New Practices to Enhance Perforating Results/media/Files/resources/oilfield...Autumn 2006 21 high-energy jets from explosive charge detonations generate shock-wave damage and create

18 Oilfield Review

New Practices to Enhance Perforating Results

Frederic BruyereTotal E&P UK plc Aberdeen, Scotland

Dave ClarkGary Stirton CNR International Aberdeen, Scotland

Aming Kusumadjaja Balikpapan, Indonesia

Dasa Manalu Muhammad Sobirin Total E&P Indonésie Balikpapan, Indonesia

Andy Martin Aberdeen, Scotland

Derek I. Robertson BPAberdeen, Scotland

Alistair Stenhouse Consultant Aberdeen, Scotland

For help in preparation of this article, thanks to DaveAtwood, Brenden Grove, Juliane Heiland and Ian Walton,Rosharon, Texas; Larry Behrmann, Kuala Lumpur, Maylasia; Alfredo Fayard, Houston, Texas; and John Wreford, BP,Aberdeen, Scotland.

eFire, FIV (Formation Isolation Valve), HSD (High ShotDensity), IRIS (Intelligent Remote Implementation System),OCD (Orientation Confirmation Device), OrientXact,PosiTrieve, PowerJet Omega, PURE, S.A.F.E. (Slapper-Actuated Firing Equipment) and SPAN (Schlumbergerperforating analysis) are marks of Schlumberger.

Recent developments in tools and techniques dramatically increase productivity

and injectivity in perforated cased-hole well completions. These advances address

a wide range of challenges from near-wellbore formation damage and perforation

damage removal, to sand influx and safe, efficient wellsite operations.

Perforating is a critical step in establishingconnectivity between subsurface zones andwellbores that are completed with cementedsteel casing. By understanding complex inter -actions between explosive shaped charges,charge carrier systems, a wellbore and thereservoir, and by applying customized perforatingsolutions, engineers can improve cased-hole wellperformance, optimize reservoir production andmaximize hydrocarbon reserve recovery.

To achieve these objectives, engineers nowincorporate reservoir parameters and well-specific conditions into fit-for-purpose perforatingdesigns. The results are proven processes andprocedures that generate addi tional productionrevenue for operators. Recently introduced toolsand techniques help operators increaseproductivity or injectivity, prevent sandproduction and improve the safety and efficiencyof perforating operations.

Deep-penetrating charges can bypass for-mation damage, increase the effective well boreradius and reduce the need for additionalperforating operations, acid washes or otherperforation cleanup techniques. Recent advancesin explosive shaped charges, charge manufac -turing and gun systems have yielded increases in

perforation penetration of 20 to 30% evencompared with the deep-penetrating charges thatwere introduced in the late 1990s and early 2000s.Perforating farther into a formation beyond near-wellbore damage caused by drilling or completionoperations is one of the key factors in improvingthe productivity of cased-hole wells.

Surge flow through perforations after shaped-charge detonation is critical inminimizing flow impairment and reducedconductivity caused by perforating-induceddamage. Schlumberger researchers found thatlarge-diameter, deep-penetrating, clean perfora -tion tunnels could be created by controlling thetransient, or dynamic, pressure differentials thatoccur in a wellbore immediately after thedetonation of perforating guns.

An innovative design process and specializedsystems exploit rapid changes in pressure thatoccur between perforating gun systems, a wellboreand a reservoir within a few hundred millisecondsafter charge detonation. This dynamic under -balanced technique uses customized perforatingdesigns, special shaped charges and fit-for-purposegun configurations to generate a large transientunderbalance from modest static underbalancedor overbalanced pressures.

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This technique consistently generates cleanperforation tunnels and optimizes resultsobtained with the latest extradeep-penetratingcharge designs and advanced perforatingsystems. Downhole gauges with extremely fastdata-sampling rates are used to capture transient-pressure data in the field and verify the dynamicunderbalanced pressure differential. Moredetailed information is helping engineers furtherenhance perforating operations and results.

In high-angle and horizontal wells whereoverburden pressure dominates in-situ stressconditions, vertical perforations typically are themost stable. Under these conditions, perforationsoriented more than 25° away from vertical mayincrease the risk of perforation collapse and sandinflux. A new orienting system for tractor-

conveyed, coiled tubing-conveyed or tubing-conveyed perforating (TCP) can accurately andreliably align shaped charges within 10° of aspecified direction, usually vertical. This systemalso provides postjob confirmation of perforationorientation. These capabilities help completionengineers reduce the risk of sand production, evenin wellbores with extreme variations in trajectory.

Other innovations in perforating systemsincrease wellsite safety and efficiency. The mostrecent downhole firing heads combine provendrillstem test (DST) technologies with detonatorsthat are unaffected by radio frequency (RF). Thiselectronic firing system increases wellsite safetyby eliminating primary explosives, by providingdirect operator control and by allowing gundetonation to be aborted at any time.

These electronic firing heads increasewellsite efficiency and reduce the rig-operatingtime required for well-completion operations byeliminating the need for parameter-gatheringsurveys and for radio silence during perforatingoperations. New detonation-initiation systemsalso allow selective firing or activation of two gunor tool systems during a single run.

New electronic firing heads also have asufficiently high data-sampling rate to capturetransient-pressure events that cannot berecorded by standard gauges. That feature isimproving our understanding of wellbore physicsduring dynamic underbalanced perforating.

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This article presents perforating designsbased on specific reservoir properties and wellparameters, advances in orienting anddetonation initiating systems, and recentimprovements in deep-penetrating shapedcharges and gun systems. It describes dynamicunderbalanced perforating and oriented TCPoperations in the North Sea and in SoutheastAsia. We conclude by discussing researchcapabilities and laboratory facilities that areessential in the development and evaluation ofperforating techniques, systems and practices.

Maximizing Well Performance To produce oil and gas, every well with steelcasing cemented across productive subsurfaceintervals must be perforated. The perforatingprocess connects subsurface formations with awellbore, allowing hydrocarbon inflow or fluidinjection downhole (right).1

Clean perforation tunnels with minimalperforating-induced damage are essential tomaximize well performance. Unfortunately, the

20 Oilfield Review

1. Cosad C: “Choosing a Perforation Strategy,” OilfieldReview 4, no. 4 (October 1992): 54–69. Behrmann L, Brooks JE, Farrant S, Fayard A,Venkitaraman A, Brown A, Michel C, Noordermeer A,Smith P and Underdown D: “Perforating Practices That Optimize Productivity,” Oilfield Review 12, no. 1(Spring 2000): 52–74.

2. Behrmann LA, Pucknell JK, Bishop SR and Hsia T-Y:“Measurement of Additional Skin Resulting fromPerforation Damage,” paper SPE 22809, presented at theSPE Annual Technical Conference and Exhibition, Dallas,October 6–9, 1991. Pucknell JK and Behrmann LA: “An Investigation of theDamaged Zone Created by Perforating,” paper SPE 22811, presented at the SPE Annual TechnicalConference and Exhibition, Dallas, October 6–9, 1991. Swift RP, Behrmann LA, Halleck PM and Krogh KE:“Micro-Mechanical Modeling of Perforating ShockDamage,” paper SPE 39458, presented at the SPEInternational Symposium on Formation Damage Control,Lafayette, Louisiana, USA, February 18–19, 1998.

3. Chang FF, Kageson-Loe NM, Walton IC, Mathisen AMand Svanes GS: “Perforating in Overbalance—Is ItReally Sinful?,” paper SPE 82203, presented at the SPEEuropean Formation Damage Conference, The Hague,May 13–14, 2003.

4. Bell WT: “Perforating Underbalanced—EvolvingTechniques,” Journal of Petroleum Technology 36, no. 10 (October 1984): 1653–1662. King GE, Anderson A and Bingham M: “A Field Study ofUnderbalance Pressures Necessary to Obtain CleanPerforations Using Tubing-Conveyed Perforating,” paperSPE 14321, presented at the SPE Annual TechnicalConference and Exhibition, Las Vegas, Nevada, USA,September 22–25, 1985. Crawford HR: “Underbalanced Perforating Design,”paper SPE 19749, presented at the SPE Annual TechnicalConference and Exhibition, San Antonio, Texas, USA,October 8–11, 1989. Tariq SM: “New, Generalized Criteria for Determining theLevel of Underbalance for Obtaining Clean Perforations,”paper SPE 20636, presented at the SPE Annual Technical Conference and Exhibition, New Orleans,September 23–26, 1990. Hsia T-Y and Behrmann LA: “Perforating Skin as aFunction of Rock Permeability and Underbalance,” paper SPE 22810, presented at the SPE Annual TechnicalConference and Exhibition, Dallas, October 6–9, 1991.

