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Copyright 2004, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the SPE Annual Technical Conference andExhibition held in Houston, Texas, U.S.A., 2629 September 2004.

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AbstractWhen performing hydraulic fracturing treatments, thecompletion community widely recognizes that if theperforations can be oriented to the maximum principle stressplane and the shot density can be maximized accordingly, thesuccess of the fracture treatment will be much greater. Thesebenefits result from the lower breakdown pressure and theminimal tortuosity effects experienced during the processwhen the above conditions are met. Also, if the perforationsare oriented in the maximum principle stress plane for anaturally completed, cased, and perforated well, the resultingperforation tunnels will usually be more stable than otherperforations that are placed away from this preferred stressplane.

In natural completion scenarios, oriented perforatingbecomes especially important if 1) the formation integrity isquestioned, and 2) sustained production without themobilization of sand that could impact production ischallenged. Normally, when faced with this dilemma,operators have automatically defaulted to a conventional sand-control treatment to ensure that production is not interruptedand that costly well interventions to clean out wellbore sandaccumulation will not be required. While traditional sandcontrol methods; i.e., installation of screens, proppants, andcompletion fluids used in the completion, are capable ofcontrolling unwanted sand production; significant productivitypenalties are often experienced with their use.

Now, there is another viable sand-control option that canbe considered by the industry. If the completion is a candidatefor sand management, a gravity-force-oriented perforatingstrategy, which will eliminate traditional sand controltreatments, can be planned. Since the system relies on gravityfor proper orientation, the most important consideration forusing this concept is that the wellbore under consideration

must have a minimum deviation of 25 degrees to orient tomaximum principle stress or the overburden gradient.

The new perforating system was used in a North-Sea fieldand resulted in a 37,600 BOPD sand-free production rate. Thenew gun system can be conveyed on wireline or coiled tubingand can also be used in conventional tubing-conveyedperforating (TCP) applications.

IntroductionThe relationship between geomechanics and completionoptimization has been studied extensively and welldocumented in the literature.1 For a vertical completion, it isadvantageous to define the minimum and maximum horizontalstresses and orient the perforations to the maximum horizontalstress to improve the hydraulic fracturing treatments andincrease the probability of more stable perforation tunnels fora perforated only completion. Generally, when designingthe perforation strategy for a cased horizontal completion, thedominant stress field will be the vertical or overburdengradient. Therefore, it is important that the perforations in thehorizontal completion be directed to the top and bottom (180phasing) of the wellbore to the maximum stress field.

Modeling tools are available to assess well productivityassociated with perforating with higher-shot-density, spiral-phased gun systems versus reduced shot density at 180 phase.It is very important to evaluate the differences in wellproductivity that can occur when selecting an orientedperforating strategy and select the perforating strategy that willmaximize reservoir recovery.

After the decision has been made to adopt an orientedperforation strategy, the design of the program must bedesigned to assure that the perforations are accurately placedin the proper stress orientation. Minor errors in perforationplacement can lead to inefficiency in initiating hydraulicfractures and perforation tunnel collapse or production of sandwith a natural completion. Diagnostic tools are readily

available during the drilling phase to determine the state ofstress in the near wellbore region (as described by Brudy2);however, the perforating hardware and accessories required toposition the perforation planes have been less than reliable.

Orienting TechniquesThere are several existing orientation methods that have beenavailable to the industry; however, each has certainlimitations. For example, for perforating smaller intervals,

SPE 90164

Novel Perforating System Used in North Sea Results in Improved Perforation for SandManagement StrategyEivind Hillestad and Paal Skillingstad, Statoil, Kent Folse, Oyvind Hjorteland, and Janne Hauge, Halliburton EnergyServices, Inc.

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orientation with gyroscopic tools with guns conveyed onelectric line are quite common and offered by many vendors.These techniques are generally quite accurate; however, theynormally require multiple wireline runs to confirm orientationand are limited to the length and weight of guns that can besafely conveyed. In addition, the wireline techniques are notapplicable to horizontal completions where the gun systemmust be conveyed into the wellbore on some type of pipe.

