(in)

Base-pipeThickness

(in/#/ft)

2.375 77,200 2,090 7.9 9,100 1,080 .190 4.62.875 103,800 3,600 10.2 8,700 910 .217 6.43.5 145,300 6,300 13.5 8,500 760 .254 9.24.0 206,000 10,500 18.1 7,500 680 .330 13.24.5 204,700 12,000 18.1 6,800 600 .271 12.65.0 244,800 16,200 20.8 6,700 550 .296 15.05.5 275,500 20,500 23.2 6,100 500 .304 17.0

6.625 378,700 34,900 31.1 5,800 420 .352 24.07.0 414,400 40,400 32.9 6,700 320 .408 29.0

Table 191: Standard Stratapac screen safe application limits1 All calculations are based on 80 Ksi minimum yield strength material @ 70F2 Screen body only; reduce as appropriate for threaded connections3 Static loads - multiply by .50 for dynamic applications4 Based on standard Range II Stratapac screen, no couplings or centralisers5 Outer cage only, static applications; needs to be reduced for cyclic service.


NominalSize

MaximumTensileLoad(Lbf)2


Lbf)3

Weight(Lbm-Ft)4

MaximumCrush

Delta-P(psid)

MaximumBurst

Delta-P(psid)5

Base-pipeThickness

(in)

Base-pipeThickness

(in/#/ft)

2.375 67,900 1,900 7.9 7,700 1,080 .190 4.62.875 87,900 3,300 10.2 7,200 910 .217 6.43.5 126,700 6,000 13.5 7,000 760 .254 9.24.0 174.000 9,700 18.1 7,900 680 .330 13.24.5 171,800 11,000 18.1 5,800 600 .271 12.65.0 209,000 15,100 20.8 5,700 550 .296 15.05.5 231,300 19,000 23.2 5,300 500 .304 17.0

6.625 319,200 32,300 31.1 5,100 420 .352 24.07.0 344,900 37,100 32.9 4,900 320 .408 29.0

Table 201: Hi-Flow Stratapac screen safe application limits1 All calculations are based on 80 Ksi minimum yield strength material @ 70F2 Screen body only; reduce as appropriate for threaded connections3 Static loads - multiply by .50 for dynamic applications4 Based on standard Range II Stratapac screen, no couplings or centralisers5 Outer cage only, static applications; needs to be reduced for cyclic service

Dog-Legs, Casing Windows and High Build AnglesStandard sand control screens can be subject to a number of specific damage mechanismswhen used in irregular bore-holes and “bending” applications:

• Localised damage to the wire and opening of the slots due to scraping against thecasing or bore-hole irregularities, and subsequent loss of sand if pre-packed.

• Structural damage to the screen jacket (parting of the screen jacket at the end ringdue to stress accumulation in the bend).

• Cracking and voiding in pre-packs. Losing pre-pack material (voids) throughstretched wire-wrap gaps even when bending elastically on the wire-wrap.

• Coupling/connection yield.• Permanent set or yield on the perforated base pipe (ie plastic deformation) leading

to difficulties in deployment in subsequent straight bore-hole sections.• Buckling instability failure on base pipe/cage.• Weld failure at the jacket end-ring attachment to the base-pipe.

In the case of high build angles or severe hole geometries, Weatherford recommends theuse of Torque & Drag modelling prior to completion equipment final selection. This is offeredas an additional service and can be used also to select the most appropriate centralisationsystem.

Stratapac Screens – Bend LimitsMost of the above damage risks have been eliminated or at least mitigated in the design andconstruction of Stratapac screens:

• The external cage or shroud protects the filter medium from any contact with thecasing and sharp edges in the well bore.

• The inherent ductility of the PMM or PMF filter medium and the unique constructionof Stratapac screens allow substantial deformation of the assembly without risk ofshearing the filter medium or end-rings.

• In addition, the rugged construction of the Stratapac screens expands the range ofoperational loads that can be applied to the completion string. This allows “roughertreatment” during installation (screen rotation, push & pull) and hence reduces the


probability of getting stuck in the hole and also screen damage from combinationloads and localised stresses.

There are hence only five main potential failure modes for Stratapac due to short radius wellsin order of probability:

• Permanent set or yield on perforated base pipe (plastic deformation)• Coupling/connection yield• Media deformation (plastic or shear)• Buckling instability failure on base pipe/cage• End-ring weld failure

The coupling/connection bend limit is determined by the coupling and the thread formselected. This factor is therefore not considered in the following table as the coupling type isnormally selected by the operator. Please note that in some cases this can be thedetermining limit on well geometry and the operator is well advised to refer to the pipe/threadmanufacturer.The lowest mode of failure has been determined by engineering calculations and confirmedin tests to be always the permanent set to the perforated base pipe. Table 21 below istherefore prepared using 70% of the base-pipe minimum yield strength. This value isselected to allow for the reduction in strength due to perforations and to keep the stressesbelow the elastic limit of the base pipe. The base pipe is assumed to be 80ksi minimum yieldin all cases. Please note that this is not a product claim and is only a recommendation sincethere are many other factors that may affect the limit, including combined loads and localstress concentration factors.

Minimum AverageBend Radius

Maximum AverageBuild AngleNominal

ScreenSize

(inches)

ScreenOD

(inches)

BasePipe Wt

#/ft

No ofBase-PipeHoles

#/ftFt M Deg

/100ft Deg/m

2.375 2.98 4.6 22 53 16.2 108 3.52.875 3.48 6.4 28 64 19.6 89 2.93.50 4.11 9.2 34 78 23.8 73 2.44.50 5.12 12.6 40 100 30.6 57 1.95.0 5.63 15.0 46 112 34.0 51 1.7

5.50 6.13 17.0 52 123 37.4 47 1.56.625 7.27 24.0 64 148 45.1 39 1.37.00 7.65 26.0 64 156 47.6 37 1.2

• Maximum OD may change depending on coupling OD• Limits are based on permanent set (plastic deformation) of the Stratapac base pipe (minimum yield

80,000 psi with standard pitch base-pipe perforations. Perforation hole size is 0.375” standard).• No account is made of pipe coupling and thread types. Weatherford Completion Systems refers the

operator to the appropriate thread specification or proposed thread/pipe manufacturer for theconnection bending limits.

• Note that all screen sizes can be used in short radius wells (defined as 0.5 to 1.5 degs/ft) but there isa risk of plastically bending the screen which will make running the screen further through anydifferent sections more difficult.

• Standard base-pipe perforation density and pattern.• These values are guidelines only. The limits require to be reduced under combination loads.

Table 21: Recommended Stratapac base-pipe safe build angle application guidelines


Please note that this bend limit guideline above is independent of hole size as this is thebend applied to the screen itself. Well geometry calculations (bore hole size, centralisersand screen OD) may allow the use of screen in higher hole build angles than recommendedin the table above.If pipe permanent set can be ignored, the recommended safe build angle application limit forStratapac is shown below in Table 22. From Table 22, all Stratapac screen sizes usingappropriate threaded connections can be actually used in any short radius well (defined as50 to 150 degs/100ft), but note there is a risk of plastically bending and hence permanentlydeforming the base-pipe. This should be allowed for in the well design. The values in Table22 have been derived using 50% of the yield value of the media. If these values (afterallowing for the safety margin) are exceeded over the build section of a hole, there is a risk ofplastically deforming the media and thereby inducing a premature screen failure.

Nominal Screen Size(inches)

Maximum Average Screen JacketBuild Angle*Deg/100ft

2.375 5022.875 4243.50 3554.50 2825.0 255

5.50 2336.625 1967.00 186

• These values do not account for permanent set to the base pipe.• Please use Table 21 wherever possible.

Table 22: Safe build angle application limits for Stratapac screen jackets

Screen Pore Size Distribution TechnologyThe high porosity of the PMF-II (PMF) media ensures that even with its higher particleretention efficiency, the media experiences less plugging than mono-pore sized media. Inthe filtration industry, this is referred to as a high “dirt holding capacity”. Based on this, PMFwas selected over woven mesh media as the primary filtration medium for Stratapac and thiswas confirmed as appropriate by a major in-house testing programme.Generally, operators are critical of “in-house” testing (which in fairness can be very biased)and are therefore more inclined to trust third party testing or their own field experience/labresults. Consequently, we include such third party data in the subsection Comparison Tests(page 87) from independent tests by major operators and users of sand screens. The testdata indicates that the PMF Stratapac provides generally better sand control overcomparable multi-layer (all-metal) premium mesh screens. When the sands are mixed orheterogeneous, this difference in sand control performance is even more pronounced infavour of Stratapac.

PorosityThe Stratapac® PMF media in Figure 63 is composed of fine metallic fibres sintered to asubstrate mesh for high tensile strength. The fine fibres result in a highly porous media withmuch higher porosity than typical pre-pack material or woven wire meshes (see Figure 68).


Figure 68: Construction and porosity of non-woven media

This is because the woven media requires the weave wires to maintain the structuralstrength and so they cannot be reduced in diameter to increase throughput or porositybeyond a certain limit. The woven media have very regular pore throat sizes (especiallysintered meshes), but note that there may be more than one size depending on the directionof particle impingement and type of weave.

Pore Size DistributionThe PMF media was designed to mimic the pore size distribution of typical pre-pack gravelmaterial. To illustrate the similarities in distribution, 180 random pores of 20/40 gravel andPMF2040 media were measured under a calibrated microscope and plotted in thehistograms shown in Figure 69 (see also SLS Report 6617 for a more complete description).Comparison of the histograms show that the PMF2040 has similar pore size distribution andaverage pore size as 20/40 gravel, but with better uniformity and much greater density ofpores due to PMF’s higher intrinsic porosity.

Figure 69: Comparison of PMF and pre-pack pore size distributions

Microns

20/40 GRAVEL

PMF2040

0 100 200 300 400 500

Freq

uenc

y

Gravel

Porosity 30%

Woven Media

Porosity 45%

PMF Media

Porosity 70%


Sand RetentionThe pore throat size distribution of PMF media emulates a pre-pack distribution and filterssand particles efficiently.The standard filtration ratings of standard Stratapac media are tabulated as follows:

Media 90% Rating Gravel PackEquivalent Sizing

PMM 60µm 40/60

PMF-II 2040 125µm 20/40

PMF-II 1220 200µm 12/20

Table 23: Standard Stratapac media ratings

In order to be as consistent as possible between this technology and earlier gravel packtechnology, the Stratapac media are commonly designated as follows: PMM4060, PMF2040and PMF1220.