6. Behrmann et al, reference 2.Pucknell and Behrmann, reference 2. Mason JN, Dees JM and Kessler N: “Block Tests Model the Near-Wellbore in a Perforated Sandstone,”paper SPE 28554, presented at the SPE Annual Technical Conference and Exhibition, New Orleans,September 25–28, 1994.

7. Behrmann LA, Li JL, Venkitaraman A and Li H: “BoreholeDynamics During Underbalanced Perforating,” paperSPE 38139, presented at the SPE European FormationDamage Control Conference, The Hague, June 2–3, 1997. Walton IC, Johnson AB, Behrmann LA and Atwood DC:“Laboratory Experiments Provide New Insights intoUnderbalanced Perforating,” paper SPE 71642, presentedat the SPE Annual Technical Conference and Exhibition,New Orleans, September 30–October 3, 2001. Behrmann LA, Hughes K, Johnson AB and Walton IC:“New Underbalanced Perforating Technique IncreasesCompletion Efficiency and Eliminates Costly AcidStimulation,” paper SPE 77364, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, September 29–October 2, 2002.

Behrmann LA, Pucknell JK and Bishop SR: “Effects ofUnderbalance and Effective Stress on PerforationDamage in Weak Sandstone: Initial Results,” paper SPE 24770, presented at the SPE Annual TechnicalConference and Exhibition, Washington DC, October 4–7, 1992. Bartusiak R, Behrmann LA and Halleck PM: “ExperimentalInvestigation of Surge Flow Velocity and Volume Neededto Obtain Perforation Cleanup,” paper SPE 26896,presented at the SPE Eastern Regional Conference andExhibition, Pittsburgh, Pennsylvania, USA, November 2–4,1993; also in Journal of Petroleum Science andEngineering 17, no. 2 (February 1997): 19–28.

5. Behrmann LA: “Underbalance Criteria for MinimumPerforation Damage,” paper SPE 30081, presented at the SPE European Formation Damage Conference, The Hague, May 15–16, 1995; also in SPE Drilling &Completions 11, no. 3 (September 1996): 173–177. Behrmann LA and McDonald B: “Underbalance orExtreme Overbalance,” paper SPE 31083, presented at the SPE International Symposium on FormationDamage Control, Lafayette, Louisiana, February 14–15,1996; also in SPE Production & Facilities 14, no. 3 (August 1999): 187–196.

> Shaped-charge performance. Perforating charges consist of four elements—a primer, the mainexplosive, a metal or powdered metal liner and steel case—connected to a detonating cord (top left).A conical cavity shape maximizes depth of penetration through steel casing, cement and rockformations (bottom left). As explosive shaped charges detonate, the liner collapses to form a high-pressure, high-velocity jet of fluidized particles (right).

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Autumn 2006 21

high-energy jets from explosive chargedetonations generate shock-wave damage andcreate fine particles and residual debris byfragmenting and loosening formation grains.2

In the 1960s, engineers recognized thebenefits of perforating with an initial staticunderbalance—a wellbore pressure that is lowerthan the formation pressure. With theintroduction and wider utilization of TCPsystems in the 1970s, underbalanced perforatingbecame the most accepted technique forpreventing post-perforating invasion of fluidsinto a formation, for mitigating crushed-zone damage around perforation tunnels and for removing debris from perforation cavities (below).3

In the 1980s and 1990s, perforating researchconcentrated on defining underbalanced criteriaand on predicting the pressure differentialrequired to generate clean, effective perfora tions.4

Based on experimental work at the SchlumbergerReservoir Completions (SRC) Technology Centerin Rosharon, Texas, researchers developed aminimum underbalance equation that is includedin the SPAN Schlumberger perforating analysisdesign program.5

Application of this equation led todevelopment of extreme underbalanced (EUB)perforating, a technique that applies staticpressure differentials two to four times greater

than those previously used in conventionaloperations. The EUB technique is designed togenerate surge flow from the formation and cleanout perforation tunnels.6

However, this technique has limitations andsafety concerns related to wireline operations.Under high differential pressures, unanchoredwireline-conveyed guns can move, or be blown,uphole during perforating, which can damage theelectric cable or cause toolstrings to become stuck.

In most cases, both conventional under -balanced and EUB perforating require coiledtubing and pumping operations to establish initialhydrostatic conditions by displacing fluids tounload wellbore fluids. Also required is a wirelinerun to set a mechanical tool that anchors theperforating guns and several wireline or slicklineruns to deploy and retrieve gun strings, and torecover the anchor. For long completion intervals,these combined operations may take three ormore days.

In addition, underbalanced and EUB tech -niques sometimes yield inconsistent results anddisappointing levels of productivity or injectivity,even in adjacent or similar wells. In contrast,perforating with initially balanced or evenoverbalanced pressures may yield surprisinglygood results. Until recently, only minimalresources were focused on determining why theeffectiveness of underbalanced perfor atingvaries so much, or on the degree of pressure differential that is actually achievedduring perforating.

The availability of pressure gauges withextremely fast data-sampling rates facilitatedmuch needed research in this area. These newhigh-resolution gauges can record wellborepressure variations during the first second afterperforating. In the late 1990s and early 2000s,researchers at SRC conducted single-shot testsusing high-resolution gauges.7

These studies found that for a few hundredthsof a second after shaped-charge detonation,wellbore pressure oscillates as high-velocity jetsand shock waves pass through wellbore liquids.Test results indicated that perforation cleanupdid not depend solely on initial static wellboreconditions before perforating, whether under -balanced, balanced or overbalanced.

The maximum pressure differentialgenerated in a wellbore during the first 100milliseconds (ms) after perforating directlyinfluenced variations in perforated coreproductivity during post-perforating flow tests.Higher dynamic underbalanced pressuresgenerated better flow efficiencies in perforatedcores. Subsequent laboratory evaluations

> Perforation damage and underbalanced pressure. Perforating shock waves and pressure shatter rock grains and break down cementation between grains, creating a low-permeability crushed zonearound perforation tunnels that is about 0.25 to 0.5 in. [0.6 to 1.3 cm] thick. This induced damage,debonded clays and mobilized fine particles reduce pore-throat openings and decrease in-situpermeability. Micrographs show undamaged rock (top left thin-section) versus crushed-zone damage (top right thin-section). Crushed-zone damage limits perforation productivity and injectivity; residualdebris further restricts injectivity. After overbalanced or balanced perforating and before productionflow, shattered rock and loose, high-permeability debris often plug perforation tunnels (middle leftand right). Underbalanced perforating and subsequent surge flow from the reservoir erodes thecrushed zone and removes residual debris from perforation tunnels. However, conventional operationsmay require extremely high static differential pressures to effective clean out perforation tunnels(bottom left and right).

Undamaged Rock

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Casing Undamaged formation 3,000-psi Underbalanced Perforating

Cement

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Formationdamage

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confirmed that removal of perforating damageand perforation cleanup were directly related tothe maximum dynamic underbalance and thetiming of surge flow (above).

Collectively, these results formed the basisfor dynamic underbalanced perforating, a newapproach to perforation cleanup.8 This PUREperforating system for clean perforationsspecifies unique wellbore conditions and gunconfigurations to generate an instantaneous drop

in pressure around the perforating guns duringshaped-charge detonation.9

Dynamic underbalanced perforating can beperformed independent of initial wellboreconditions to create the drop in pressure andrapid surge flow required to generate high shearstress around perforation tunnels immediatelyafter charge detonation. Shear failure of thecrushed zone caused by dramatic reduction in

wellbore pressure rather than tunnel erosion dueto fluid influx from the formation appears to play an important, perhaps vital, role inperforation cleanup.10

For PURE wireline perforating applicationswith expendable capsule, or strip guns, thewellbore must be close to balanced or slightlyunderbalanced conditions, so that there is somepositive flow from the formation after chargedetonation. During wireline operations with

22 Oilfield Review

> Static versus dynamic underbalance. About 95% of conventional perforating jobs do not achieve adequate underbalanced pressure or surge flow afterperforation tunnels are created. The actual static underbalanced perforating pressure often is not great enough and is not applied rapidly enough to cleanout perforation tunnels; the wellbore also may quickly return to a balanced or overbalanced condition (top left). Dynamic underbalanced perforatinggenerates a rapid drop in pressure around the guns (top right). Micrographs show a thin cross section of visible damage after a static underbalancedperforating test (bottom left). In Zone 1, which is about 3 mm thick, formation grains and grain boundaries have been crushed or fractured (red line). InZone 2, which is about 7 mm thick, perforating-induced damage is less extensive, confined mostly to fractured individual grains. Porosity and permeabilityare essentially intact (green line). In Zone 3, beyond the green line, perforating-induced damage is negligible; only a few fractured grains are visible. Thisis essentially unaltered rock. During PURE perforating tests, rapid application of large underbalanced pressure differentials and instantaneous surge flowremoves all of the crushed-zone damage and most of the damage in Zone 2 (green line); a narrow 2.5-mm band of slightly fractured grains is visible(bottom right). The PURE technique creates large-diameter tunnels with minimal permeability damage, which correlates with an essentially undamaged,or unaltered, rock matrix and extremely good core-flow efficiency.