For pipe or tubing-conveyed perforating (TCP) systems,the orientation has generally relied on gravity-based systemsto orient or rotate the guns to the proper direction.3A typicalTCP-oriented system, which consists of mechanical tubingswivels, explosive transfer swivels, eccentered gun tandems oroffset tandem fins, is shown in Fig. 1. This type of system isdesigned to allow the entire gun carrier to rotate such that theeccentric tandem travels on the low side of the wellbore or theoffset fins travel on the high side of the wellbore. Theinherent problem with this type of setup is that long gunassemblies experience significant torque and drag as they areconveyed into the wellbore, leading to potential error inperforation placement on the order of 20 in some cases.Assuming a perfect wellbore with no torque or drag placed on

the gun assembly, the potential error band is still on the orderof 10 due to the fact that enough gun-to-casing standoff isrequired to allow the guns to rotate freely.

In the past, some completion strategies have called forpositioning guns on depth and running wireline gyro tools toreference off a known point in the workstring in order todetermine current position; then, the entire workstring isrotated at surface and orientation is confirmed. This techniqueworks well for shorter gun lengths where the torque on the gunassembly is negligible. The ideal orienting system would be asetup that eliminates the adverse torque and friction thatinduces unwanted rotation of the gun system.

New Gravity-Force Orienting System

A novel orientation system that is still based on the force ofgravity to position the shaped charges has been developed;however, the orientation now is accomplished in a verypristine environment where wellbore effects are eliminated.The shaped-charge orientation is accomplished by moving theswivels from the external gun tandems to the shaped-chargetube holder inside of the scalloped gun carrier. The scallopedgun carrier is an atmospheric chamber with essentially apolished bore that is free from any pipe dope, residualcement/mud, mill scale or any other types of debris that arenormally found in the wellbore.

The orienting charge tube configuration used in the newassembly is illustrated in Fig. 2 and consists of multiplecharge tubes per 22-ft gun carrier that rotate independently of

each other. Orientation is achieved by mounting the shapedcharges relative to a series of weight strips attached to thecharge tubes. The exterior of the gun carrier, illustrated inFig. 3,is modified with bands cut on the circumference of thegun to replace conventional scallops that are required toprotect the completion from gun burrs.

The primary benefit offered by this new system is thatorientation accuracy is not dependent on well path, fluid ordebris. In addition, increased orientation accuracy is nowpossible in completions with restrictions; this assembly can be

conveyed through the completion to orient perforations inlarger-casing profiles. Previously, this type of perforatingwould not have been possible because of the requirement toposition the guns with orienting fins and mechanical swivels.The elimination of the orienting fins and other associatedhardware required with the old system also speeds up themake-up and break-out of the perforating assemblies, thusreducing flat-line time during the completion.

The reduction of the aforementioned hardware also servesto reduce the make-up length and weights of the orienting gunassemblies, allowing longer gun lengths to be conveyed onwireline in vertical wells or horizontals wells with tractorassistance. In addition, the new orientation system allowsinstallation of roller tandems to facilitate conveyance onwireline applications where high well angle prevents gravityalone from positioning gun assembly on depth. The reductionin hardware also reduces the distance between each loadedgun assembly at the gun connectors; therefore, the dead zoneor non-perforated sections of the casing are reduced. Forinstance, for a fully loaded 112-ft interval, the maximumnumber of perforations at 4 spf, oriented at 180, would be 382perforations with the old system. For the newly developed

gravity force system, the total shots would be 425 In the sameperforation scenario, which would be an increase of 10%.Increasing the number of perforations results in a potentialimprovement in well productivity because of the better flow-convergence to the wellbore.

In the horizontal completion, one of the major drawbacksto prior orienting techniques has been that the gun systemmust be positioned on the low side of the well to orient theguns. When the guns are lying directly on the casing, the rowof perforations at 0 phase will not have any gun-to-casingclearance, which is required in many cases to allow theshaped-charge jet enough time to develop fully as it exits thegun scallop. This can result in a possible reduction information penetration and casing-entrance hole size. The

other major problem with the gun on the low-side of thecasing is that even if the well is perforated with optimumunderbalance pressure to surge the perforation debris from theperforation tunnels in the formation, the gun directly on the 0phase row of holes prevents efficient surging of theperforations.