Stable Sand filter cake with PMF-IIThe PMF media have a broader pore throat pore size distribution than the equivalent meshtype screens. The PMF therefore catches a slightly broader distribution of impinging sandparticles and hence forms a stable and permeable sand filter cake relatively quickly. Thesintered construction means that the pore size is fixed and does not alter with changes in ∆p.This means that the filtration medium is not subject to continual flexing and unloading whichwould otherwise lead to metal fatigue and premature failure. The construction and closeproximity of the drainage mesh (and to some extent the outer cage) contribute to maintainingthe sand filter cake in place even with changes in flow-rate and fluid phase. It should bepointed out that with these coarser screens (100µm and above), fines can be expected to beproduced especially early on in the installation’s life. If the sand filter cake is not stable, finescan be expected with every change of ∆p (well shut-ins, etc) and fluid phase change anderosion can be expected to be more of a problem. The outer drainage mesh & perforatedcage provide mechanical support to the sand filter cake during its formation and increasesthe sand filter cake stability during multi-phase and changeable flow condition.

Screen & Media Selection GuidelinesThe completion method and completion fluids should be considered in order to select theright filtration media to control sand production from a producing well. If gravel packing orinserting beads between the screen and formation, the media may be designed to control thegravel as the gravel is in turn already controlling the formation sands. Sometimes,depending on an assessment of the risks involved, the screen media is sized for theformation sands even when gravel packing because the operator is not convinced that aperfect gravel pack can be assured. This is especially so in the case of long openholehorizontal completions.Using the Coberly d10 rule for poorly uniform sands can lead to unwanted sand production.Also, the sand bridges can be made up of much smaller particles than for a uniform sand andtherefore may not be as stable. Thus, one screen sizing criteria is not appropriate to allsands. Therefore, a screen selection chart was devised which selects a screen pore sizebased on both the average sand size and uniformity coefficient.


Stratapac Media Selection ChartUsing a combination of standard gravel pack and screen selection rules (Saucier, Coberly,Schwartz), as well as laboratory experiments, a chart providing simple media selectionguidelines is provided in Figure 70. Qualitatively, these guidelines are explained as follows:

• For very well sorted with low uniformity coefficients (2 < Cu < 3), the recommendationsfollow the Coberly rule, where the screen is sized to the d10 of the sand (the d10 isextrapolated from the d50 and the Uniformity Coefficient (Cu), assuming a unimodal sandfollowing a standard log-normal distribution). Although 90% of the sand is smaller thanthe d10, the high amount of sand uniformity yields rapid sand filter cake formation andeffective sand control.

• At medium Uniformity Coefficients (3 < Cu < 7), the Coberly rule leads to too much sandproduction, especially in an open annulus configuration or in an injector well. TheSaucier rule (screen pore size = df50) is applied where the average sand size is at orgreater than the average screen pore size.

• For large Uniformity Coefficients (Cu > 7), sizing the screen according to the Saucier rulemay not prevent long-term sand production. The Schwartz rule (where screen pore size= df70) is applicable for very poorly sorted sand and leads to the selection of a very finescreen that may be susceptible to fines plugging. To avoid premature plugging silt andclays (particles less than 44 µm) should be less than 20wt%. If formation fines aregreater than 20wt%, a gravel pack is recommended to lock formation sand in place, andprovide a barrier to sand migration as far away from the well as possible to reduce fluidvelocity.

Figure 70: Stratapac media selection chart

To accommodate these different sand characteristics, the screen media selection chartcompares the average sand size (d50) with the distribution (d40/d90) and makes a screenselection based on the most appropriate sizing criteria.

1

2

3

4

5

6

7

8

9

10

0 50 100 150 200 250 300

Average Sand Size (D50) in microns

Uni

form

ity C

oeffi

cien

t (D

40/D

90)

Gravel Pack

PMM PMF 2040

PMF 1220


Comparison TestsIn addition to our in-house testing, the concepts and design principles used in Stratapac &Stratacoil have been confirmed independently by a number of operators. Some of the resultsof such evaluations are presented below.

Fine Sands – 40/60 Comparison TestA major oil producer tested the relative plugging tendencies of various types of screendesigns13. Included in the tests were 40/60 single screen consolidated pre-pack, 40/60 dualscreen consolidated pre-pack, low profile 12/20 consolidated pre-pack, low profile 20/40consolidated pre-pack, and the Stratapac Screen with PMM media. This technical reportreviews the results of this paper with respect to the relative plugging tendencies of thesescreens.For these tests, the apparatus consisted of a 4.75" ID casing which contained a 2-7/8"nominal OD base-pipe screen sample. The screen sample contained a 4' length of actualscreen material. A water-based slurry consisting of a 50/50 blend of SAE Coarse Test Dustand 70/140 mesh sand was made up to a 1500 ppm concentration. The slurry was pumpedthrough the screen sample at 55 gpm. The test was run until the maximum differentialpressure across the screen was 1500 psi.The results of the tests are shown in Figure 71 below. The Stratapac Screen reached theterminal differential pressure after being challenged with approximately twice the amount ofcontaminant as the 40/60 screens. The effluent from the Stratapac Screen visually appearedto be very clear, similar to the effluent from the 40/60 screens.

Figure 71: Fine screens - plugging and retention

Interestingly, the Stratapac Screen, which has superior sand retention than the 40/60 pre-packs, also took longer to reach terminal DP than either the 12/20 or 20/40 low profilescreens. The tests for the latter screens were terminated around 900 psid because gravelwas produced through both screens at that pressure. 13 S.A. Ali and H.L. Dearing, Petroleum International, July, 1996

0 0.5 1 1.5

Lbm Sand/Ft²

0

200

400

600

800

1000

1200

1400

1600

Diff

eren

tial P

ress

ure

(psi

d)

PMM 4060 Stratapac

12/20Pre-pack

40/60 Thin Pack 40/60 Pre-Pack

20/40 Pre-Pack

2.0 2.5

Effluent Particle Size Analysis

Screen Type 1% CutPoint

40/60 Thin Pack 35640/60 Pre-Pack 35520/40 Pre-pack12/20 Pre-packPMM 40/60 Stratapac 89


In summary, the test results showed that the Stratapac Screen has excellent resistance toplugging. The screen has the retention rating characteristics of a 40/60 pre-pack screen, butthe plugging characteristics of coarser screen materials. This is due to the higher porevolume achieved in the PMM material (52% vs. 32-36% reported for sand-packs made out ofsieved gravel14) and the thin cross section of the PMM filtration material which has very littledepth and therefore has a much reduced tendency to plug internally with solids. For a moredetailed discussion on the effects of media porosity, please refer to Coarse Sands – 12/20Comparison Test.

Medium Sands - 20/40 Comparison TestA major Gulf Coast operator performed a series of plugging/sand retention tests on a varietyof screen media test disks. The tests included PMF2040 (sintered metal fibre), a sinteredlaminate wire mesh screen with a manufacturer’s rating of 125 microns, 25/35 Pre-Pack,4 gauge wire-wrap screen (WWS), 6 gauge WWS and an 8 gauge WWS. The sand used inthe tests was a synthetic mixture simulating a Gulf of Mexico sand with a D50 of 70 micronsand D40/D90 ratio of 15 with 35% fines less than 44 microns in size. The plugging tendencywas determined by measuring the pressure across the screen with respect to cumulativesand volume, and the removal efficiencies were measured during the course of the tests.Figure 72 & Figure 73 show some of the results of the tests. The removal efficienciesavailable are reported only for the first five minutes of the test to minimise the effect of cakeformation on the apparent sand retention.

Figure 72: Plugging potential - 20/40

14 W.L. Pemberthy and C.M. Shaughnessy, “Sand Control”, Society of Petroleum Engineers, 1992

Plugging Potential

0

20

40

60

80

100

120

Cumulative Weight

Del

taP

(Psi

)

WWS (4 ga) Pre-Pack 25/35

Mesh 125 PMF2040

WWS (6 ga)


Figure 73: Sand retention efficiencies

The results show that the PMF2040 media had a lower pressure drop than the sintered125 micron mesh and better removal efficiency below 100 microns. The 25/35 Pre-Pack hadbetter removal efficiencies below 100 microns than either of the metallic media but of coursewith much higher plugging tendency.

Coarse Sands – 12/20 Comparison TestA series of tests was performed by a major North Sea Operator to compare screenperformance in terms of sand retention and plugging resistance. The originality of thesetests was to address several issues that had been overlooked in other tests reported in theliterature:

• Screen performance was evaluated simultaneously in terms of sand retention andplugging resistance

• The sand tested was coarse and relatively poorly sorted while most published tests dealwith either fine, poorly sorted sand (Gulf of Mexico type) or coarse and well sorted(Norwegian type)

• The tests were a third party comparison of all new screen technologies recentlyintroduced to the marketplace.

The data summarised below is more completely presented in SPE Paper 54745 (EuropeanFormation Damage Symposium, Den Haag 1999) and compares many screen types withequivalent ratings. Additional information provided by Weatherford Completion Systems isbased on our understanding of the technology and the test results as presented to us byNorsk Hydro. The notes below can be inferred from the test data and competitor informationas published in the public domain.

Removal Efficiencies

20

30

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60

70

80

90

100

0 20 40 60 80 100 120Size (microns)

Rem

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Pre-Pack 25/35 PMF2040 Mesh (125) WWS (8 ga)


Code Description

Sintered Fibre Mesh Stratapac PMF 1220 and 2040 screensSintered Twill Mesh Poroplus 250 mediaDutch Twill Weave (non-sintered)

Baker Excluder 230

Pre-pack Standard 12 gauge generic pre-pack screens (from Baker)Wire-wrap Standard 12 gauge generic wire-wrap screen (from Baker)Wire-wrap 2 USF Johnson Superflo screen

Table 24: Key to screen types

Test MethodThe tests consisted of challenging a screen test disc (approximately 95 mm) with water at aflow-rate of 5 l/min contaminated with a sand slurry suspended in a polymer solution injectedat 10 ml/min into the water stream. Slurry viscosity and mixing conditions were such that thefluid reaching the screen had a viscosity close to water with a well dispersed solidssuspension at a sand concentration of 200 mg/l with the flow going downward.Sand tested: d50 = 160 µm; d40 /d90 = 3.8 (see sieve analysis - Figure 74: Challenge sandsieve analysis).