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Autumn 2006 23

hollow-steel carrier guns and in TCP appli -cations, the initial hydrostatic pressure can beunderbalanced or overbalanced. Establishing aclosed-chamber TCP system with a retrievabledownhole packer can quickly generate a dynamicunderbalance and also prevent the wellbore fromreturning to an overbalanced condition whenwell and pore pressures equalize.

Wells perforated using the PURE processhave performed significantly better thancomparable wells perforated under a staticunderbalance. In the USA, dynamic under -balanced perforating eliminated the need forsecondary perforation cleanup with acid in somelow-permeability, or tight, reservoirs.11 Offshoreoil wells in Norway perforated using the PUREtechnique had an average productivity index (PI)three to six times greater than other wells in thesame fields that were perforated under an initialstatic underbalance.

Engineers specifically design PURE gunstrings, the type and number of shaped charges,and the initial wellbore hydrostatic conditions to

control the magnitude of dynamic under -balanced pressure and the rate of surge flowfrom the formation. A nearly instantaneousunderbalance and influx are created around theguns when high-pressure wellbore fluids rapidlyfill spent charge carriers immediately afterperforating (below).

Shaped charges that do not penetrate thewellbore casing can be interspersed along a gunstring. These PURE charges open additionalholes in conventional charge carriers or in PUREchambers to control the underbalanced pressuredifferential and the rate of influx through thenewly created perforations.

Compared with conventional underbalancedperforating, dynamic underbalanced designsincrease well productivity and injectivity, andimprove operational efficiency. Operators haveapplied PURE perforating designs andtechniques to complete or recomplete more than500 wells worldwide, including extensive use inIndonesia and the North Sea.

Enhancing Production in a Mature FieldDuring 2003, CNR International applied PUREperforating in five mature Ninian field wells.12

Located northeast of the Shetland Islands in theUK sector of the North Sea, this mature field hasproduced oil since 1978. The reservoir is about22,000 ft [6,706 m] deep with multiple pay zonesacross long gross intervals. CNR completed thesewells with cemented steel liners.

A typical completion operation established ahydrostatic pressure in the wellbore that waslower than the reservoir pressure, detonated theTCP system under this static underbalance andthen flowed the wells to clean up beforeretrieving the spent guns. However, establishingan optimal static underbalance beforeperforating was difficult and often involvedchanging out, or displacing, wellbore fluids,which was time-consuming, expensive and, insome cases, impractical. Perforating would bemore efficient and less expensive if this stepcould be avoided.

8. Dynamic underbalanced perforating is a Schlumbergerpatented process marketed under the Schlumbergermark PURE perforating system for clean perforations.

9. Johnson AB, Brooks JE, Behrmann LA, Venkitaraman A,Walton I, Vovers AP, Vaynshteyn V, Patel DR and Fruge MW: “Reservoir Communication with a Wellbore,”U.S. Patent No. 6,598,682 (July 29, 2003); also InternationalPublication No. WO 01/65060 (September 7, 2001). Brooks JE, Yang W, Grove BM, Walton IC andBehrmann LA: “Components and Methods for Use With Explosives,” U.S. Patent Application Publication No. 2003/0150646 (August 14, 2003). Johnson AB, Behrmann LA, Yang W and Cornelis FH:“Controlling Transient Underbalance in a Wellbore,” U.S. Patent Application Publication No. 2003/0089498(May 15, 2003). Bakker E, Veeken K, Behrmann L, Milton P, Stirton G,Salsman A, Walton I, Stutz L and Underdown D: “TheNew Dynamics of Underbalanced Perforating,” OilfieldReview 15, no. 4 (Winter 2003/2004): 54–67.

10. Walton IC: “Optimum Underbalance for the Removal ofPerforation Damage,” paper SPE 63108, presented at theSPE Annual Technical Conference and Exhibition, Dallas,October 1–4, 2000. Subiaur ST, Graham CA and Walton IC: “UnderbalancedPressure Criteria for Perforating Carbonates,” paper SPE 86542, presented at the SPE InternationalSymposium and Exhibition on Formation DamageControl, Lafayette, Louisiana, February 18–20, 2004.

11. Stutz HL and Behrmann LA: “Dynamic UnderbalancedPerforating Eliminates Near Wellbore Acid Stimulation in Low-Pressure Weber Formation,” paper SPE 86543,presented at the SPE International Symposium andExhibition on Formation Damage Control, Lafayette,Louisiana, February 18–20, 2004.

12. Martin AJ, Clark D and Stirton G: “DynamicUnderbalanced Perforating on a Mature North SeaField,” paper SPE 93638, presented at the SPE EuropeanFormation Damage Conference, Scheveningen,The Netherlands, May 25–27, 2005.

> Optimal dynamic underbalance and perforation damage removal. In addition to conventional shapedcharges (blue), PURE perforating systems can include special PURE chambers and PURE charges(yellow) interspersed along a gun string (far left). The PURE charges do not penetrate the wellborecasing, but instead open extra holes in conventional charge carriers or additional PURE chambers tomaximize the transient pressure differential and optimize perforation cleanout (center left and centerright). Immediately after charge detonation, high-velocity perforating jets generate perforation tunnelsin the formation (0 to 100 µs). PURE designs manipulate wellbore conditions and gun parameters toinstantaneously create an optimal underbalance across a perforated interval (100 to 200 ms). Tensilefailure of the crushed zone around perforation tunnels and surge flow from the formation removeinduced damage and residual debris (300 to 400 ms). Rapid application of a high differential pressure isthe key to PURE perforating. Perforated laboratory cores examined under hydraulic stress with a colorvideo probe show a perforation filled with pulverized formation material and surrounded byfragmented quartz grains (top right); a perforation without fragmentation, but with pulverized materialremaining along the bottom of the tunnel (middle right); and a clean tunnel (bottom right).

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In Well 1, CNR needed to complete severalzones across a 2,200-ft [671-m] interval.Engineers decided to perforate using the PUREprocess with TCP guns and a DST string to createa closed-chamber system. The SPAN programindicated that perforating with a dynamicunderbalance could improve well productivity by15% or more compared with conventional staticoverbalanced or underbalanced operations.

The key was to generate a rapid dynamicunderbalance across the entire completioninterval. The primary gun design called for a 33⁄8-in. TCP system to perforate 992 ft [302 m] ofnet pay. As a contingency, engineers prepared adesign for 27⁄8-in. guns in case a smaller liner hadto be run.

CNR and Schlumberger estimated the forma -tion pressure to be 4,500 psi [31 MPa]. For thisdesign, a hydrostatic wellbore pressure of8,000 psi [55.2 MPa] was required to generate adynamic underbalance of 3,500 psi [24.1 MPa].This required the wellbore to be pressured upbefore perforating. CNR used a DST tool anddownhole packer with the test valve closed to forma sealed chamber before perforating and toquickly create a dynamic underbalance (left).

This DST configuration ensured that thewellbore would not return to an overbalancedcondition after gun detonation. CNR perforatedthe first well in August 2003. A conventional TCPfiring head with a delay provided enough time to

24 Oilfield Review

> Shoot-and-pull operations in the North Sea. A static overbalance of 3,500 psi was required to generate an effective dynamic underbalanceduring tubing-conveyed perforating in Ninianfield wells. An estimated reservoir pressure of4,500 psi required that CNR increase the initialhydrostatic pressure in these wellbores to 8,000 psi before perforating. To create a closedchamber, CNR used a DST configuration. Fromthe bottom up, this assembly included a pressuregauge and carrier, 33⁄8-in. tubing-conveyed guns, a hydraulic delay firing head (HDF), a retrievablePosiTrieve downhole packer with hold-downsection, and an electronic Intelligent RemoteDual-Valve (IRDV) tool that could be closedbefore the guns fired and reopened afterward by low-pressure pulses from surface.

