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Abstract

The choice of how to perforate an interval is considered bymany engineers as a straightforward exercise. Typically, anengineer takes API gun performance data from RP-43, usessome nodal analysis program and determines the "optimum"shots per foot, phasing and shaped charge that provides themaximum inflow performance for the completion. Thisinformation in combination with operational constraints isthen used to design the perforation operation.

As more and more is understood about the perforation process,some operators are becoming aware of the value ofapproaching the perforation operation as a system instead of asimple engineering exercise. However, one aspect that has notreceived very much attention is the make-up of the reservoir inthe near wellbore area, and how this affects the overallperformance of the perforated interval after the perforationevent.

This paper presents the results of a program to optimize theperforation strategy for a major gas field in the North Sea. Anemphasis is placed on understanding the effect of theperforation operation on the deliverability of the reservoir nearthe perforation. The discussion in this paper includespetrographic analyses of the near perforation rock before andafter the perforation event. In addition, CT (ComputerTomography) scans and core flow tests were done in order tounderstand observations from whole core tests. Productiondata from several wells is presented that documents thesuccess of the system approach to developing the perforationstrategy for the field.

Introduction

A study was commissioned by Chevron and Conoco todetermine the optimum underbalance and potential for sandproduction for a major North Sea gas field. The optimumunderbalance was initially calculated using theoretical modelsbased on log derived formation properties. Experiments werethen conducted to confirm the theoretical models. The initialresults indicated a positive skin after perforating up to 3000psi underbalance (1). The objective of this study was todetermine the cause of the positive skin.

It is well established that perforating process causes a changein the matrix rock immediately surrounding the perforationtunnel that results in a decrease in the matrix permeability inthis region (2-4). The magnitude of this permeability change isa function of many parameters such as rock properties, matrixand perforating fluids, and hydrostatic pressure in the wellbore(5). The general practice to minimize the detrimental effect ofthe damaged zone around the perforation tunnel is to perforateunderbalanced. Several studies have been done to try toquantify and model the optimum amount of underbalanceneeded to minimize the effect of this damaged zone (6-12).

It has been postulated that the combination of underbalanceand fines in the matrix can also contribute to magnitude ofdamage around the perforation tunnel (13). If there are porelining fines in the formation and the velocity of the surgetoward the perforation tunnel caused by the underbalance isgreater than the critical velocity for fines movement, it ispossible that the migration of these fines into the 'crushedzone' around the perforation tunnel can cause a furtherdecrease in the permeability.

Perforation Tests

A series of whole core perforation tests were done in order tooptimize the perforation strategy for a major gas field in theNorth Sea. One part of the optimization process involvedevaluation of the effect of underbalance on the potentialproduction from the perforation. Details of the procedure arepresented in previous work (1). In general, the test wasaccomplished by shooting a shaped charge into a sample ofcored formation material at reservoir condition of net

SPE 64425

Considering The Reservoir in Determining a Perforation StrategyD.R. Underdown, SPE, Chevron, P. Mariotti, Chevron, A. Venkitaraman, SPE, Schlumberger Reservoir Completions

2 D.R. UNDERDOWN, P. MARIOTTI, A. VENKITARAMAN SPE 64425

overburden conditions in the fixture shown in Figure 1. Theperforation test consisted of initially measuring thepermeability to a synthetic formation brine in the productiondirection, perforating the core up to 3000 psi underbalancedand re-measuring the brine permeability in the productiondirection. A positive perforation skin was calculated fromregain permeability flow tests of the reservoir sample tests.

In an effort to understand the cause of the positive skin,several tests were done on the formation core after the initialperforation test. These included a CT scan of the perforatedcore to determine the shape, penetration and amount of debrisin the perforation tunnel. Additional tests were done todetermine if fluid compatibility or fines movement was thecause of the positive skin.