Figure 74: Challenge sand sieve analysis

1. Produced sand was collected downstream of the screen in a sand trap throughout thetest and its average particle size distribution was measured by Coulter Counter.

2. Sand retention was measured by weighing the sand that passes through the screenduring the first minutes (first 20 litres) and corresponds to the sand retention of the actualscreen before the sand filter cakes forms.

3. Based on the Media Selection Chart, PMF1220 medium was selected as mostappropriate for this sand (Figure 75 below).

d10 = 500 µmd50 = 160 µmd40/d90 = 3.8d10/d95 = 115% fines0

20

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60

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101001000

Sand Size (µm)

% W

eigh

t Ret

aine

d


Figure 75: Media selection chart with challenge sand plotted

Results• Of all screens tested, the Stratapac screen exhibited the best resistance to plugging

(Figure 76).

• Stratapac PMF1220 screen retained sand very efficiently (Figure 77)

• Stratapac PMF1220 controlled fines production (Figure 78).

1.0

2.0

3.0

4.0

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9.0

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0 50 100 150 200 250 300

Average Sand Size (d50), microns

Uni

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oeffi

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Brent Sand

PMF1220

PMF2040

PMMGravelPack

Required


0

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7

0 10 20 30 40 50

sand reaching screen (g)

pres

sure

dro

p (b

ar)

1. Prepack2. wire wrap3. Wire wrap24. Dutch twill5. Sintered twill

h6. Sinteredfibre mesh

1 2 3 4 5 6

Figure 76: Coarse sand screen plugging resistance test

Figure 77: Coarse sand - sand retention efficiency

91 88

7165 62

50

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90

100

Stratap

ac PMF12

20

Dutch Twill

Weave

Sintered

Twill Mes

h

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rap 2

(12ga)

Wire-w

rap 12

ga

Scre

en R

eten

tion

Effi

cien

cy (%

)


0

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25

30

101001000

Sand Size (µm)

Wei

ght %

Stratapac PMF1220

Dutch Twill Weave

Sintered Twill Mesh

Wire-wrap 2

Wire-wrap

Figure 78: Coarse sand - effluent particle size distribution

Normally for screens with equivalent open area (connected porosity), a screen with biggerpores would resist plugging better as it would allow more sand to pass through. Referring toFigure 77, Excluder (88%) and Stratapac (91%) had roughly the same sand retentionefficiency and by inspection of Figure 78 retained the same size particles. All things beingequal, the screens should therefore have a very comparable plugging resistance. However,when looking at Figure 76, Stratapac has a markedly better plugging resistance. Referring toFigure 79, when Excluder and Stratapac screens are normalised to the same equivalentporosity, their plugging resistance curves closely overlay. This illustrates the significance ofopen area or porosity in filtration/screen design. In general, the more heterogeneous thesands the larger the performance difference is between PMF and the other screen types.

Screen TypeApproximate

OpenArea/Porosity

Slotted liner 1.5% – 6%

Wire-wrap (90µm wire) 6 – 25%

Pre-pack 35 – 45%*

Multi-layer wire-wrap (45µm wire) 11% – 40%

Metal mesh (Dutch twill weave, etc) 40% – 50%*Sintered powder mesh – PMM 52%*Sintered fibre mesh – PMF 68% – 72%*

Table 25: Media porosity comparison


*NB: It is not practical to measure the open area of meshes or pre-packs as the actual openarea would change with the depth and plane of the section and hence be open tointerpretation/dispute. The asterisked values are hence (near absolute) porositymeasurements and the effective porosity (connected pore space) could be expected to bemarginally less. Certainly in the case of fibre mesh, it is not thought that this reduction isvery significant as evidenced by Figure 79.

Figure 79: PMF & DTW plugging resistance - normalised for porosity

Longer Service LifeThe tests detailed above confirm the principle that ‘filter’ porosity is key to service life. Bydelaying the onset of plugging and by providing a more permeable sand filter cake, PMFmedia will provide greater fluid throughput in the same time period or the same fluidthroughput rate over an extended period of time. Due to the highly permeable nature of mostmodern screen designs, the well screen is not generally the rate limiting factor in acompletion. Therefore, the main advantage to selecting PMF screen media is extendedservice life. All things being equal in terms of retention and flow-rates, the PMF screen couldbe expected to have a service life 50% to 100% longer than other screens performing thesame job.

0 1 2 3 4 5 6 7 8

0 5 10 15 20 25 30 35 40 45 50 Weight of Sand/Porosity of Screen

Pres

sure

Dro

p (b

ar)

Dutch twill weav e Sintered f ibre mesh


7. Gravel Pack IssuesAs Weatherford does not offer the complete range of gravel packing systems and servicesin-house, the procedures and processes are discussed in the main sand control trainingcourse.This section will point out some of the issues involved in the more complex gravel packingoperations.

Horizontal Open-Hole Gravel Packing

Figure 80: Openhole gravel pack

In some situations, gravel packing canbe simple and straight-forward. Thegravel (with flush and push pills) intothe screen/well bore annulus and thecarrier fluid either leaks off into theformation or returns through the wash-pipe inside the screen. The gravelsupports the borehole wall andprovides a permeable mediumthrough which the hydrocarbons can flow (Figure 80).

Figure 81: Angle of repose (dry sand)

It is important to get a good pack. If the annulus is not completely packed and voids areevident, then the reservoir fluids will either travel up the unpacked annulus and not thescreen, or will be funnelled through the void directly against the screen. The screen is oftensized to retain the gravel and in such a situation, the formation sands will pass through athigh rate often eroding the screen in short order. This phenomenon is called “hot-spot”erosion.The angle of repose of dry sand is 28° (see Figure 81). By inference, anytime the hole angleis greater than 62° (90° - 28°), the sand will have difficulty to propagate along the well bore.


Figure 82 illustrates a horizontal well gravel pack with a partial pack resulting from higherthan expected leak-off. The operation requires to be planned and executed very carefully.The gravel quantities and fluid volumes are calculated and pumped in controlled amounts.The mud cake on the well bore walls is scoured and reduced to a minimum by pumping athigh rates (>300 fpm). The gravel carrier fluid is selected to be compatible with the drill-inand completion fluids to avoid emulsions. For most horizontal wells, the carrier fluid is waterbased and hence for best results, the drill-in fluid is a water based system. Again inhorizontal situations with longer intervals, the pumping requirements are considerable as thegravel is pumped at high rate (>5 bpm). The gravel and carrier fluid exit the work stringbelow the packer and form series of gravel dunes (the alpha waves). The design of thepack, fluid and pumping programme can vary the size of the dunes and the angle of reposeto some extent. The variables in the programme are the amount of carrier fluid leaking off,the borehole stability and the borehole size. An excess of gravel is allowed for in the case ofwash-outs.Carrier fluid can also bypass the annulus and flow through the wash-pipe/screen annulus.This also needs to be controlled (a wash pipe OD to Screen ID ratio of 0.8 is optimum). Thescreen OD should allow about 1” (0.75” minimum) radial clearance to formation for effectivepacking.Many long horizontals (>2000ft) are gravel packed, although there is much evidence ofpartial packs in these cases. Schlumberger have introduced the All-Pak system of shunttubes to try to achieve a full pack. This requires a greater screen/well-bore annulus to allowfor the tubes. Weatherford manufactures the Advance-Pack system for OSCA whichachieves similar results with a smaller OD, and Halliburton also market the CAPS system.

Figure 82: Partial pack of long horizontal well

Gravel packing in horizontal wells therefore has the following issues that the operator shouldbe aware of.

• Fluid compatibility issues with OBM drill in fluids. In certain situations, this may entail acomplete (and expensive) swap out of drilling fluids between intermediate and final wellsections

• The risk of a partial pack is high, especially with high leak-off zones and poor well borestability

• The completion ID and hence production may be reduced (especially with shunt tubes)

• Skin may be increased with sand/gravel mixing

• Depending on rate, inflow profile and coning issues may be apparent

• Safety (personnel & high pressure pumping) & logistics


Cased Hole Gravel Packing

Figure 83: IGP cross-section

Cased hole gravel packs as shown in Figure 83 are cased and perforated wells which arethen internally gravel packed. Flow from the reservoir sweeps into the well area and is thenforced to flow radially and then hemi-spherically into the packed perforation tunnel. Flowthrough the tunnel is linear until it hits the gravel pack annulus where it may disperse until itpasses into the screen. Once into the screen it is again concentrated over a base-pipeperforation and eventually enters the well-stream. Energy is lost in each flow regime. Eachflow regime therefore has its own associated pressure drop. As the flow is concentrated intothe perforation tunnel, the pressure drop associated with this flow regime is significant andalso through the gravel pack into the screen. In a properly packed IGP, most of the gravel inthe annulus will not see much production flow.The perforation tunnels require to be cleaned efficiently and packed completely withoutmixing with formation sands in an IGP. Figure 84 shows a typical perforation tunnel. Theperforation tunnel should pass through the invaded zone (often called the mud filtrate orformation damaged zone) and into the virgin formation. The tunnel is lined with a small layerof highly compacted or fused rock from the high temperatures and pressures exerted by the


explosive charge and the tunnel is filled with debris from the charge, casing, cement androck. This debris needs to be removed and the tunnel cleaned. The tunnel should then bepacked during the gravel packing process as long as the carrier fluid can leak off into theformation through the compacted zone.