Intelligent Remote DualValve (IRDV)

Jar

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> Ninian field Well 1 perforating. A standard downhole gauge recordedwellbore pressure during the perforating operation on Well 1. The 5-seconddata-sampling rate was too slow to capture the actual magnitude of thedynamic, or transient, underbalanced pressure. However, these data showthe operational sequence, including deploying guns; setting the DST packer;triggering the time-delay firing head; closing the test valve; detonating thecharges; creating a dynamic underbalance; building back up to formationpressure; and controlling, or killing, the well (top). A closer evaluation of thedata indicates that the wellbore hydrostatic pressure dropped dramaticallyfrom 8,000 psi to less than 5,100 psi [35.2 MPa] after the guns fired. Thepressure then built up quickly and equalized with formation pressure(bottom). The wellbore pressure did not return to an overbalanced condition.

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close the test valve before the guns fired.Schlumberger used a standard downhole gaugewith a 5-second data-sampling rate to recordwellbore pressure (previous page, right).

At this sampling rate, the gauge could notcapture detailed transient-pressure responsesduring PURE perforating. However, the low-resolution data indicated that the wellborepressure dropped dramatically from 8,000 psiwhen the guns detonated and then built back upquickly to equalize with the reservoir pressure,an indication that this operation achieved aneffective dynamic underbalance.

Surface gauges registered a steady pressureof 850 psi [5.9 MPa] after the test valve wasreopened. The initial pressure drop, rapidpressure buildup and high surface pressureindicated that the perforations cleaned upquickly and had little or no damage. Afterobserving the well for several hours, CNRreleased the DST packer, circulated anondamaging fluid to control the well andretrieved the TCP guns.

CNR installed permanent completionequipment and produced the well at an initial oilrate of 9,500 bbl/d [1,510 m3/d], significantlyhigher than the rate projected for conventionalunderbalanced perforating. The recovered gunscontained large volumes of shot debris andformation sand, indicating a rapid and effectivedynamic underbalance that caused the spentcharge carriers to fill quickly, which helpedretain shrapnel and other residual perforatingdebris inside the guns.

CNR perforated Well 2 using a similar PUREprocess and system. This design called for 31⁄2-in.guns loaded to perforate three zones comprisingabout 910 ft [277 m] of pay across a 1,600-ft [488-m] gross interval. This well was drilled as aninjector, but produced oil for a short time beforeconversion to water injection. The initial surfacepressure after perforating indicated a reservoirpressure of more than 6,100 psi [42.1 MPa],significantly greater than in the first well (above right).

CNR also perforated Well 3 and Well 4 using aclosed-chamber DST string to trap high pressurearound the guns before perforating. Standardgauge data from these two wells indicated that adynamic underbalance was achieved (right).

Perforating operations on Well 5 followed thesame procedures as on the previous four wells.This time, however, CNR used a PURE design anda DST system that included an eFire electronicfiring head system (see “Improving OperationalEfficiency and Safety,” next page).

> Ninian field Well 2 perforating. An evaluation of standard pressure-gaugedata from Ninian field Well 2 perforating also indicated a dramatic pressuredrop from 8,000 psi to less than 5,320 psi [36.7 MPa] followed by a rapidequalization with the reservoir pressure. The wellbore pressure did not returnto overbalanced conditions.

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> Ninian field Well 3 and Well 4 perforating. Standard pressure-gauge datafrom Well 3 indicated that pressure dropped from 6,340 psi [43.7 MPa] to lessthan 5,900 psi [40.7 MPa] and then built up to the formation pressure (top).The pressure in Well 4 dropped from 7,700 psi [53.1 MPa] to 4,800 psi[33.1 MPa] and then built back up to formation pressure (bottom). Thewellbore pressure did not return to overbalanced conditions in either well.

Guns fire

Guns fire



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, psi




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(continued on page 28)


1. Huber KB and Pease JM: “S.A.F.E. PerforatingUnaffected by Radio and Electric Power,” paper SPE 20635, presented at the SPE Annual TechnicalConference and Exhibition, New Orleans,September 23–26, 1990.

2. Healy JC, Maratier JP and Fruge MW: “Testing GreenCanyon Wells with a Pressure-Pulse-Controlled DSTSystem,” paper SPE 22720, presented at the SPEAnnual Technical Conference and Exhibition, Dallas,October 6–9, 1991.

3. Taylor N, Guevara J and Sabine C: “A New ElectronicFiring Head for Slickline Explosive Services,” paperSPE 72325, presented at the IADC/SPE Middle EastDrilling Technology Conference, Bahrain, October22–24, 2001. King J, Beagrie B and Billingham M: “An ImprovedMethod of Slickline Perforating,” paper SPE 81536,presented at the SPE 13th Middle East Oil Show &Conference, Bahrain, June 9–12, 2003.

26 Oilfield Review

Explosive services and other downholeoperations now can be initiated safely andefficiently using advanced electronic firingheads. Schlumberger uses eFire electronicfiring head systems to detonate perforatingguns, to set packers or bridge plugs and toactivate chemical cutters, mechanical tubingpunchers or other downhole tools, such astesting or sampling devices.

These programmable systems combine twoproven technologies: the IRIS IntelligentRemote Implementation System and S.A.F.E.Slapper-Actuated Firing Equipment for initi -ating wireline perforating.1 Dual-valvedrillstem testing (DST) tools use the IRISintelligent controller to open and close a test,or flow, valve and a circulating valve. Bothtechnologies have been used extensively inharsh well conditions since the early 1990s.2

The operation of eFire systems is similar tothat of a DST tool. A computer in the IRIScontroller detects a unique sequence ofsignals from the surface. These coded pulsesare recognized and interpreted as commandsto initiate downhole operations through aS.A.F.E. system for wireline perforating.

The S.A.F.E. system uses an exploding foilinitiator (EFI), which is reliable and fail-safe,to initiate a detonation chain. Developed formilitary use, EFI technology eliminated theneed for primary high explosives in detonators.An EFI initiator is immune to radiofrequencies and stray electrical voltage fromwelding operations, corrosion protectionsystems, electrical transmission lines andradio communications at a wellsite.

Previous firing heads required extra equip -ment and supporting wellsite services andoperations, such as nitrogen tanks andpumping equipment. These systems operatedautomatically under preset pressure or temper -ature parameters, which often required thatoperators perform an initial parameter surveyto define the existing wellbore conditions.

Conventional firing heads rely on wellcondi tions remaining static during alloperations and must be removed frompredefined operating depths or pressureconditions to abort detona tion. Alternatively,these systems might fail to detonate becauseof changing downhole conditions, which wouldrequire a new param eter survey. Of greaterconcern, downhole sampling or testing toolsand perforating guns with conventional firingheads also might activate at the wrong depthin a wellbore or detonate prematurely.

An eFire system overcomes these disadvan -tages and limitations by providing totalcontrol of an operation from the surface.Wellsite personnel can arm, fire or abortoperations at any time, eliminating the needfor a parameter survey and allowing moreperforating runs to be made in a day.

The firing head detects changes in pressureor flow through the tubing or tubing-casingannulus, changes in slickline cable tension or changes in wireline electrical current. By using different sensors, Schlumbergerdeveloped a family of electronic firing headsfor tubing-, coiled tubing-, slickline- andwireline-conveyed operations (next page).3

Command signals require relatively lowpressure, flow, tension or currentdifferentials. This feature reduces the needfor supporting surface equipment or pumps,and sources of nitrogen or other gases. Rapidcommand-execution times and the real-timecapability to abort detonation provide morereliable control of explosive operations, whichnow can be performed safely even in low-pressure wells.

The IRIS controller recognizes a distinctivesequence of changes as coded signals thatform a unique command structure for theIRIS controller. These specialized commandsensure that eFire systems are insensitive tounrelated surface or subsurface wellsiteoperations, such as jarring or movingequipment, and random pressure variations in the subject well or adjacent wellbores.

To enhance safety, two separate processorsindependently verify each command. Tooloperators perform a setup and function testusing a laptop computer before the eFire headis connected to the EFI or any explosive devices.

In addition to a fail-safe command structure,the eFire heads have to be enabled by a presethydrostatic pressure followed by an armingcommand from the surface before the systemwill accept a detonation command. The eFirehead then converts battery power to a highervoltage that activates the EFI initiator.

During 2002, BP identified a number ofwells in the southern North Sea that could bereperforated or recompleted. This areaincluded 39 platforms, primarily unmannedstructures, with minimal surfaceinfrastructure and facilities. Many of thesewells were perforated more than 30 years agoat only 1 shot per foot (spf). Engineersdetermined that adding perforations couldsignificantly improve well productivity.

Slickline perforating, which requires fewerpersonnel and simplifies both pressure-control and equipment requirements, was themost cost-effective method for performingthese remedial well interventions. A singleunit and one crew can perform all of therequired work, reducing the number of cranelifts and associated risk to personnel.