Computer Tomography Evaluation

The CT scan process is a non-destructive x-ray imagingmethod that provides 3D imaging of the subject whole core.This evaluation was done within hours of the completion ofthe perforation test at the Schlumberger facility. Figure 2shows that the perforation does contain a large amount ofdebris. The "solid" mounds in the center of the CT scan is thedebris left in the perforation tunnel. This debris ispredominantly made up of perforation generated rock grainslying loosely in the perforation tunnel. The very dark spots inthe debris are part of liner material from the shaped charge.Figure 3 is a photograph of the core impregnated with epoxyand split to allow physical inspection of the perforation tunnel.Even though this evaluation shows a short perforation tunnelpartially filled with debris, a negative skin was expected.Therefore, further investigation was necessary to understandthe explanation for the positive skin.

Fluid Sensitivity Tests

One of the first explanations considered for the positive skinwas the possibility of an adverse fluid/rock interactionbetween the brine and the core material. It was thought that anadverse interaction of the interstitial brine may have causedpore lining fines to become mobile and cause the observeddecrease in permeability after the underbalance perforationtest. There are two reasons to discount this possibility. Thefirst is that the synthetic brine was formulated to have a Cl-

content similar to the interstitial brine of the formation. Eventhough the Cl- ions do not provide a stabilization effect, theassociated cations do stabilize clays. The compatibility of thesynthetic brine with the rock used in the perforation test isdemonstrated by the stable brine permeability using 3.1%NaCl before the test.

In support of this observation, an independent fluid sensitivitytest was done by an independent outside lab using theCapillary Suction Time (CST) technique (14). The basicconcept is that the longer the Capillary Suction Time, thegreater the interaction of the fluid with the subject rock. The

test measured the change in CST time as a function ofconcentration of NaCl in the brine. Inspection of Figure 4shows that the interval tested is only moderately sensitive tofresh water. A very sensitive formation will have as much as a1000% change in CST time; therefore, the low percent changein CST time for the 3% NaCl indicated that this brineminimizes the interaction of the brine and the rock. Based onthis information, it was concluded that the concentration of thebrine used in the perforating tests did not cause anydestabilization of pore lining material.

Fines Migration Tests

Another possibility for creation of formation damage is finesmigration caused by the surge of formation fluids into theperforation tunnel due to the underbalance. A sample of theproducing interval was examined with the Scanning ElectronMicroscope(SEM) and petrographically in thin section. TheSEM in Figure 5 shows abundant grain-coating illite/smectite,fibrous illite and pore-filling kaolinite booklets. The kaoliniteis coarse enough to be seen in thin section as shown in theFigure 6 photomicrograph. All of these clay types can becomemobile.

In addition, the SEM and thin section both show abundantquartz overgrowths. Quartz overgrowths are known to cause asignificant reduction in pore-throat diameter which wouldpossibily increase the likelihood of plugging from migratingfines over a comparable rock without quartz overgrowths.

The next step in the investigation to determine the potentialfor fines movement involved Critical Rate tests by anindependent lab. The Critical Rate tests are designed todetermine if fines will move within a core plug by flowing afluid through a core plug at increasing rates. The details of thetest procedure are given in reference (15). There are twoaspects of the potential for fines movement which wereinvestigated: 1) the effect of the mobile phase used in theperforation test, and 2) the effect of gas production rate on thepotential for fines movement. Two core plugs from theformation core used in the perforation test were used in theCritical Rate tests.

One of the major concerns with the initial perforation testswas the effect of the mobile phase. The mobile phase used inthe perforation tests was 3.1% NaCl. This was a concern sincethe actual mobile phase in the reservoir was gas. It wasspeculated that since the mobile phase in the perforation testwas also the wetting phase, there was a tendency for finesmobilization during the perforating process similar to theincrease in the potential for sand production once a well startsto make water.