Figure 84: Typical perforation tunnel

The choice of perforating gun is therefore important to

• Reach past the cement and damage zone to the virgin formation

• Minimise the compaction zone

• Minimise debris

• Maximise the exposed production surface area

• Maximise the exit hole diameterIf the formation is unconsolidated, the tunnel can be expected to collapse, in which case theexit hole diameter (ie the hole size through the casing) can be maximised at the expense ofdepth of penetration through the use of big-hole charges, as the tunnel will need to be pre-packed.

Figure 85: Perforation packing


The tunnel should be pre-packed (Figure 85) to achieve maximum permeability. Studiesshow that if the gravel and formation sands get mixed in the tunnel, permeability and henceproduction is severely impaired. Hole deviation, perforation orientation and poor leak-off intothe formation also severely affect tunnel packing efficiency.

Figure 86: Effective of formation damage

Gun centralisation, perforation charge performance and formation damage can alsosignificantly affect productivity as shown in Figure 86. A study of the various gravel packingtechniques is presented in Table 26. This illustrates the increase in skin that can beexpected even over EGP techniques. Interestingly, chemical consolidation gave goodresults, but this is effective over relatively short intervals (typically 10ft with epoxy resins).The IGP completions however allowed the client to produce at a higher sand free rate thanwithout packing and so were considered successful.

Gravel Pack Productivity Comparison(same field & similar lengths)

Method Skin Flow Efficiency

IGP 25 22%

EGP 11 38%

Under-Reamed GP 6 53%

Sand Consolidation(short lengths) 2 75%

Table 26: GP productivity comparisons

Gravel Sizing and Screen SelectionThe next issue is to select the most appropriate gravel. Incorrect sizing of gravel and screencan cause many problems with well productivity and premature completion failures.

Sieve Analysis DataThe proper sampling of the formation sand is critical in determining the gravel size. Theessence of sieve analysis is to obtain the correct formation grain size and size distribution.


Therefore, correct procedure and interpretation of the sieve analysis results are crucial to theselection of the appropriate gravel size. The following gravel sizing is based on three fieldsamples and are assumed to be representative of the formation sand (see Figure 87).

Figure 87: Example formation sieve data

Gravel Size SelectionGravel pack sand must meet the requirement of API RP58 recommendation (sorting,sphericity, roundness). The correct choice of gravel size will ensure the following:

• Preventing formation sand intrusion into the gravel pack

• Minimising the permeability impairment within the gravel pack

• Providing maximum productivity by minimising the skin.

Proposed Gravel Sizing CriteriaThere are many gravel sizing formulae published to date which are defined based on adiameter ratio of gravel/sand at certain percentile. At present, the most popular criterion isthe Saucier’s formula that is the diameter of gravel should fall within a range of 5~6 times thediameter of formation sand at 50 percentile (d50). The Saucier’s formula is fine for uniformsand.The dg/df ratio and the uniformity coefficient (Cu=d40/d90) are the key parameters to design thegravel size and screen size. Ren proposes the design criteria should be adjusted for poorlysorted sand (Cu > 5), and d75 should be used instead of d50 to design the gravel size.

Limitations of the existing gravel sizing formulaeFor example, the Saucier formula is used to illustrate the shortcomings of the existingformulae for gravel size selection. Sand A and sand B have the same 50 percentile diameter(105 microns). However, the sorting is different. According to Saucier’s rule, mesh 20/40gravel will be selected for both the sand A and sand B (see Figure 88).

0102030405060708090100

1101001000

Sieve Size (mm)

Cum

Wt %Depth 6370'

Depth 6460' Depth 6270'


Figure 88: Limitations of Saucier's rule to gravel sizing

It is obvious that the existing formulae are too general to meet the requirements of variousformation sand controls. Post placement evaluation also shows that there are certainlimitations of the existing gravel sizing formulae. This is mainly due to the lack ofconsideration of the geometry of the pack structure, the impairment mechanism, and thesorting of both gravel and formation sand.

Pack Structure ArrangementsSuccessful gravel packs require good gravel size selection. In order to have a betterunderstanding of the fundamental properties of the gravel pack, it is important to investigatethe actual packing structure arrangements. Ren has developed a modelling technique thatcan simulate 3D gravel pack structure (see Figure 89), which assists in understanding theeffects of the pack structure arrangement and its properties. The model-predicted resultsagree well with the experimental data.

Figure 89: 3D Gravel pack structure generated by 3D modelling

0102030405060708090100

1101001000

Sieve Size (micron)

Cum

Wei

ght (

%)Sand A

Sand B


Pore Throat DistributionThe gravel pack structure determines its porosity, permeability and pore throat distribution.For a given sand size distribution, the pore throat distribution dictates the bridging/pluggingability of the gravel pack. The size of the formation sand, which can invade the gravel pack,will depend upon the pore space/throat of the gravel pack structure (see Figure 90).

Figure 90: Gravel pack Pore Throat Distribution (PTD)

New Gravel Size CriteriaBased upon the pore throat distribution (PTD) of the pack structure, the candidate gravel isselected based upon the closest curve match of the formation sand size distribution and thepore throat distribution of the gravel pack. This new criteria, which considers both gravel andformation sand distribution, guarantees that the formation sand forms an interface/externalcake on the candidate gravel. Hence mesh 40/60 and 50/70 gravel should be the candidategravel sizes for the example field sand control by using the new criteria. Mesh 40/60 gravelsize was eventually used successfully due to availability issues.

Figure 91: Curve match of the gravel pack PTD with formation sand PSD

0

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40

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80

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101001000

Sieve Size (microns)

Cum

Per

cent

age

(%)1630 GP

2040 GP3050 GP4060 GP5070 GP

1220 GP

0

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40

50

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1101001000

Sieve Size (m icrons)

Cum

Per

cent

age

(%)1630 GP

2040 GP3050 GP4060 GP5070 GP1220 GPDepth 6460' Depth 6270'Depth 6370'


Sand Invasion MechanismProper prediction of gravel pack permeability impairment requires good understanding of thesand invasion mechanism. The sand invasion mechanism has a strong relationship with thefinal permeability of the gravel pack, the well life and well performance (PI). The idealinvasion mechanism is interfacing bridging, which will form an external filter cake and preventthe formation sand from invading the gravel pack deeply (see Figure 92).

Figure 92: Schematic of interfacing bridging

The bridging effects can be modelled by computer. For example, the above arrangementwas checked by generating the 3D packing structure of the 40/60 mesh gravel. Then,individual grains of the formation sand (based on the formation sieve analysis) wererandomly selected by the model and tracked as they flowed into the gravel pack sand. Theformation sand grains either bridged at the surface of the gravel pack, invaded and bridgedwithin the pore throat structure of the gravel pack, or flowed completely through the gravelpack. The model kept track of all the particles and calculated the permeability of the gravelpack sand as it was invaded. The model was run until the permeability through the gravelpack stabilised. Based upon the simulation, it can be seen clearly that mesh 40/60 gravelcan form an interfacing bridging, which will form an external filter cake and prevent theformation sand from invading the gravel pack (see Figure 93).

Figure 93: Bridging mechanism with example data

In conclusion, 40/60 gravel was selected over 20/40 and this choice was confirmedexperimentally and in the field. The wells had previously been completed with 20/40 systemsand performed at high rates initially until the fines invaded the pack. Although 40/60 has

FormationSand

Screen

GravelPack Sand


initially less permeability than 20/40, the total system permeability after sand filter cakestabilisation was superior.Two important conclusions can be drawn from this exercise:

• Interface bridging is achieved by more closely matching the gravel pore throats withthe particle size distribution. Note this is a non-uniform approach and has parallelswith Stratapac media selection

• Using standard gravel pack sizing systems based on the Saucier criteria (orderivative thereof) may give non-optimal well performance in non-uniform sands.

• Modelling can lead to non-intuitive results which need to be supported by lab testing.Testing should be as holistic and realistic as possible.


8. Expandable Sand Screens

IntroductionFigure 94 illustrates the differences between EGP and ESS in the case of a 20 API oilreservoir drilled with a 6” open-hole. ESS when installed would have an ID of at least 5” andup to 5.5”. For a GP, the options are to use a 3.5” screen (or a 2.7/8” screen if using shunttubes). The tubing intake curve (VLP - Vertical Lift Performance) is assumed to be the samefor all completion types. The Inflow Performance (IPR) curves for the various ESS/EGPcompletions indicate the ability of the reservoir to produce through the sand-facecompletions. A “solution point” is where the VLP and IPR curves intersect and this indicatessteady natural flow. The solution points vary by over 4,000 bpd between ESS and the 2.7/8”EGP completion.

Figure 94: Productivity comparison for a various ESS & EGP completion options ina 6" heavy oil producer

Figure 95 compares the flow contribution of five equal intervals along the length of the well.The ESS completion clearly has a more even inflow, which of course has several advantagesin terms of reservoir management:

• Production rates considerably more than equivalent gravel pack rates

• Reduced and more even draw-down for production along the wellbore length (lessprone to sanding and better mud-cake lift off).

• Even production inflow giving more efficient reservoir drainage

• Reduced risk of early water & gas breakthrough

• Well slimming


Figure 95: Flow contribution along well bore of various ESS & EGP completions

These benefits are derived purely from the increased ID available for production. Theproductivity gains of using ESS in a 6” hole are roughly equivalent to that of a standard EGPin a 8.5” hole, hence the use of ESS allows the operator to slim down his well. This conceptis referred to as “well slimming” and is causing much operator interest in this product.Additionally, there are two further benefits to an ESS deployment. Firstly, the more evendraw-down along the well should improve the consistency of mud cake removal along thewell bore length. Secondly, there will be some skin improvements in the near well bore areaas less fluids will have been pumped with ESS than in an EGP. In horizontal wells, thisbenefit is less apparent however.The reduction and elimination of the annulus gives the following benefits:

• Formation sands/clays/fines do not mix in the annulus and plug completion

• Zonal isolation & control is possible

• Increased number of remedial options

• Remedial options still provide a sufficient ID for high rate production

• Production logging data is useful.


ESS ConstructionThe ESS is made of metallic components designed to withstand the toughest wellenvironments. It combines four basic elements to deliver sand control in various wellconditions while maintaining high reliability, longevity and optimum hydrocarbon production.