Improving Operational Efficiency and Safety

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Autumn 2006 27

BP engineers decided to use the eFire-slickline head for perforating after success-fully performing several other operations withthe system, including setting a packer andpunching holes above a plug stuck inside atubing string. This approach improved

operational efficiency and significantly reducedcosts by eliminating the time-consuming andcostly prejob surveys, or parameter runs, thatare required with conventional time-delayfiring heads.

BP correlated perforation depths by marking,or flagging, the slickline cable during aninitial logging trip. A memory gamma ray andcasing-collar-locator tool was run in an emptycarrier that replicated the length and weightof the actual gun system. The loaded gunsthen were run to the flagged depth andarmed. The depth counter was reset based onthe correlation log, the firing command wasinitiated, and the guns were positioned at thetarget perforating depth before detonation. A disarm command was sent before retrievingthe guns.

Initially, BP verified slickline depth correla -tions by running a 40-arm caliper acrossnewly perforated or reperforated intervals.These surveys indicated that slickline-conveyed guns could be detonated on depthwith real-time control. BP realized significanttime and cost savings by using the eFireslickline system, completing as many as three perforating runs to depths greater than10,000 ft [3,048 m] in less than 12 hours using40-ft [12-m] gun strings.

Compared with wireline operations, eFiretechnology for slickline perforating proved tobe extremely efficient and led to a cost savingof more than 15% in the southern North Seabusiness unit of BP. The resulting productionincrease of about 10% represented a signif-icant achievement in this mature area wherefields have produced for more than 35 years.

The eFire technology provides a stepchange in well interventions. For example,slickline perforating saves about one day perwell during abandonment operations. Morethan 500 perfor ating operations have beenperformed with eFire slickline systems in theNorth Sea since this technology was intro-duced in 2001. In addition, about 50% of TCPoperations in the UK sector now use the eFiresystem, primarily because of improvedwellsite efficiency and safety.

> Electronic firing systems. With their simple command structure, eFire systemsare compatible with various systems and conveyance methods, includingpermanent completion perforating (PCP), conventional tubing or drillpipe,coiled tubing, slickline and wireline. The tubing-conveyed perforating headmeasures pressure (left). The coiled tubing head measures pressure changesassociated with variations in flow rate (middle left). The slickline systemincludes a strain sensor, or tension converter, that translates vertical cablemovement into pressure pulses that are recognized by the IRIS controller(middle right). The wireline head detects changes in electrical current (right).The latest eFire systems also have the capability to capture pressure data at a high sampling rate during PURE perforating operations.

Tubing-conveyed Slickline-conveyed

Wireline-conveyed

Coiled tubing-conveyed

Controllermodule

EFI detonator

Initiatormodule

Battery

Pressuretransducer

Differential-pressuresensor Pressure

transducer

Currentsensor

59100schD05R1.qxp:59100schD05R1 11/21/06 6:34 PM Page 27

New versions of the eFire head recordwellbore pressure at a 1-kHz rate and cancapture transient-pressure events during thefirst few milliseconds after shaped-chargedetonation. After perforating, fast-gauge data

indicated that a dynamic underbalance of morethan 5,000 psi occurred within 100 ms of gundetonation (above).

Fast-gauge data confirmed the magnitudeand timing of the dynamic underbalance.

Standard gauges, such as those used on previouswells, could not record pressure data fast enoughto fully evaluate PURE operations. Achieving rapid, high-pressure underbalanced conditionsand nearly instantaneous surge flow ensured the removal of perforating damage and debris.Clean perforation tunnels are essential formaximizing oil production and water injection,and also for optimizing gas-well productivity.

Optimizing Gas-Condensate Output As in the North Sea, dynamic underbalancedperforating also has achieved significant successin Southeast Asia where Total E&P Indonésieoperates the Tunu field. Located on the easternside of Mahakam delta in East Kalimantan,Indonesia, the reservoirs of this field compriseinterbedded sandstones from 2,300 to 4,500 m[7,546 to 14,764 ft] deep. Since 1990, more than370 wells have been drilled in the Tunu field(next page, top left).

In 1999, Total began completing these gas-condensate wells using extreme underbalanced(EUB) perforating.13 This approach requiredpumping operations to unload wellbore fluidsand a wireline run to set an anchor, along withmultiple slickline runs to position and recoverstacked guns, and to retrieve the anchor.

In November 2004, Schlumberger recom -mended dynamic underbalanced perfor atingusing wireline-conveyed guns for two new wellswith 41⁄2-in. casing. Engineers used SPAN softwareto account for wellbore geometry, fluid density,gun configuration, shaped-charge performanceand reservoir properties.

Based on this analysis, Schlumbergerdeveloped a 27⁄8-in. system with deep-penetratingshaped charges specifically for this application.Engineers designed the gun string so that thetotal number of holes in these chambers wouldgenerate the required underbalance to facilitateperforation cleanup. Total began performingthese wireline jobs in November 2004.14

Pressure gauges with a high data-samplingrate that could record pressure events during thefirst second after detonation were not availablefor these jobs. However, wellhead pressures inboth wells increased immediately afterperforating as wellbore fluids began to flow backand unload. Within 30 minutes, gas began flowingto the surface.

Well 1 and Well 2 produced gas at 15 MMcf/d[424,753 m3/d] and 27 MMcf/d [764,555 m3/d],respectively, with a flowing wellhead pressure of435 psi [3 MPa]. Pressure buildup data acquiredwith a downhole gauge indicated skin values of

28 Oilfield Review

13. Behrmann et al, 2002, reference 7.14. Minto D, Falxa P, Manalu D, Simatupang M, Behrmann LA,

Kusumadjaja A: “Dynamic Underbalanced PerforatingSystem Increases Productivity and Reduces Cost in EastKalimantan Gas Field: A Case Study,” paper SPE 97363,presented at the SPE/IADC Middle East DrillingTechnology Conference and Exhibition, Dubai, UAE,September 12–14, 2005.

15. Almaguer J, Manrique J, Wickramasuriya S, Habbtar A,López-de-Cárdenas J, May D, McNally AC andSulbarán A: “Orienting Perforations in the RightDirection,” Oilfield Review 14, no. 1 (Spring 2002): 16–31.

16. Sulbarán AL, Carbonell RS and López-de-Cárdenas JE:“Oriented Perforating for Sand Prevention,” paper SPE 57954, presented at the SPE European FormationDamage Conference, The Hague, May 31–June 1, 1999.

> Ninian field Well 5 perforating. Pressure data show the sequence of eventsduring perforating operations on Ninian field Well 5, including low-pressurepulses to activate the eFire firing head, closing the test valve to trap pressure,the time delay before guns fire, gun detonation, achieving a dynamicunderbalance, a pressure buildup after the dynamic underbalance, openingthe test valve and a buildup back to the formation pressure (top). High-resolution data recorded by the eFire system at a 1-kHz sampling rate indicatethat pressure dropped from 7,000 psi [48.2 MPa] to less than 2,100 psi [14.5 MPa]in 100 ms (bottom). The actual magnitude of dynamic underbalance was notcaptured by standard gauges used on the previous four wells. The eFire dataprovided CNR International with conclusive evidence of the extremeunderbalanced pressures that can be achieved during PURE dynamicunderbalanced perforating.

Fire guns

Trap pressure below tester valve

Open tester valve

eFire command

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Pore pressure, psieFire data at 200 ms, psi

Pore pressure, psieFire data at 1 ms, psi


Autumn 2006 29

1.1 for Well 1 and zero for Well 2. Well 3 produced29 MMcf/d [821,189 m3/d] with a skin value ofzero. Well 4 produced gas at 35 MMcf/d[991,090 m3/d] with a skin value of negative 2.25.

The average skin value for 35 wells perforatedfrom 2000 to 2004 using the conventional staticEUB technique was 4.73. Reliable pressure buildupdata from the first four Tunu field wells perforatedwith the PURE technique had an average skinvalue of negative 0.29. These low skin valuesyielded a cumulative increase in gas production ofmore than 200% in those four completions.

The jobs were performed efficiently withsignificant cost savings compared to conven -tional EUB operations, yielding increased wellproductivity. Perforating and cleanup operationsfor each well required only one day to complete.Total perforated six more wells during this initialphase of PURE perforating, which resulted in acumulative cost-saving of about 43% comparedwith previous EUB operations (above right).

Since 2004, more than 40 wells have beenperforated in the Tunu, Tambora and Pecikofields using dynamic underbalanced designs thatachieved an average productivity gain exceeding150%. To date, there is no evidence of sand influxin these wells after perforating with the PUREtechnique. In some applications, accuratelyoriented perforation also may be required toprevent the production of sand.