The newly developed gun system can actually bepositioned or centralized as needed to ensure that the low-sideperforations are perforated at optimum gun clearance and thatperforations are cleaned up effectively. Centralization or gunpositioning can be important to allow unobstructed flow fromall the perforations into the wellbore if it is necessary to leavethe gun assemblies across the perforated interval.

Most orienting methods require very complex gyro-type

tools to verify that proper orientation has been achievedfollowing the perforation event. With this system, theorientation is confirmed when the guns are retrieved from thecompletion by examining the exterior of the gun carriers.When the gravity-force assembly orients properly, all of theindependent charge-tube assemblies in each 22-ft gun carriershould result in a straight line of gun exit holes in the bandsdown the length of the gun assembly. Furthermore, as theguns are retrieved, a visual observation between each 22-ftgun carrier should show that all the exit holes in each gun

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section are in a straight line, provided the guns are conveyedwithout any type of conventional swivel sub.

Gravity-Force Orientation VerificationTo validate the new internal orienting system, a series of testswere performed to ensure gun survivability and accuracy ofperforation orientation. A fully loaded 22-ft, 3

-in., 4-spf gunassembly was perforated at surface conditions in a surface

pond in a horizontal position, and then, at an angle of 30. Thesurface pond test (as shown in Fig. 4) resulted in a straight linedown the length of the 22-ft gun carrier for both gun angles asshown in Fig. 5. The deflection in the gun exit holes from thestraight line resulted in an orientation error band close to 5with the measurement technique used. Analytical calculationsprior to the development of the banded gun system indicatedthat the rated gun collapse would be well within the currentcollapse rating for a conventional scalloped gun system. Theinternal orienting gun system was placed in a pressure testvessel, and the banded gun assembly survived an appliedpressure of 30,000 psi at 450F.

The other test criterion for the new system was to ensureorientation accuracy for various wellbore dogleg severities.

To accomplish this test, two 22-ft gravity-force gunassemblies were connected and subjected to a maximumbending or torque of 13/100 ft as illustrated in Figs. 6 and 7.Gun orientation was determined visually by machiningwindows at various positions on the exterior of the gun carrieras shown in Fig. 8. The shape-charge orientation (per thevisual orientation shown in Fig. 9) was deemed to beessentially the same as observed during the surface pond test.Thus, gun bending due to well path orientation error caused bypotential lockup of orienting charge-tube assemblies waseliminated.

Case History: Gravity-Force System, North SeaThe Norne field is located approximately 125 miles from the

Norwegian coast and 50 miles north of Heidrun field. Statoildiscovered this Norwegian Sea field in December 1991, andan appraisal well drilled the following year confirmed that thefield was the group's biggest oil find in several years. Thereservoir consists of a 443-foot hydrocarbon-bearing columnin sandstones of Lower and Middle Jurassic age. The waterdepth is 1,250 ft.

The field has been developed with a production andstorage ship tied to five subsea templates. A total of 12 oilproduction, 7 water injection and 1 gas injection slot areavailable at the templates. All production wells have beencompleted with long horizontal or near horizontal linersections, which are perforated on drill pipe before running theupper completion. The water/gas injectors were drilled

vertically or slightly deviated through the reservoir and wereperforated on wireline.Well 6608/10-B-3 H is a horizontal oil producer, which

penetrates two sandstone formations the Ile and Tofte. Itwas set on stream in July, 1999 and experienced waterbreakthrough in August of 2001. After water breakthrough, arapid decline was noted in the oil production rate. The wellwas shut-in in April of 2002 due to a combination of lowreservoir pressure and a high water cut.

A multidisciplinary group was brought together toinvestigate the problem and to suggest possible solutions forbringing the well back on stream. It was decided to carry out awell intervention to detect and shut off water-flooded zonesand perforate oil-bearing zones in the heel section of the well.