Its basic components are:

• Base pipe

• Filtration media (Petroweave)

• Outer protection shroud

• Integral expandable connector

Figure 96: ESS Construction

Filtration Media(Petroweave)

ExpandableBase-Pipe

Petroweaveattached tobase-pipe

ExpandableProtective Shroud


Figure 97: ESS Assembly

Top connector houses the expansioncone.

ESS Joints are normally supplied in11.6m lengths and are 11.5m whenmade-up (4.71” make up loss)

Shoe assembly with cone catcherAfter expansion the cone is leftsecured in the cone catcher at thebase of the ESS assembly.


Base Pipe and connectionsThe ESS base pipe is a robust Expandable Slotted Tube (EST ) capable of expanding up to80% in diameter, depending on the size of base pipe selected. The base EST provides avery large inflow area for the produced fluids.The base pipe has the ability to go through short curvature radius making it an ideal choicefor deployment in horizontal holes. Because the pipe is slotted, the ESS becomes more

tolerant to larger bending moments, displaying classleading flexibility (ie 5-1/2” ESS can be deployedthrough 30°/30m doglegs; it will fail only at 43°/30mdog legs).The ESS joints have integral expandableconnectors with no blank areas to obstruct flow.Each joint has both a pin and a box connection,which are designed to provide a mechanicalinterlock (for strength) during deployment andexpansion. The Petroweave filter on the connectorsoverlap after screwing together the ESS joints,providing a sand tight connection. The connector isa critical element of the ESS system and has beenextensively tested to prove sand exclusion capabilityafter expansion.The flush connectors are made of Super DuplexStainless steel to provide high tensile and bendingstrength (ie tensile yield of the 5-1/2” ESS is230,000lbf).

Figure 98: Stabbing in ESS pin connection

Figure 99: ESS Connections

Filter Medium (Petroweave)The filter medium Petroweave is a metal weave designed to provide maximum filtration/flowarea, thus maximising resistance to plugging effects. The Petroweave is manufactured inboth 316L and Nickel Alloy (Incoloy 825) offering compatibility with most corrosive wellenvironments. Following extensive testing to select the filter medium, Petroweave filters withnominal sizes ranging from 150 micron to 270 micron were chosen as elements of the ESSto suit different sandstone types and operational preferences.


The Petroweave is attached to the base-pipe using a processthat ensures the integrity and uniformity of the sand exclusionapertures. The filters overlap each other along the length of thebase pipe and accommodate the circumference increase duringexpansion while remaining sand tight. Correct selection of thePetroweave filter should aim to restrain the load bearing grains ofthe formation sand from passing into the well-bore. These grainswill naturally bridge the formation sand against the ESS ,controlling sand influx in the process.

Figure 100: Expanded ESS

The sand control capability of the Petroweave is enhanced because the screens are placedin direct contact with the well-bore, reducing movement of formation sands in the annulus.As a result, the well-bore remains stable throughout the life of the well and risk of holecollapse is decreased. Furthermore, with restricted fines migration, near well-boreimpairment is reduced. In cased hole applications, the ESS will outperform traditionalscreens by eliminating annular fill.

Outer Protection ShroudThe outer protective shroud ensures the filter medium is not damaged when running thescreens in hole. It also acts as the encapsulating layer, ensuring the filter media remaintightly sandwiched together following the completion of ESS expansion.

Expandable Isolation Sleeve (For Information)The Expandable Isolation Sleeve (EIS) is a complimentary product to the ESS designed foruse in wells where zonal isolation may be required during the well life. Current practices foropen hole zonal isolation involve the use of External Casing Packers, which have provenwith time to be unreliable due to cement shrinkage and complex installation procedures. TheEIS is constructed in a similar manner to ESS enabling it to be made up in the ESS string inthe same manner as the ESS at the correct space out. EIS uses a HNBR rubber coating onthe protective shroud, which is energised against the formation during the expansion processpreventing annular flow. Should it be necessary later in the field life the EIS may be re-energised against the formation using inflatable packer set inside the EIS.

Expansion SystemsSolid ConeThe solid cone (shown below) is pre-installed in the ESS ETC (Expandable Top Connector),fitted with the standard 7.375” OD tungsten carbide cone ring (to suit 8.500” hole). The solidcone is easily removable from the ETC if so desired. The body is a two part assembly, which

holds the cone ring. The cone holder is supplied with asuite of cone rings, to suit irregular hole sizes. On the outersurface of the cone body are four slip segments. Theselocate in profiles in the EBC (expandable bottom connector)to ensure the cone remains at the bottom of the ESS .Tests have shown that this feature is NOT necessary, but isincluded as an additional safety feature.

Figure 101: Expansion Cone

To date, the majority of ESS field expansions have beenperformed using this method. A second expansion method


integrates the cone with the expansion mandrel so that the cone is retrieved. Whatevermethod is utilised it is essential that the borehole geometry is maintained within the surplusexpansion limits of the ESS in order to achieve both full expansion and borehole support.

Expansion MandrelThe expansion mandrel shown below is deployed on the end of the expansionstring on all solid cone, two trip systems. The mandrel passes through the conebody, centralises in the unexpanded ESS and engages with the inner shoulderin the cone body.

Figure 102: Expansionmandrel

Rotary ExpansionA range of Rotary Expansion tools is in development, which can be deployedon drill pipe or coil tubing. Driven by a PDM mud motor, they improve thedeployment options and expansion performance of the expandable product line.The rotary expansion tool is available as a selective device which will passthrough restrictions (eg packer bores). Following deployment and expansionthe tool can be functioned to “collapsed mode” allowing retrieval. Experience suggests thatthe use of rollers for expansion reduces the required expansion forces by 66 – 75 % byeliminating friction, thus extending the length of ESS that can be expanded in extendedreach wells. Additionally, this system can be retrieved and reset to a smaller open ODshould a restriction in the well be encountered.

Figure 103: First 4” Compliant Rotary Expansion System (CRES) tool as used on Shell Brigantine

Figure 104: Non Compliant Rotary Expansion Subassembly


Figure 105: Compliant Rotary Expansion System

The CRES tools consist of roller cones in the nose which expand the EST to a fixed size.The balls in the compliant section above then move out under hydraulic pressure and forcethe EST outwards still further to ensure it conforms to the bore-hole wall. This system is stillunder intensive development and its first application occurred in October 2000 when it wasused to expand 4000ft of 4” ESS in a North Sea well. This first use of the system actuallyused two trips, the first trip used a “conventional” fixed cone system to expand the ESS outinitially as a precaution. The second trip was with the CRES device which expanded thescreen out to conform to the well-bore.The intention is of course for rotary expansion in one trip (two trip deployment) and thisshould be achievable by year end 2001. Longer term, the intention is to deploy ESS andexpand in one trip (single trip deployment/expansion).

ESS Mechanical PropertiesTESTED MECHANICAL FIGURES

The following figures are the results of testing to destruction under laboratory conditions.These figures are for information only and on no account should be quoted for operationalissues. A safety factor should be adopted at all times, and as such, the rule of thumb foroperations is to divide these figures by 2.

2 7/8" 3.5" 4" 4.5" 5.5"

Tensile Yield (lbf) 100,000 100,000 125,000 175,000

Tensile failure (lbf) 134,000 140,000 188,000 230,000

Compressive Failure (lbf) 63,000 66,000 107,000 122,000

Bend (°/100ft) 53 43 42 43

Point load collapseresistance (psi) 2,240 2240 2240

Rotational Torque (Ft-lbf) 3,500 3,500 3500

Expansion Force (lbf) 30,000 20,000 20,000 25,000 35,000

Table 27: ESS application limits


ESSSize (in) Pipe OD Wall

Thick Pipe ID Wt/Ft Box OD Box ID FilterWidth

MaxFinalOD

2.875 73.03 7.01 59.01 2.67 83.8 59 200 114.422.875 0.28 2.32 5.89 3.30 2.32 7.87 4.50

3.5 88.9 5.49 77.92 3.49 99 74.5 120 133.353.500 0.22 3.07 7.70 3.90 2.93 4.71 5.25

4 101.6 5.74 90.12 4.2 112.4 87.3 140 158.754 0.23 3.55 9.26 4.43 3.44 5.49 6.25

4.5 114.3 6.02 102.26 4.97 124.5 100 160 184.154.5 0.24 4.03 10.96 4.90 3.94 6.28 7.25

5.5 141.3 6.55 128.2 6.74 151.5 127 200 232.415.56 0.26 5.05 14.86 5.96 5.00 7.87 9.15

Table 28: ESS dimensional data (mm & inches)

ESS Installation ProceduresThe ESS system can be installed using either a one-trip or a two-trip system. Note that theone trip system requires further development to allow this to be applied over all ESS sizesand systems. The two-trip system is currently used as standard.In the two-trip system, the ESS is conveyed in the same manner as conventional non-gravelpacked screen, ie below a liner hanger or packer. The ESS and liner hanger can be run witha concentric wash string which allows circulation through the ESS via a wash down shoestring. The wash-string makes the running string stiffer and this factor should be consideredin Torque & Drag (T&D) modelling. The use of the wash-string, although a complication,may be required for well control and mud conditioning considerations and this is a drilling &client decision.

Torque and DragBefore any ESS installation, it is necessary to run a Torque & Drag simulation to ensure:

• The ESS can be safely deployed to TD without exceeding its mechanical limits orinducing helical buckling into the string

• The ESS can be safely expanded in its entirety without the expansion stringexceeding its mechanical limits or inducing helical buckling into the string.

• Hydraulic factors do not limit the activation of key system components, eg linerhangers or expansion cones.

Weatherford Completion Systems make extensive use of the Landmark Torque and Dragsoftware package to determine whether a particular ESS can be safely deployed in the well.The interval length is not limited by hydraulics as is the case with gravel packing. Rather thelimit to the length of ESS that can be expanded using conventional “weight on bit” from the


drill-string (or with mud pump hydraulic power) is limited or dependent on a number offactors:

• The trajectory of the entire well to surface

• The availability of required drill-collars, drill-pipe, etc

• Prevailing hole sizes and friction factors, often dependent on mud types, formationtypes, casing grades

• The type of expansion mechanism used.The use of the CRES tools is expected to extend the envelope of possible candidate wells.Physical testing representative of field conditions is difficult to simulate in test wells as theydo not contain long reach sections where achieving WOB is difficult. To date, the validationand fine-tuning of the T&D model has been achieved via observation of expansion performedin real applications. Most wells and certainly all highly deviated or horizontal applicationswould be modelled when full well data was made available to ensure the appropriate weighton bit (no of drill collars) can be achieved.