Oriented TCP Operations Oil and gas operators recognize that orientedperforating is an effective technique formitigating sand production.15 In high-angle andhorizontal completions and under normal in-situstress conditions—the maximum stressdirection is vertical—shooting along the highside of a wellbore improves the stability ofperforation tunnels in a formation. Thistechnique also prevents debris from blockingperforations on the low side of the hole.

However, completing extended intervals ininclined wellbores often requires tubing-conveyed systems with hundreds of gun sectionsthat must remain closely aligned to maintain anear-vertical perforation orientation. Largecompressional loads generate slight clockwiserotation and gradual misalignment at each gunsection that accumulates over long strings withconventional connections.

Using caliper logs, North Sea operators foundthat past attempts at oriented perforatingresulted in perforations that were misaligned byas much as 45° from the desired verticalorientation. In many of these completions,perforations aligned more than 25° from verticalin weakly consolidated formations have anincreased risk of collapsing and producing sand.16

Alignment errors were greatest in wellboreswith significant variations, or doglegs, in thewellbore path. Operators needed a TCP orientingsystem that would maintain a vertical chargealignment independent of changes in well

> Tunu field, Indonesia. The Mahakam production-sharing contract, operated by Inpex and Total E&PIndonésie, includes the Bekapai and Handil oil fields and the Peciko, Sisi–Nubi, South Mahakam andTunu gas-condensate fields. Tunu field, located along the eastern limit of the Mahakam delta, is themajor supplier of gas to the East Kalimantan pipeline system. Operators have drilled more than370 wells since the field began producing in 1990.

Tunufield

K A L I M A N T A N

A S I A

I N D O N E S I A

> Reducing operational costs. By using PUREdynamic underbalanced perforating on morethan 40 wells, Total reduced costs by more than40% compared with previous EUB operations.Operational expenses include perforating servicesand coiled tubing operations to displace, or unload,wellbore liquids and to clean out wellbores.

-43%

Dynamic underbalanceStatic underbalance

Coiled tubing costsPerforating costs


trajectory. In response to a request from Hydro,Schlumberger designed, tested and deployed anew TCP orienting system for Norwegian North Seawells within a five-month period (above).17

In addition to weighted spacers for passiveorientation, this OrientXact tubing-conveyedoriented perforating system combined innovativealigning-locking adapters with special high-load, low-friction roller-bearing swivels that reduced average misalignments between guns toabout 0.17°.

The OrientXact aligning-locking adapters aremanufactured to extremely tight tolerances toeliminate rotational misalignment caused byclearances and spaces between tool parts andcomponents. The new swivel design provided lowtorsional friction under compressive or tensile

loads up to 55,000 lbf [244,652 N] withsimultaneous bending of 10° per 100 ft [30.5 m].

Bending also generated torsional forces thatrotated the gun string away from a verticalorientation. Charge carriers and weightedspacers were manufactured to bend uniformly,independent of changes in wellbore trajectory.This design eliminated the tendency for guncomponents to rotate the shaped charges awayfrom vertical when the string is bent.

After gun detonation, the weighted spacerskeep spent charges and carrier exit holes pointedtoward the high side of a wellbore, minimizingthe chance that debris inside a carrier will fallout during retrieval of spent guns.

The OrientXact system includes an OCDOrientation Confirmation Device that recordsperforation orientation during charge detonationto the nearest 0.5°. Two OCD units per gunsection confirm perforation directions after theguns are fired and retrieved. A pendulumassembly inside the OCD unit contains a free-rotating collar, a detonation-cord port, a bullettube, a bullet and an angular scale.

Explosive energy from the detonation cordforces the bullet through the barrel tube towardthe inside wall of each OCD unit, where itrecords gun alignment relative to a vertical, or0°, orientation on the scale. Operators read thesescales after spent guns are retrieved todetermine the perforating orientation for thatsection of guns.

30 Oilfield Review

> Tubing-conveyed oriented perforating. The OrientXact system aligns TCP guns with charges at 10° and 350° phasing angles to shoot in the verticaldirection (top right). Aligning and locking adapters with interference-fit notches and keys that are manufactured to extremely close tolerances connecteach gun section to eliminate the rotational and cumulative misalignments inherent in conventional threaded connections (top left). Low-friction swivelswith a high-load capacity in both tension and compression support individual sections of guns and passive orienting weights (bottom left). An OCDOrientation Conformation Device at the end of each swivel-supported section verifies perforation orientation to within 0.5° (bottom right).

Drillpipe(closed string) Spacer Bottom nose7-in. linerOrienting weight

Dual hydraulic-delayfiring head (HDF)

Pressure gauges

Sealed ballistictransfer

4.5-in. oriented perforating gun,low debris, 4 shots per foot, 10° and 350°

Low-friction swivel

OCD Orientation Confirmation Device

Aligning-locking adapter


Autumn 2006 31

The OrientXact system uses guns with 4 to6 shots per foot (spf) and a 20° phasing anglebetween charges that shoot on either side ofvertical to maximize perforation density, spacingand stability. OrientXact systems have been usedto perforate within 10° of vertical regardless ofwell trajectory, even with more than 1,600 ft[488 m] between OrientXact swivels.18

This advanced orientation system consistentlyperforates along the high side or the low side ofinclined wells to prevent sand production. Whenthe angle between a wellbore and the maximumstress direction, typically vertical, is greater than75°, this oriented perforating technique helpsprevent sand production.

To date, Hydro and Statoil have used thissystem to perforate more than 50 wells in theNorwegian sector of the North Sea. TheOrientXact system is available for 27⁄8-in., 33⁄8-in.and 41⁄2-in. TCP guns. A 27⁄8-in. OrientXact systemwas used by BP to perforate Andrew field wells inthe North Sea UK sector.

Preventing Sand Production BP began developing the North Sea Andrew fieldin 1996. Water production from some wells in thisUK sector reservoir began to increase during 1998,and the field declined from peak output in 2000.BP first detected sand in two wells during 2001,three years after water breakthrough. Horizontalwells in the Andrew field were completed withcemented liners to facilitate future interventions,recompletions and water control.

Sand production appeared to be related topressure depletion and perforation stability. BPperforated these wells using TCP systems and

underbalanced perforating operations, whichminimized perforation damage and routinelyresulted in production from more than 90% of thehorizontal section. The production tubingincluded an FIV Formation Isolation Valve below apermanent packer to provide well control duringgun deployment and after perforating (above).19

Initially, BP used 33⁄8-in. TCP guns with 4 spfat 60° phasing angles to perforate consolidatedsands. A higher perforation density reduced therate of flow through each hole and decreased thechance of flow-induced formation failure andsand influx.

In an attempt to minimize sand production,BP perforated some of the less consolidatedintervals with oriented TCP guns usingconventional passive weights and swivels withcharges at phasing angles of 25° and 335°.However, orientation accuracy was uncertain,and actual perforation alignment on either sideof vertical could not be verified.

Schlumberger sand-prediction modelsindicated that at about 32° from vertical,perforations could collapse and initiate sandinflux.20 The onset of sand production fromAndrew field wells could have resulted frommisaligned perforations at 60° phasing angles or

17. Benavides SP, Myers WD, Van Sickle EW andVargervik K: “Advances in Horizontal OrientedPerforating,” paper SPE 81051, presented at the SPELatin American and Caribbean Petroleum EngineeringConference, Port-of-Spain, Trinidad and Tobago,West Indies, April 27–30, 2003.Stenhaug M, Erichsen L, Doornbosch FHC andParrott RA: “A Step Change in Perforating TechnologyImproves Productivity of Horizontal Wells in the NorthSea,” paper SPE 84910, presented at the SPEInternational Improved Oil Recovery Conference in AsiaPacific, Kuala Lumpur, October 20–21, 2003. Bersås K, Stenhaug M, Doornbosch F, Langseth B,Fimreite H and Parrott B: “Perforations on Target,”Oilfield Review 16, no. 1 (Spring 2004): 28–37.

18. Bersås et al, reference 17. 19. Kusaka K, Patel D, Gomersall S, Mason J and Doughty P:

“Underbalance Perforation in Long Horizontal Wells inthe Andrew Field,” paper OTC 8532, presented at theOffshore Technology Conference, Houston, May 5–8, 1997. Mason J and Gomersall S: “Andrew/Cyrus HorizontalWell Completions,” paper SPE 38183, presented at theEuropean Formation Damage Conference, The Hague,June 2–3, 1997.