The new gravity-force-orienting perforating guns wereselected because they provided capability to:

Orient perforations in vertical plane (0-180 degrees

phasing) to minimize risk of sand production. Orient guns in wells where sand and scale may be

present in the wellbore.

Use high-shot-density premium deep-penetratingshaped charges to maximize oil production.

Run long perforating strings (110 ft) in each wirelinerun (additional orientation subs not required).

Be dispatched within the required timeframe.With rig operating costs close to $400,000 US per day, the

majority of the intervention costs are related to rig time.Efficient rig and equipment utilization can representsignificant overall cost savings. As a consequence,considerable time and money were spent on the planning ofthe offshore operation. Dedicated personnel from perforating

and the wireline conveyance vendors were appointed for jobfollow-up early on in the project.

A dynamically positioned vessel was brought on locationin February 2003. A production log was recorded across thereservoir at shut-in conditions. Log data showed that allcurrent perforations were fully or partly water flooded. Theupper areas of both the Ile and Tofte formation were believedto contain oil. Reservoir characteristics for the latter formationare known from core samples. Tofte 3 has a very goodporosity (30-40 %) and permeability (k ~ 1-2 darcy) and hasbeen known to produce sand.

The gravity-force-orienting perforation tool stringconsisting of 5 gun sections (22-ft each) was deployed using7/16-in. cable combined with a well tractor pulling the guns in

place. A gamma ray collar locator (GR/CCL) was used tocorrelate the guns, and the overall toolstring length came closeto 138 ft. After perforating in the Ile Formation, an attemptwas made to clean up the well against the rig. Due to lowreservoir pressure (close to hydrostatic) and water flooding, itwas decided to install a permanent bridge plug above Ile.

A second perforation run was then performed in the upperTofte 3 formation. The guns were set off with 290 psi underbalance in the well.

Case History Summary: Gravity-Force System, NorthSeaAn oriented perforating job was performed at the Snorre fieldto perforate in the direction of the maximum principle stress to

minimize the production of formation sand. The Snorre P-41well was a 5-1/2 mono-bore type completion with a wellangle ranging from 60 to 65 degrees. Due to the well angleand the desire not to mobilize coiled tubing or perform aconventional well workover; the guns were to be conveyed toformation depth with the assistance of a wireline tractor. Atotal of seven perforating runs were planned to restoreproduction to the P-41 completion; reference Table 1 forintervals. After the second perforating run the well was

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producing 18,875 bopd versus an expected production rate of3,150 bopd. The remaining perforating runs were performedwith the well producing approximately 9,450 bopd to achievesome underbalance pressure during perforation event.

All seven perforating runs were completed with 100% success achieved on desired orientation. The maximum gunlength conveyed was 110 ft plus the associated tractor andcorrelation tools. A complete gravity force orientation system

summary for the North Sea is shown in Table 2.

Field ResultsThe result of the job was that an initial sand-free oilproduction rate in excess of 37,740 BOPD was obtained. Thisrepresented a 20% increase in the overall Norne Fieldproduction potential at the time. Pressure build-up analysisgave a skin equal to 0 and a productivity index value of 58bbl/day/psi. The combination of a high permeability zone,perforation in under balance, vertical oriented perforations andlong wireline perforation intervals are all factorscontributing to the high initial oil production rate.

The operation was done with a dynamically positionedvessel according to plan. Particularly significant was the fact

that the operating efficiency exceeded 96% when compared tooriginal time estimates. All well objectives were met, andpayback for the operation was attained within approximately6-days of production from the well.

A total of 5 oriented perforating jobs have been performedwith the new system with orientation verification confirmedon 100% of the total of 20 individual perforating runs.

Conclusions1. A new internal orienting gun assembly that can facilitateimproved orientation accuracy for long perforating assemblieshas been developed and tested.2. The new internal orienting gun assembly offers orientingperforating solutions for completions that previously could not

be perforated efficiently because of constricting configurationssuch as when smaller guns had to be run through a completionto perforate in larger casing.3. Improved perforation orientation can result in effectivesand-management strategy with proper characterization offormation stresses and dependence of reservoir depletion.