ESS Expansion ForcesESS Expansion is achieved by slacking off weight from the expansion string to lay downweight on the expansion cone. For 5.5” ESS, the following forces can be expected.

• Unlubricated expansion forces, as expected in testing, can range from 35k – 50k Lbf

• Lubricated expansion forces, as expected down hole, can range from 35k – 40k Lbf.The above information is relevant to a one or two trip system with a solid Tungsten Carbideexpansion cone. Tests conducted with the 3.5” roller cone show expansion forces of 4000lbf, compared with the 12,000 – 16,000 lbf expansion force using a solid cone.

First TripHaving made up all the T&D pre-determined ESS joints with blank pipe and packer/hanger,the entire system is run to depth on a drill pipe work string. When on depth, the packer is setwithin the liner, using conventional procedures; ie drop ball, pressure up to set, anchor testand pressure test. The running tools would then be released, and the work string retrieved tosurface.

Second Trip (Conventional Cone System)The expansion string would consist of the Expansion Mandrel and appropriate drill collarsand heavy weight drill pipe determined by T&D simulation. Once the Expansion Mandrel islanded on the Expansion Cone located on the top of the ESS, slacking-off weight shears outthe locking screws of the Expansion Cone, and the expansion would be initiated. Expansionwould continue until the cone lands at the bottom of the ESS. The expansion string wouldthen be pulled and retrieved back to surface.


Typical running time for 150 m of ESS (13 joints) set at 2,500mWith the Two-Trip System

OPERATIONS TIME1. Make-up-up bottom screen joint c/w cone catcher profile and bull plug2. Make up ESS joints3. Make-up top joint of ESS complete with pre-installed expansion cone4. Make-up crossover to blank pipe

3 hrs.

5. Make-up blank pipe space out pipe as required6. Run inner string/space out7. Pick up Packer (or liner hanger)8. Make up inner and outer string connections

2 hrs.

9. Run entire System to depth on Drill Pipe 8 hrs.

10. Drop ball/Carry out setting sequence for Packer11. Close the pipe rams and pressure test the annulus to ensure packer seal

integrity12. Confirm anchor/test and pressure/test, release running tools

1 hr.

13. POOH with Running tool 5 hrs.

14. Make up expansion string with enough drill collars/drill pipe to provide35,000 lbs. Net down-hole force and RIH

7 hrs.

15. Latch into expansion cone / shear pins16. Initiate expansion17. Expand ESS joints

1 hr.

18. Land out expansion cone to bottom ESS joint19. POOH with expansion string20. END

8 hrs.

Total time 35 hours

Table 29: Operation time estimate


Figure 106: Two trip installation: Trip 1 - Set Hanger

1) Make-up screens2) Make-up Packer setting tool3) Run in the hole4) Correlate depth5) Set packer and test6) Release running tools

ESS Deployment (Two Trip System)


Figure 107: Two trip installation: Trip 1 - Expand screen

8) Make up Expansion string 9) Run in the hole10) Engage expansion cone

11) Expand ESS

12) Pull out of the hole


Pre-Installation, Well Prep, Clean-Up & LoggingIn order to drill and maintain an optimum hole through the reservoir, it is important that all therelevant personnel have an early involvement in planning the operation and developing theprogramme. The relevant people should perform hazard identification and developcontingency plans, select the most appropriate fluid, determine the data acquisitionrequirement, etc, etc.Hazard identification and mitigation needs to be carried out on the processes required toinstall an ESS, highlighting the critical issues affecting the installation such as:

• The quality of the reservoir section; require a gauge hole, a clean hole (cuttings free).

• Fluids strategy from drilling to well clean-up.

Proper selection and design of the drilling fluid has a major impact in the success of theoperation. The mud type selected must be optimised to achieve the following:

• Aid the drilling of a close to gauge hole, including the prevention of interstitial clayexpansion to enable the maximum cone size selection in order to achieve boreholesupport

• Controlled maximum particle size during the drilling of the reservoir section requiringthe use of sized barite or bentonite. Solids control activities should include the use ofthe finest mesh screens practical to maintain a particle size less than 1/3rd andpreferably 1/6th the size of the ESS mesh. The centrifuges typically should be run inbarite recovery mode continuously while drilling the reservoir section. It isrecommended that while drilling the reservoir section, particle size tests are made atregular intervals to ensure the maximum size is maintained below the pre-determinedmaximum. Test disks can be provided to allow this to be checked and monitored atthe well site.

• To achieve little to no residual cuttings or debris and a thin and tight wall cake, themud should be treated to provide minimum fluid loss (good mud filter cake), optimumrheology for hole cleaning, optimum solids content and quality.

If any well-bore areas logged indicate potential problem zones, a check trip to TD isrecommended to ensure there is a clean hole with no suspected hole problems. A heavyweight pill may be pumped around to ensure the hole is clean. Additionally, considerationshould be given to provide on-site hole cleaning tuition to the offshore personnel.

Data Acquisition For ESS DeploymentThe data acquisition should aim to deliver a good well and to identify potential problems thatmay arise during the sand face completion. The information should help to assess how thewell is being drilled in relation to the plan. It is recommended that caliper information beobtained prior to a gravel pack or an ESS installation. The caliper information allows thewell-site personnel to select the correct fixed cone prior to running the completion. With theuse of the compliant rotary tools and experience in a particular field, the importance of thisinformation will be reduced. Note that for gravel packing, an estimate of borehole volume isalso of importance to check that the required quantity of gravel has been used and that thepack is complete.

• Mud Logging - helps to identify any possible fluid problems that could affect thedeployment and expansion of ESS.


• Navigation tools such as Rotary Steerable Systems coupled with GR/Resistivity andNear Bit inclination sensors (ie Autotrack) result in better hole quality since theconstant rotation improves hole cleaning. Furthermore, it avoids the micro-doglegscaused by rotating and sliding directional motor systems.

• Modular Drilling Dynamics and Pressure - this information could be used to optimisedrilling and to avoid excessive vibration, possible tool failure and over-gauge holes. Italso assists in identifying possible problems removing cuttings from the well-bore.

• Electric Line Logging - Caliper data used for the selection of cone size can beobtained during open hole logging. A high mud weight may preclude the use ofultrasonic imaging tools, but in general these are easier to obtain in horizontal wells.An independent six arm caliper tool, such as can be provided on dipmeters, providesa good indication of borehole shape. Borehole image logs (eg FMS, STAR, CBIL,UBI) have been proven in previous applications to have adequate accuracy todetermine hole size/shape and rugosity prior to expansion.

Use of ESS in Combination with Other EST ProductsESS was designed to be run in combination with other Weatherford expandable products toprovide sand face completions that meet different reservoir management needs. It can berun in combination with Expandable Completion Liner (ECL ), Expandable Isolation Sleeve(EIS), Rubber Coated ECL-R, and blank pipe.For instance, blank pipe would be deployed across the unstable cap rock to prevent collapsewhile ECL would provide hole support in the formation below the cap rock. Rubber coatedECL could provide an annular seal after expansion to make selective treatments acrossdifferent zones possible using inflates or packers. Finally, ESS would be deployed to controlsand in zones with weak compressive strength.

Accessory EquipmentThe ESS system can be run with standard completion equipment such as circulating shoes,blank pipe, seal-bores, fluid loss control devices, packers and liner hangers. Weatherfordcan supply the entire system requirements. It is recommended that a packer or liner hangerwith the largest bore be selected to run with the ESS system to avoid restrictions that couldchoke production or limit the capability for future intervention work. Weatherford hasdesigned and can provide a range of EXP packers to fulfil this requirement and has availablea wide range of suitable liner hangers.

Technical IssuesMinimum rat-hole requirementsThere is normally a requirement to have the ESS expanded over the entire sandface lengthand sometimes, due to the active aquifers for example, the rat-hole must be minimisedTo minimise the rat-hole would require the length and number of components beneath theESS to be limited. The most effective way to do this is to run the ESS without a concentricinner string for circulation. If the benefits of an inner string are minimal, this string should beeliminated if possible. Without an inner string there is a requirement only to run a x-over withbullnose on the bottom of the ESS system in order to make the system sand tight. Thiswould result in an approximate length below the fully expanded ESS section of 2 – 3 ft.The issue of expanding the ESS from a semi-submersible rig whilst ensuring that maximumsit down forces are not exceeded on the ESS can be addressed over short sections by


introducing a no-go to the expansion string which lands on top of the packer or liner hanger.This will physically prevent the expansion tool from travelling further than required, althoughextreme caution would still be required not to exceed the maximum set down weight on theliner hanger or packer.

Mud filter cake removalOne of the primary requirements for the drill-in fluid, alongside hole cleaning properties andwell control properties, is to produce a thin cake, which minimises leak-off and henceformation damage. Normally, during the well clean up, this filter cake is back-producedthrough the ESS.A series of tests carried out by an operator on producing filter cake back through the ESShas provided a useful insight into the filter cake behaviour as ESS shroud and mesh ispressed into the cake and a draw-down applied. The test set used a ceramic disc with a5mm filter cake, a layer of expanded perforated plate as used for the outer shroud of theESS and a layer of ESS weave, all pressed together. Oil was then flowed through theceramic disc and through the ESS. Although it is not possible to release the results of thetest, the picture below shows the test piece at conclusion of the testing.In clockwise order from top left the pieces are, ceramic disc, perforated plate, lower screen,upper screen. It can be seen from the pictures that an absence of filter cake is noted on theweave over the area that the perforated plate sat and that the filter cake is still in place in theperforated plate.