20. Venkitaraman A, Li H, Leonard AJ and Bowden PR:“Experimental Investigation of Sanding Propensity forthe Andrew Completion,” paper SPE 50387, presented atthe SPE International Conference on Horizontal WellTechnology, Calgary, November 1–4, 1998.

> Perforating procedures in the North Sea. BP typically perforates horizontalcompletions in the Andrew field without controlling, or killing, the well. Thisapproach maximizes production and is more cost-effective than othertechniques. BP installs a FIV Formation Isolation Valve in the productiontubing below a permanent packer to serve as a downhole lubricator duringgun deployment and to provide well control after perforating. After installingdownhole completion equipment, permanent production tubing and thesurface wellhead, BP deploys TCP guns with a hydraulic snubbing unit andperforates wells in underbalanced conditions. A shifting tool on the end of theTCP string closes the FIV tool as the spent guns are retrieved. An inflow testconfirms that the valve has closed and that the spent guns can be removedfrom the well safely. Pressure pulses applied from the surface reopen the FIVtool to initiate production without performing a rig-assisted intervention.

Engage shifting tool Close valve, remove guns Reopen valve with pressurepulses applied from surface

Workstring

Production tubing

Production casing

Permanent packer

FIV Formation Isolation Valve

Cemented liner

Guns

Shifting tool


from inability to consistently achieve near-vertical perforation orientations using 25° and335° phasing angles with a conventional TCPorienting system (above).

BP decided that a new gun design was neededto minimize orientation errors.21 Engineers choseTCP guns with charges at 10° and 350° phasingangles to help align perforations closer tovertical for increased stability. Optimally spaced,near-vertical perforations would improve totalinflow, reduce the flow rate through each holeand maximize the distance between perforationtunnels in the formation for increased stability(next page, left).

This system included passive orientationweights and swivels to align the charges and anOCD system in each gun section to recordperforation direction. Deep-penetrating premiumshaped charges were used to reduce near-wellbore pressure drop during production and minimize formation stresses on perfor -ation tunnels.

BP initially planned to complete the new Well A-15 with a 51⁄2-in. liner, so Schlumbergerbuilt 33⁄8-in. guns with 5 spf at 10° and 350°phasing angles. Difficult drilling conditionscaused significant variations in wellbore trajec -tory that forced BP to run a 41⁄2-in. liner and use asmaller perforating system.

In April 2002, BP perforated Well A-15 bydeploying more than 1,000 m [3,281 ft] of 27⁄8-in.oriented TCP guns with 6 spf at 10° and 350°phasing angles. However, this gun systemperformed below expectations. The OCD devicesrecorded an average alignment error of 26° fromthe vertical (right).

Confirmation of perforation orientationallowed engineers to assess the risk of sandinflux and helped BP make production decisions.This information provided a benchmark forevaluating future developments in tools andtechniques. The large alignment error in Well A-15 raised concerns that perforations inweaker intervals might fail and produce sand asreservoir pressure decreased.

To achieve accurate near-vertical perforationorientations, Hydro and Schlumberger hadjointly developed OrientXact technology. The

new OrientXact swivels reduced friction by 90%and could withstand the high loads generated bylong gun strings in both tension and compres -sion. In addition, improved connections withtight manufacturing tolerances further reducedalignment errors between gun sections.

BP applied OrientXact swivels for the firsttime on two existing Andrew field wells toperforate bypassed oil zones identified by 4Dseismic data and cased-hole logs. BP planned toadd perforations at the upper end, or heel, of thehorizontal sections in Well A-8 and Well A-7 using

32 Oilfield Review

> Perforating strategies. BP perforated Andrew field Wells A-2 through A-6 using 33⁄8-in. HSD High Shot Density guns with 4 spf. The weaker, or lessconsolidated, sandstone intervals were perforated using conventional oriented guns with charges at 25° and 335° phasing (left) to align perforations oneither side of vertical and shoot along the high side of a wellbore. However, orientation accuracy was unknown (middle). BP perforated the moreconsolidated intervals in Wells A-7 through A-14 using standard fully phased TCP guns with charges at 60° phasing (right).

25° and 335° phasing0°

3 3⁄8-in. HSD gun5 1⁄2-in. liner6 1⁄4-in. open hole

Unknownaccuracy


60° phasing


25° and 335° phasing0°

> Conventional oriented TCP guns. BP perforatedAndrew field Well A-15 using more than 1,000 mof 27⁄8-in. conventional oriented guns. These gunswere deployed and retrieved through an FIV tool.The orientation data from OCD systems includedin this gun string indicated an average perforationmisalignment of 26° away from vertical. Sand-production models predicted that at thisorientation, perforations in weakly consolidatedsands could become a source of sand influx inweakly consolidated sandstones.

10° and 350° phasing26° error

26°

2 7⁄8-in. HSD gun4 1⁄2-in. liner

> Accurate oriented perforating with coiled tubing-conveyance. The OCD systems in the perforatingsystem used on Well A-8 verified that improvedOrientXact swivels between gun sections couldincrease orientation accuracy even with analternative coiled tubing-conveyance method.The average perforation alignment error in thiswell was 12°, a 14° improvement compared withWell A-15. Sand-prediction models indicated that perforations at this near-vertical orientationwould prevent sand influx for several years.


12°



Autumn 2006 33

33⁄8-in. TCP guns with the new swivel design andHSD High Shot Density charges at 10° and 350°from vertical.

Schlumberger deployed, fired and retrievedthis TCP system in Well A-8 with coiled tubingand a surface deployment system. Afterrecovering the guns, the OCD devices confirmedan average alignment error of 12°, 14° less thanin Well A-15 (previous page, bottom right).

BP and Schlumberger projected thatperforations at this orientation would notproduce sand for several years, even in weakersand intervals. Based on the success of Well A-8,BP planned to use the same technique on Well A-7. However, design models predicted thatcoiled tubing alone could not reach theperforating depth because of helical buckling.

These complications and a longer perforatinginterval required BP to make two perforatingruns. BP chose a combination of coiled tubingand two downhole tractors to convey the guns.

The average gun alignment error was 11° on thefirst gun run; on the second gun run, the gunalignment error was less than 8° (above).

BP subsequently perforated the new A-16well using a 33⁄8-in. TCP system with newOrientXact swivels and shaped charges at 10°and 350° phase angles. Orientation accuracy wasexpected to be high because this wellbore had noextreme variations in trajectory. The average gunalignment error was less than 5° (left).

During cleanup, however, Well A-16 produceda substantial amount of debris back to surface,causing a processing shutdown. As a result, BPand Schlumberger focused additional engi -neering efforts on reducing debris in futureperforating operations. A potential solution wasthe 41⁄2-in. OrientXact system developed forHydro. This system used low-debris shapedcharges that do not fragment into small pieces,and the large fragments remain inside the gun.

BP agreed to develop this technology for asmaller gun system to use on the next new well.Well A-17 would encounter the same difficultdrilling conditions as Well A-15, so BP decided todevelop a 27⁄8-in. OrientXact system similar to the41⁄2-in. guns developed for Hydro. This designincluded OCD systems, low-debris charges at 10°and 350° phasing angles, and connections thatreduced alignment error between gun sections.

21. Martin AJ, Robertson D, Wreford J and Lindsay A:“High-Accuracy Oriented Perforating Extends Sand-FreeProduction Life of Andrew Field,” paper SPE 93639,presented at the SPE Offshore Europe Conference,Aberdeen, September 6–9, 2005.

> Optimizing shot density and phasing angles. BP elected to use a new perforating strategy forWell A-15 in the Andrew field to maximize thedistance between perforations and to reduce the flow rate through each perforation (top).Engineers initially designed a 33⁄8-in. oriented TCPsystem with charges at 10° and 350° phasingangles and 5 spf (bottom). By reducing the anglebetween perforations from 50° to 20°, this designwas expected to align the tunnels closer tovertical than previous guns with 25° and 335°phasing angles and avoid tunnel collapse.Difficult drilling conditions required BP to run a41⁄2-in. liner instead of 51⁄2-in. casing and changeto smaller 27⁄8-in. TCP guns with 6 spf.

10° and 350°phasing

Perforation tunnels at the sandface

0°

> Accurate oriented perforating with tractor-assisted coiled tubing. Because of the depth and a long completion interval, BP perforated Andrew field Well A-7 using a combination of coiled tubing-conveyance and two downhole tractors. This operation required two perforating runs. Both gun runswere successful with an orientation error of 11° on the first run (left), and an error of 7.5° on thesecond run as verified by OCD systems (right).