4. The new internal orienting gun assembly has beensuccessfully implemented in a North Sea completion, resultingin an initial sand-free production rate of approximately 37,000BOPD.

AcknowledgementsThe authors would like to thank the management ofHalliburton Energy Services, Statoil, and its Norne field

partners for permission to publish this paper. Special thanksare given to Melissa Allin and Flint George of HalliburtonTubing Conveyed Perforating Engineering Department forengineering and technical support during the development ofthe new internal-orienting-gun system.

References1. Eriksen, J.H., Sanfilippo, F., Kvamsdal, A. L., George, F., Kleppa,

E.: "Orienting Live Well Perforating Technique ProvidesInnovative Sand-Control Method in the North Sea, SPE 73195,presented at SPE Annual Technical Conference and Exhibition,Houston, 3-6 Oct 1999.

2. Brudy, M.: Determination of the State of Stress by Analysis ofDrilling-Induced Fractures-Results from the Northern NorthSea, paper SPE 47236 presented at the SPE/ISRM Eurock 98,

Trondheim, Norway, 8-10 July 1998.3. Benavides, S. P., Myers, W. D., Van Sickle, E. W., Vargervik, K.:

Advances in Horizontal Oriented Perforating, SPE 81051,presented at SPE Latin America and Caribbean Petroleum

Engineering Conference, Port-of-Spain, Trinidad, West Indies,27-30 April 2003.

SI Metric Conversion Factorsbar x 1.0* E + 05 = Pabbl x 1.589 873 E 01 = m3F (F 32)1.8 = Cft x 3.048* E 01 = min x 2.54* E + 00 = cmmL x 1.0 E + 00 = cm3mile x 1.609 344* E + 00 = km

psi x 6.894 757 E + 00 = kPa

* Conversion is exact.

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Table 1: Case History Snorre Field

Perforated Interval Formation

Run No. Top (ft) Bottom (ft) Name

1 18,761 18,840 Lunde 4

2 18,570 18,636 Lunde 3

3 18,423 18,505 Lunde 2

4 18,308 18,406 Lunde 2

5 18,160 18,259 Lunde 2

6 18,088 18,131 Lunde 1

7 18,013 18,045 Statfjord S5

Table 2: Case History Summary

Field Well TypeNumberof Runs

TotalPerforatedInterval (ft)

ConveyanceMethod

ExpectedRate(bopd)

ActualRate(bopd)

Norne B-3H Subsea 2 219 Wireline Tractor 19,000 37,000

Norne B-1H Subsea 5 398 Wireline Tractor 7,800 9,400

GullveigK2-AH Subsea 3 86 Wireline Tractor 3,100 3,100

Statfjord C-19 Fixed 3 132 Wireline Tractor 2,800 4,400

Snorre P-41 Fixed 7 531 Wireline Tractor n/a n/a

Totals 20 1366

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Fig. 1 A Typical Gravity-based TCP-Oriented System

Orienting Fin

Lock Ring

Gun Orienting Fin

Lock Ring

shooting Swivel

Lock Ring

Orienting Fin

CN02129

Retrievable Packer (optional)

Ported Nipple and TDF

Tubing Swivel

Orienting Subs

Explosive TransferSwivel Subs

VannGun Assembly Por ted Nippleand TDF

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Fig. 2

The new Gravity-Force Orienting System

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Fig. 3

Exterior of the Gun. Note the bands that replace the traditional scallops required to protect the

completion from gun burrs.

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Fig. 4

Surface Pond Test to Verify Orientation

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Fig. 5

Straight line gun angles determined by pond test.

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Fig. 6

Visual of second test criterion to ensure orientation accuracy for wellbore dogleg severities.Subjected to 13 /100-ft force

Fig. 7

Drawing of test to ensure orientation accuracy

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Fig. 8

Close-up of machined window

Fig. 8 Machined windows on exterior of the gun

carrier for gun orientation.

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Fig. 9

Visual shaped-charge orientation essentially the same as observed during the surface pond test.