Figure 108: Photographs following mud filter cake removal testing


It has been concluded as a result of this testing, that during phase from ESS expansion tofilter cake production the following occurs:

• The ESS is expanded to contact the borehole with a radial force of 2,000 lbf. As thishappens the filter cake adjacent to the gaps in the perforated plate is pressed into thedepth of the outer shroud down to the mesh

• The filter cake adjacent to the solid area of the perforated plate is pressed firmly intothe formation

• Following the running of the upper completion, the well is drawn down to below thepre-determined mud cake pop off differential and the well is cleaned up. From thepictures, it can be seen that the mud cake trapped against the weave begins to beproduced (lower left and right). But where the filter cake has been squeezed betweenthe expanded shroud and the ceramic disc / formation, the filter cake remains in placethough considerably less thick.

• Consequently, there is no loss of borehole support in the ESS system occurs as aresults of the filter cake removal.

ESS Media TestingThe following tests have been performed on both the individual elements and the completeESS system to verify sand exclusion capability and mechanical properties to resist variouswell environments.

Filter media plugging testsDifferent filter media types were tested to identify a media with the best sand controlcharacteristics whilst maintaining high resistance to plugging.

Figure 109: Comparative screen plugging tests

S O U T H E R N N O R T H S E A P E R M I A N S A N D S T O N E

( P . I . = 3 6 0 B B L / D / F T )

0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

0 1 0 2 0

F L O W I N G T I M E ( M I N S )

SCR

EEN

PR

ESSU

RE

DR

OP

(PSI

)

2 0 0 M IC R O N S IN T E R E D W E A V E

2 0 / 4 0 P R E - P A C K E D S C R E E N S

2 0 / 4 0 S IN T E R E D M E T A L S C R E E N S

2 0 0 M IC R O N E S S M E M B A N E


Filter media erosion testsErosion tests were performed using sand particles accelerated with compressed air througha gun nozzle. Based on the results of the tests, it was concluded that a specific weave hadthe best erosion resistance over similar ‘family’ filters and this was selected as the basedesign for Petroweave meshes. The photographs shown below depict the results ofdestructive testing.

Hollander Plain Weave PetroweaveFigure 110: Erosion testing

ESS sand exclusion/integrity testsThese tests were performed on both preand post expanded ESS to document itscapability to exclude sand grains withparticle size equalling the nominalapertures of the screen. All the testsshowed absolute sand retention,confirming the sand tight effectiveness ofthe ESS connector and weave overlapdesign.

Figure 111: Sand exclusion testing

ESS mechanical testsThis set of tests was aimed to identifying the mechanical strength of the ESS undercompression, tension, bending, crush, and collapse loads. This information is used tooptimise the planning and execution of jobs.

Figure 112: ESS connector strength testing


Figure 113: ESS mechanical testing

ESS expansion testsNumerous expansion tests have been performed for various ESS sizes whilst simulating awide range of expansion conditions in the expansion test bay and at a test drill rig inAberdeen.

Borehole SupportBorehole support was discussed in Section 3. The derivation of the computer model used todetermine ESS suitability in a highly deviated or horizontal well is discussed later in thissection.

Surplus ExpansionOne of the key features of ESS is that it eliminates the annulus, even when the borehole isnot ‘’gun barrel gauge”. This property is known as ‘surplus expansion’. The configuration ofthe slot pattern causes the pipe to flow over the expansion cone when driven though it, givinga larger ESS bore than the cone OD plus screen thickness. The amount of surplusexpansion is related directly to the expansion cone lead angle and whether the expansionedge is sharp or graded.Much research has been conducted on ascertaining the most effective cone design.Reducing the cone angle decreases the expansion force and the surplus expansion;increasing the cone angle increases the expansion force and the amount of surplusexpansion. All fixed cone configurations have been standardised to give appropriate surplusexpansion without excessive expansion forces. Based on theoretical models, test resultsand field experience, the amount of surplus expansion for the 5.500” ESS expanded with astandard cone will be a minimum of 4%.The following is an example calculation for 5.500” ESS in 8.500” open hole:

Cone OD 7.375 inchesplus 5% Surplus 0.300 inches (4% of cone OD)ESS Thickness 1.000 inch (0.5” per side)

Table 30: Surplus expansion (i)

Therefore, expanding a 5.500” ESS using a 7.375” cone, the minimum theoretical ESS ODwill be as follows:

(Cone OD) + (ESS thickness) + (Surplus Expansion) = Expanded ESS OD7.375” + 1.000” + 0.300” = 8.675”

Table 31: Surplus expansion (ii)


Therefore open hole size can vary between 8.375” minimum to 8.675” maximum toguarantee full bore hole support and full system expansion.

C =ESS Thickness = 1.00in

A = Surplus Expansion = 4%

B = Cone OD = 7.375in

A + B + C = Expansion OD

Figure 114: Surplus expansion

A

B


As the surplus expansion can vary, usually the worst case is used for prediction purposes.However, in a measured sample piece surplus expansion was found to average 5.3% overthe joint and 4.1% at the connector.Note that for general purposes, surplus expansion is quoted as being 4%. This is theamount of minimum expansion, which could be achieved at the connector.The use of the rotary compliant tool may change this value slightly. The initial roller nosesection will result in surplus expansion similar to the fixed cone. The compliant sectionshould expand the screen still further.

Benefits of Borehole SupportThe interaction between the Expandable Slotted Tubulars (EST ) and formation rock hasbeen studied. Note that this data is not limited to ESS sand screens. Specifically,experimental tests were carried out and analytical solutions in combination with empiricalmethods were used to study the EST rock interaction.A preliminary quantification of the borehole stabilisation effect of the EST when it is applieddirectly against the open hole has been derived. The results obtained so far have shown thatthe stabilisation effect of EST is significant. Experimental thick-walled cylinder tests withEST support suggests that the EST can increase load bearing capacity of the borehole byapproximately 70%. An analysis of typical field data shows that the use of EST wouldimprove significantly the hole stability of horizontal open-hole completions. The typical fieldinput parameters required are the in-situ stress (effective overburden stress) and thick-walledcylinder strength.

Borehole Support TestingA number of scouting experiments have been performed to provide a basis for understandingrock/EST interaction. In the following sub-sections, experiments relevant to the rock/ESTinteraction are presented. They include thick-walled cylinder tests with EST support andborehole collapse tests with inner hole pressure support.

Expansion/Contraction behaviour of ESTTo obtain an insight into theforces involved in ESTexpansion, a sleeve has beeninflated around the EST. Thepressure contraction response ofthe EST can be measured and itis shown below. Figure 115shows the response of ESTunder several cycles of pressureloading and unloading. Fromthe “stress-strain” curve, a valuehas been derived for thepressure response of the ESTwithin its elastic range. It canbeen seen that the limit of theEST elastic behaviour isapproximately 10 to 12 Bar.

Figure 115: Change of EST Radius with external pressure

0

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6

8

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12

14

16

18

0 1 2 3 4 5 6 7 8

Radial displacement (mm)

Exte

rnal

pre

ssur

e (B

ar)


TWC experiments with ESTThe thick wall cylinder (TWC) experiments involved installing an EST in an 85 mm outerdiameter TWC sample with a 33 mm inner diameter hole and a length of 170 mm. Thesample was then pressure tested in ROXELL with an outer rubber membrane isolating thesample from the cell’s pressurising fluid. Isotropic compressive pressure was applied on theoutside (ie outer wall, top and bottom) of the sample while the inner hole was maintained atatmospheric pressure. The external pressure was applied incrementally. A rough indicationof the inner-hole deformation was obtained by visual inspection using an endoscope placedwithin the hole. The external non-linear deformation was indicated through thepressure-volume response of the cell fluid.

Sandstone testA 32% expanded EST was inserted into a Castlegate sandstone TWC sample. The externalcell pressure was increased smoothly up to approximately 500 bars at which point thesample began to fail (see Figure 116). No prior indication of initial failure was indicated bythe pressure curve, although visual inspection of the inner hole using an endoscope showedsome break-out of sand grains at about 300 bars. Once the load capacity of 500 bar wasreached, a pressure drop of 80 bar was observed, followed by a stage of post-failure‘stabilisation’ in which the sample could withstand further pressure build-up to 690 bar(maximum allowable cell pressure). At this pressure the EST had deformed into an ovalshape, but the slots were not fully closed. The additional stabilisation effect of the EST wasestablished by comparison with an unsupported Castlegate TWC. Failure of the TWCwithout EST support occurred at approximately 300 bar, which is in agreement with previousCastlegate TWC testsIt was observed that the EST support was able to keep the failed zone in place and hence,the composite structure sustained higher pressure than the TWC without EST support.

Borehole collapse testThe Borehole Collapse (BHC) test is essentially the TWC test with internal hydraulic supportpressure. By comparing the BHC test to the TWC test with EST support, it is possible toexpress the EST support in terms of equivalent (mud) pressure support.The loading procedure of the BHC test is different from the TWC test. In the BHC test, theTWC sandstone sample is loaded hydrostatically to simulate in-situ stress condition byincreasing the internal hole pressure Pin and external cell pressure Pex simultaneously, withrubber sleeves on both the internal and external of the sample. Subsequently, the externalpressure is maintained while reducing the internal pressure at a constant rate. The innerhole pressure-volume behaviour is monitored for hole deformation and failure.In both the BHC test and the TWC test with EST, the inner holes are supported. In theformer case, the inner hole is supported by fluid pressure and in the latter, by the expandedEST. Therefore, the BHC tests provide additional insights to the stabilisation effect of EST inthe Castlegate Tests.The results of the BHC tests carried out on the Castlegate sandstone are summarised inTable 32. In these tests, various loading rates were used. The results are considered to beconsistent and reproducible. The results show that with an inner hole support pressure aslow as 10 bar, the TWC samples can withstand an external pressure of almost twice theTWC strength.


Figure 116: Castlegate TWC testing with & without EST support

BHC Tests on Castlegate with Internal Pressure

Test Internal Pressure at Collapse(Bar)

External Pressure at Collapse(Bar)

1 14 6802 16 6803 10 680

Table 32: BHC testing

EST Supported Sandstone TWC

1,450

2,900

4,350

5,800

7,250

Externalpressure(psi)

5 10 2015 25 30

Up to 10,000 psi (max cell pressure)

Deformation (% cell vol.)