To test this hypothesis, two Critical Rate tests were done. Thefirst test examined if high rate gas production in this formationwould cause fine movement. The test consisted of graduallyincreasing the interstitial gas flow rate through a core plug

SPE 64425 CONSIDERING THE RESERVOIR IN DETERMINING A PERFORATION STRATEGY 3

from the formation at residual brine saturation and measuringthe retained permeability until the gas flow rate through thecore plug was equivalent to about 60,000,000 SCFPD. Theresults of this test shown in Figure 7 indicate an initialincrease in normalized permeability which is attributed to adecrease in residual brine saturation. However, at the higherrates there is no decrease in normalized permeability thatindicates that up to about an equivalent of 60,000,000 SCFPDthere is no movement of fines.

The second test consisted of taking this same core and flowing3.1% NaCl at increasing rates. The rates at which the sampleswere tested were higher than the initial core permeability testscarried out prior to perforation. In addition the smaller size ofthe samples used gives a much higher flow velocity. Figure 8shows that almost immediately upon the onset of flow of theNaCl, there is a decrease in permeability. The flow rates of theNaCl in this test are low compared to the high flow rateexperienced during the 3000 psi underbalance surge.However, these results do indicate that one of the major causesof the formation damage observed in the perforation test isprobably the result of the fines moving toward the perforationtunnel during the underbalance surge.

Analysis and Implementation

There are several aspects of this study that point out howimportant it is to know the characteristics of the reservoir, theinteraction of various fluids with the reservoir rock, andoperational parameters associated with the perforatingoperation to maximize the production from completion. Theinformation gained in this evaluation provided data specific tothe North Sea reservoir, and pointed out how important it isnot to generalize when it comes to deciding on completionprocedures.

The SEM analysis and thin section analysis did show thatthere were pore lining fines that could potentially becomemobile. However, just the presence of the pore lining andcementing fines does not tell if these fines will ever becomemobile. The fluid sensitivity tests also did not show that theformation was particularly reactive with the brine used as thetest fluid. The fines migration tests showed that interstitialflow of the wetting phase caused the fines in the formation tobecome mobile. This indicates that it was very important tokeep aqueous based fluids off the formation during the drillingand completion operation. These findings support other workthat indicated that is was necessary to drill the wells with oilbased muds to minimize the potential damage from waterbased muds (16). However, operational restrictions requiredperforating with brine as the wellbore fluid. Therefore, thewells were flowed to surface for an extended timeimmediately after perforating to minimize the potential forformation damage from brine based perforating fluids.

The initial theoretical calculations indicated that as much as4000 psi underbalance would not cause any sanding.

However, an underbalance of 1100 to 1400 psi was chosen forthe completions based on the core test results and operationalconstraints. The perforation strategy was optimized using logderived permeability data and numerous simulations to choosethe guns, charges, and shot density. One of the majorstrategies was to perforate different shot densities and chargesin different sections of the formation based on the differentlayer properties. At least 20 wells have been completed usingthe strategy resulting from this study. The results of wellperformance tests on 12 wells are shown in Table I. With theexception of three wells, all the wells show a negative skinand are producing better than expected.

Discussion

In order to understand the relevance of this study it isnecessary to take a step back and look at the overall objective;how to design a good perforating job. Nodal analysisprograms can provide a relative performance comparison ofdifferent perforator choices to assist us in the design.However, it is important to realize we that there are severalparameters in the analysis that are routinely assumed orestimated. One important such parameter is the perforationdamage. Over the last several years underbalance perforatinghas evolved as a method to minimize this damage and severalcorrelations are available to estimate the optimumunderbalance (for zero perforation skin). Each successiveeffort adds a component that was overlooked by the previousone or assumed as having a lesser importance. This studybrings out two such overlooked factors: nature of the fluid inthe wellbore and pore spaces (gas, oil, or water), and thepercentage of clays and mobile fines. These concerns wereaddressed for this specific North Sea application throughexperiments conducted at as near reservoir conditions aspractical. Even in the context of these experiments the studyshows that it is important to mimic the actual perforationprocess when conducting perforation optimization tests. It isalso important to keep in mind that "stepping out the box", andnot accepting the norm can help.