11°


10° and 350° phasing7.5° error

7.5°


> Optimizing oriented tubing-conveyed perforating.Well A-16 provided the first opportunity to testthe OrientXact swivel design in a TCP applicationon a new well in the Andrew field. The trajectoryof this wellbore had no severe doglegs, so BPexpected orientation accuracy to be high. TheOCD devices in this 33⁄8-in. TCP system, whichwas deployed through an FIV tool, indicated analignment error of less than 5°.


4.6°



In April 2004, BP perforated the new A-17well using this new 27⁄8-in. OrientXact TCP systemconveyed below 27⁄8-in. and 31⁄2-in. tubing. Afteroperators fired and retrieved the guns, the OCDsystems confirmed that the perforations wereabout 2.6° from vertical (above).

This degree of accuracy was impressive, butthe OrientXact system also retained most of theresidual charge debris inside the gun tubes. BPrecovered only a small volume of debris, mainlyrust from the casing walls, at the surface frommore than 1,200 m [3,937 ft] of guns.

Maximizing production and operationalefficiency was important, but sand preventionwas the primary reason for applying orientedperforating in this field. BP and Schlumbergerachieved this objective on the A-7, A-8, A-16 andA-17 wells. Continuous monitoring of solids sinceMay 2003 has indicated extremely low sandinflux from existing completions with neworiented perforations, and from new wells, suchas Well A-18, that were perforated with theOrientXact system (right).

In addition to sand prevention and orientedfracturing, engineers use orienting systems toachieve other objectives, including reperforatingand recompletion of wells with complexdownhole equipment configurations like those inthe North Sea Otter field.

Subsea Recompletion with Oriented TCP Total E&P UK plc needed an accurate, cost-effective method for orienting perforations in ahorizontal North Sea well. This particularcompletion in the Otter field located northeast ofthe Shetland Islands was one of three subseawells, each equipped with dual electricalsubmersible pump (ESP) artificial lift systems.Total had identified a bypassed zone that could be commingled with production fromexisting perforations.

However, the target interval was behind 75⁄8-in. casing, which presented several perfor -ating challenges. The guns had to pass throughthe ESP bypass and a 2.66-in. [6.8-cm] landingprofile and align the perforations verticallydownward toward the low side of the wellbore tomitigate sand influx and maximize perforationpenetration inside 75⁄8-in. casing (next page).

The gun design also had to minimize residualdebris to avoid perforation plugging and damageto the two ESP systems. During the first phase ofthis project, engineers assessed the risk of sandproduction. Total conducted sensitivity studies todetermine if oriented perforations would preventsand production from weak formation intervals.Sand-prediction software confirmed that

perforations aligned within 10° of vertical in theweakest pay zones would remain stable even atfull depletion.

The second phase involved designing a smallgun system with charges at a zero phasing angle. Schlumberger built and tested a highlyaccurate, low-debris 21⁄4-in. OrientXact systemthat could pass through the ESP bypass and a 2.4-in. [6-cm] restriction.

Engineers chose 2-in. PowerJet Omega deeppenetrating perforating shaped charges tomaximize penetration in the weak Otter sandsand to eliminate debris that could damage thedual ESP system. These extra-deep penetratingcharges, which achieve a 23.8-in. [60.5-cm]penetration in an API Section-1 target, alsoinclude a low-debris case that remains intactinside the carrier after perforating.

Normally perforations are oriented along thehigh side of a wellbore to optimize perforationstability and cleanup, and to avoid perforationplugging. However, firing a small gun systeminside 75⁄8-in. casing would decrease perforationpenetration depth in the formation if shapedcharges were oriented upward. Gravity wouldcause guns to lie on the low side and require thatcharges shoot across the wellbore through wellfluids. Schlumberger designed the gun system to

34 Oilfield Review

> Accurate oriented tubing-conveyed perforating.The OCD systems in the 27⁄8-in. OrientXact TCPstring that was used to perforate Andrew fieldWell A-17 verified that all of the perforationswere aligned about 2.6° from vertical. BPattributed this degree of orientation accuracy tothe OrientXact aligning and locking adapters,which minimized misalignment between gunsections, and also to the high-load, low-frictionOrientXact swivels, which maintained a near-vertical charge orientation before and afterperforating. This was the first use of a completeOrientXact system by BP in the Andrew field.


2.6°

2 7⁄8-in. OrientXact gun5 1⁄2-in. liner6 1⁄4-in. open hole

> Andrew field sand production. BP required accurate oriented perforating to extend the productivelife of maturing North Sea fields. Except for occasional spikes associated with unstable flow duringprocessing shutdowns, clamp-on monitors installed on production piping at the surface indicatedlower rates of sand production for the Andrew field A-7, A-8, A-16, A-17 and A-18 wells, which werecompleted with more accurate oriented perforating systems (green).

10,000

1,000

100

10

Cum

ulat

ive

sand

, kg

1

0.1

0.013/20/03 6/28/03 10/6/03 1/14/04 4/23/04 8/1/04

Date

11/9/04 2/17/05 5/28/05 9/5/05 12/14/05

Blue: Nonoriented 60° phasing Orange: Conventional oriented perforatingGray: Nonperforated completion Green: OrientXact oriented perforating

A-3A-6A-14A-13

A-4A-10A-12A-15

A-7A-8A-16A-17A-18


Autumn 2006 35

rotate and shoot downward, which sacrificedperforation cleanup for penetration depth, butallowed Total to perforate this subsea well withoutpulling the permanent completion equipment.

Total successfully deployed the 21⁄4-in. systemon a 21⁄8-in. wireline tractor during twoperforating runs with 12.2-m [40-ft] gun strings.The low-friction OrientXact swivel and weightsrotated the charges to the low side of thewellbore. The OCD orientation confirmationdevices verified that all perforations werealigned downward within 2° of vertical. Theadditional perforations contributed substantialincremental production from the Otter field.

Oil production increased from about 8,000 to15,000 bbl/d [1,271 to 2,384 m3/d] with noindication of sand production. The entireoperation was completed in 36 hours withminimal residual debris, no downtime and nolost-time incidents. This successful operationhelped Total avoid a difficult and expensivesubsea workover.

New Dynamics and Directions Efficient and effective perforating designs mustaddress a number of factors, including shaped-charge performance, formation characteristics—rock strength, permeability and porosity—shotdensity and transient wellbore pressures before,during and after gun detonation. Perforatingtechniques and tools, including advances such asPowerJet Omega charges, the PURE process andOrientXact systems, contribute significantly tothe success of stimulation treatments and sand-management methods.

Deep-penetrating charges can bypassinvasion; big-hole charges for fracture stimu -lations or gravel packing maximize perfor ationflow area; optimal charge orientation andspacing can prevent or mitigate sand productionand other factors that restrict production.Innovative perforating methods and techniques,such as PURE dynamic under balanced perfor -ating, ensure clean and effective perforationtunnels. By effectively cleaning up allperforations, dynamic underbalanced perfor -ating maximizes well productivity and injectivity.

Research and laboratory analysis areadditional elements in the development of newperforating techniques and systems. Only byconsidering all of the relevant factors canoperators achieve optimal perforated completions.Schlumberger research and develop ment effortsand state-of-the-art manufacturing capabilities atSRC continue to address the performance ofshaped charges and gun systems. Researchers areseeking perforating solutions for a wide range of oiland gas applications, including carbonatereservoirs and coalbed methane production.

Modern perforating solutions efficientlydeliver higher productivity and injectivity acrossmany types of completions—new or existing oil, gas or gas-condensate wells—during every stage in the life of well or field, from initial development to plateau production.Ultimately, this will mean decreased costs,additional revenue and increased profit foroperating companies. —MET

> Orienting downward vertical perforations. Total E&P UK plc operates the Otter field northeast of the Shetland Islands in the North Sea.Well T2/P1 was one of three horizontal subsea completions, each equipped with a dual electrical submersible pump (ESP) system forartificial lift. To access bypassed oil in Sand A behind 75⁄8-in. casing at the top, or heel, of the horizontal section without pulling existingcompletion equipment, Total needed an oriented gun system that could pass through the ESP bypass and a 2.66-in. landing profile.

4 1⁄2-in. screens

Subsurface safety valve

3.06-in. bypasswith flapper valves

2.66-in. landingprofile

Subsea wellhead

Blowout preventer

ESP

ESP

Lower completion Sand-screen completion

Existing D sand productionA sand

7 5⁄8-in. casing

10 3⁄4-in. casing

S C O T L A N D

N O R WAY

Shetland Islands

Otter field

Orkney Islands


Documents

New Practices to Enhance Perforating Results/media/Files/resources/oilfield...Autumn 2006 21 high-energy jets from explosive charge detonations generate shock-wave damage and create