TWC testTWC test with EST

UltimateLoad capacity

Post failurestabilization

Some sand breakoutobserved usingendoscope TWC strength


Loose-sand testTwo thick-walled cylinder (TWC) experiments were also carried out with unconsolidatedsamples. In these experiments two fractions of loose sand were mixed and compactedaround the EST which was wrapped in a mesh screen. An unconsolidated sand pack with28% porosity was achieved in this way. With the support of the EST, the TWC loose-sandspecimen sustained an external pressure of 80 bar before the slots of the EST started toclose. This initial closure of the slots corresponds with the point where the pressure-volumecurve deviates from linearity. This result was confirmed in a duplicate second test in whichan endoscope was used to observe the slot closure.

AnalysisIn order to assess the interaction between rock and EST, it is necessary first of all todescribe the structural behaviour of the EST. The EST is a fairly complex structure with thecapability to sustain a large strain deformation. An accurate description of EST and rockinteraction under all load conditions and deformations (linear, non-linear, large strain) has notbeen completed. Instead a simplified model of the EST is adopted and, based on this model,the EST interaction with rock is studied using empirical as well as analytical methods. Theanalysis described in this section is used to consider the field application of the EST.To obtain an insight into the forces involved in EST expansion, a sleeve has been inflatedinside the EST. The pressure-expansion response of the EST can be measured from the“stress-strain” like curve. In our present description of the EST behaviour, a number ofassumptions are made. Linear elasticity is assumed so that the radial expansion andcontraction of the EST are assumed to behave in an elastic manner (at least at relativelysmall strain). This is equivalent to assuming that at small strain, the EST would expand andcontract like a flexible elastic tube without slots, and the EST has an equivalent elasticmodulus, EC. The value of EC is obtained from the above experimental expansion of theEST. The EST also has an equivalent minimum yield pressure, which can be equated to theinternal support pressure that the EST can provide without permanent deformation.In the above experiment, approximately 10 bar is needed for a radial elastic displacement of0.35 mm. The minimum yield pressure can therefore be taken as 10 bar for this particularEST used in the tests.

Rock-structure interactionStresses and displacements in the rock surrounding the borehole and in the EST depend,not only on the rock mass properties and the in-situ stress field, but on the stiffness anddeformation characteristic of the EST, ie the internal support pressure provided by the EST.A typical inner hole pressure response of the BHC test is shown in the figure below. Thisindicates the collapse of the sample at an inner hole pressure of between 10 and 16 Bar.Plotted in the same figure is the TWC test result with EST support. It should be noted thatthe loading path of the BHC test is not the same as that used in the TWC test. In the latter,the rock is not pre-stressed and an external pressure is applied incrementally until collapse.The inner support pressure can be empirically equated to the stiffness of the EST. Thereforestiff sets (those with high angle finger deformations and thick walls) will have a highereffective support pressure. The EST in the test has a minimum yield pressure of 10 bar, oran internal support pressure of 10 bar without permanent deformation.


Figure 117: Increase of Castlegate TWC strength with increasing internal pressure.

Internal borehole support effect on TWCA simple empirical assessment of hole stability can be made using the thick-walled cylinderstrength. The instability of an open-hole horizontal well will occur if the effective verticalstress (or the greatest effective stress) is greater than the TWC strength (TWC). This can beexpressed as:

The horizontal open-hole will be stable if σv / TWC < 1

The TWC test with EST support exhibited a higher load capacity. Denoting this ultimate loadcapacity as TWCE, the above empirical criterion can be modified as:

The horizontal open-hole with EST will be stable if σv / TWCE < 1

Since, from the test result; TWCE = 1.7 TWC

The horizontal open-hole with EST support will be stable if σv / TWC <1.7

This empirical criterion can be used in the assessment of EST field application. It isqualitative, especially when the factor of 1.7 is based on only limited data.If the results from the BHC test and the Castlegate test are plotted in terms of internalsupport at TWC strength, a straight line relationship can be derived. Similarly, the resultsfrom the loose sand EST test can be plotted on the same graph (see figure below). It is nowpossible to infer the effects of the EST on different ‘rock strengths’.

0

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20

300 350 400 450 500 550 600 650 700

TWC (Bar)

Inte

rnal

Pre

ssur

e (B

ar)

Castlegate BHC Test Castlegate/EST Test


Figure 118: Effect of internal pressure on increase in TWC of loose sand and Castlegatesandstone

Consider an example producing formation: The overburden stress has a value of 2.2Bar/10m. The producing formation is at 3000m TVD and is hydrostatically pressured (1Bar/10m). The effective stress governing material failure is σv, ie overburden stress lesspore pressure. Initially, while the reservoir is undepleted, the maximum effective stress in thevirgin formation is likely to be:

σv = ((2000)/10)x2.2 - ((3000/10)x1

σv = 440 - 200

σv = 240 Bar

The TWC strength of the material is 300 Bar for example. The formation is drilled and isstable (σv /TWC < 1).Consider now that the formation is allowed to produce and over time the virgin reservoirpressure is halved to 100 Bar. The unsupported formation will probably fail since σv /TWC =1.13. Consider that the well had been installed with an EST after drilling with an internalsupport pressure of 10 Bar. The modified empirical criterion for an EST; σv /TWC < 1.7 andshows that the borehole would still be stable to the point of full depletion.

Benefits of borehole support in field applicationsIn field applications some variation in borehole dimension is to be expected. Boreholesupport is achieved, in the case of solid cone and roller cone expansion as a result of surplusexpansion as described above. The pre-requisites for successfully achieving this are:

0

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0 100 200 300 400 500 600 700 800

TWC (Bar)

Inte

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e (B

ar)

Castlegate BHC TestCastlegate/EST TestLoose sand/EST Test


• Borehole ID variation within the limits determined by surplus expansion• Accurate borehole geometry information to enable the optimum cone OD to be

selected.The theoretical calculation tells us that if we use a 7.325” expansion cone, variation in well-bore ID between 8.375in and 8.744in can be accommodated. Ensuring complete boreholesupport with an ESS is highly dependent on being able to maintain the drilled hole diameterwithin these constraints. Experience with early ESS installations has shown that the choiceof tool to determine borehole geometry can be of particular importance.Consider a typical four arm caliper used in dip tools. The stated accuracy for this tool istypically ±0.2in. Should the log show a minimum restriction of 8.5in in the open hole, it ispossible that the actual minimum restriction is 8.3in. In this case a 7.375in OD cone couldnot be selected despite the log showing gauge hole. A smaller cone size would be selectedto ensure that full expansion can be achieved, but the maximum hole ID limit for full supportis reduced. An accurate borehole ID can be achieved by running an ultrasonic boreholeimager such as a UBI, alternatively a caliper tool which has been proven to provide excellentresults in ESS applications, is the 6 arm Diplog (STAR or FMS) which can provide a 3-Dborehole image log.The plot below shows the actual results at one depth for a 4” ESS that has been deployed in6” open hole. Following an open-hole UBI log, a 4.625” OD cone was selected for pre-installing into the top connector. The plot below shows an internal radius of 2.36” or 4.72” ID.Surplus expansion at this depth is only 2% and hence must be constricted by the formation.In conclusion borehole support was achieved.Further to that, a UBI log taken from a North Sea well following deployment and expansion ofthe ESS indicated that in the 12.25” rat-hole below the 9.625” casing shoe where the ESSwas expanded without restriction, the internal radius was as much as 3.9” showing a surplusexpansion of:

Surplus Expansion = ((3.9 x 2) – 7.125in [cone size]) / 7.125in [cone size] = 9.5 %.

The remainder of the log in the 8.5” section showed surplus expansion between the range of3 to 5 % indicating that full borehole support was achieved.


UBI SpiralPlot ShowsOpen Slots

on High Side(Top)

UBI BoreholeRadius @4690mMD

Figure 119: UBI log data illustrating borehole support


9. Support Services

Technical & Operational SupportWeatherford has many offices and facilities located around the world. Technical, commercialand operational support is available in the first instance from the nearest location. Sandcontrol specialist personnel are located in Houston, Lafayette, Paris, Caracas, Rio deJaneiro, Aberdeen, Singapore and Kuala Lumpur. Technical and engineering support is alsoavailable from the Houston and Aberdeen. For wire-wrap based and Stratapac screens,additional offshore personnel and support are not normally required. However, personnelcan of course be provided at the well-site as the situation demands.ESS currently requires an ESS crew to supervise the installation process, though less crewis required than for gravel packing. As rig crews become more experienced with ESS in saya field development, the number of ESS personnel needed would be expected to decrease.Onshore personnel and equipment are available to assist in the completion design,programme design, well planning and sand control selection strategy.

Sand Prediction & Production Technology SupportServices are available to assist clients in making sand prediction calculations. The sand facecompletion is of course a critical component to the well and has a big impact on productivity.Computer modelling of the completion is required to ensure the well produces at an optimumrate by selecting the most appropriate equipment. Weatherford can provide such supportservices on a consultancy basis as and if required.

Engineering, Manufacturing and Service SupportConventional (eg Stratapac, Micro-Pak and Dura-Grip) screens are manufactured in fivelocations: New Iberia (Louisiana), Houston (Texas), Singapore, Batam (Indonesia) and Duri(Indonesia). ESS is manufactured in Aberdeen, UK. Manufacturing is performed toauditable and traceable standards. An Integrated Manufacturing Quality Plan (IMQP) can beprovided at time of order in which the degree of in-process inspection, material traceabilityand documentation should be specified. Weatherford Completion Systems maintains a CNCand OCTG machine shop facilities at a number of locations world-wide for the manufactureof auxiliary equipment and the replacement/refurbishment of threads, etc.Weatherford is equipped to provide many additional services and equipment options,including:

• Expandable screens and borehole liners

• Centralisation systems – bow-spring, solid rotating, roller options

• Casing and tubing running crews and equipment

• Liner hangers and EXP packers

• External casing packers

• Wash strings, seal bore subs and reaming shoes

• Thru-tubing packers and equipment

• Speciality products for zonal isolation, flow control & gravel packing.

Documents

Weatherford Sand Control Manual A4 R1