Conclusions

The following conclusions are based on the informationdiscussed in this paper:

The mineralogy and pore geometry of the formation canhave major impact on the effectiveness of the perforationoperation. And, because of the significant variationcommon to sandstones with regard to both mineralogyand pore geometry, testing an actual sample of the zone ofinterest is highly desirable.

Typical fluid sensitivity tests do not always indicate themagnitude of the potential impact of fluid/rockinteractions on the skin around the perforation tunnel.

It is important to mimic field conditions of fluid type,formation mineralogy and pressure differentials as much

4 D.R. UNDERDOWN, P. MARIOTTI, A. VENKITARAMAN SPE 64425

as possible when conducting perforation core flow tests. The result of core flow tests supports fines migration

resulting from movement of the interstitial wetting phaseas a cause of the apparent damage observed in the initialperforating test for the North Sea field.

Acknowledgement

The authors thank Chevron Production Technology Companyand Schlumberger Reservoir Completions organizations forpermission to publish the paper.

References

1. Underdown, D.R., Jenkins, W.H., Pitts, A., Venkitaraman, A., &Li, H.,"Optimizing Perforating Strategy in Well Completions toMaximize Productivity", SPE 58772, Presented at the SPEInternational Symposium on Formation Damage, Lafayette, LA,Feb. 23-24, 2000.

2. Behrmann, L.A., Pucknell, J.K., Bishop, S.R., & Hsia,T.Y.,"Measurement of Addition Skin Resulting from PerforationDamage", SPE 22809, Presented at the 66th Annual TechnicalConference, Dallas, TX, Oct. 6-9, 1991.

3. Hsia, T.Y., & Behrmann, L.A.,"Perforation Skin as a Functionof Rock Permeability and Underbalance", SPE 22810, Presentedat the 66th Annual Technical Conference, Dallas, TX, Oct. 6-9,1991.

4. Pucknell, J.K., & Behrmann, L.A.,"An Investigation of theDamaged Zone Created by Perforating", SPE 22811, Presentedat the 66th Annual Technical Conference, Dallas, TX, Oct 6-9,1991.

5. Bird, K. & Block, R.H.J.,"Perforating in Tight Sandstones:Effect of Pore Fluid and Underbalance", SPE 36860, Presentedat the SPE European Petroleum Conference, Milan, Italy, Oct.22-24, 1996.

6. Crawford, H.R.,"Underbalanced Perforating Design", SPE19749, Presented at the 64th Annual Technical Conference andExhibition, San Antonio, TX, Oct. 8-11, 1989.

7. King, G.E., Anderson, A., & Bingham, M.,"A Field Study ofUnderbalance Pressures Necessary to Obtain Clean PerforationsUsing Tubing Conveyed Peforating", JPT 38(7), June 1986, pp662-664.

8. Behrmann, L.A.,"Underbalance Criteria for MinimumPerforation Damage", SPE 30081, Presented at the SPEEuropean Formation Damage Conference, The Hague, TheNetherlands, May 15-16, 1995.

9. Behrmann, L.A., Pucknell, J.K., and Bishop, S.R.,"Effects ofUnderbalance and Effective Stress on Perforation Damage inWeak Sandstone: Initial Results", SPE 24770, Presented at the67th Annual Conference, Washington, DC, Oct. 4-7, 1992.

10. Tariq, S.M.,"New, Generalized Criteria for Determining theLevel of Underbalance for Obtaining Clean Perforations", SPE20636, Presented at the 65th Annual Technical Conference andExhibition, New Orleans, LA, Sept. 23-26, 1990.

11. Colle, E.,"Increase Production with Underbalance Perforating",Pet. Engr. Intl., July, 1988. Pp 39 - 42.

12. Bartusiak, R., Behrmann, L.A., & Halleck, P.M., "ExperimentalInvestigation of Surge Flow Velocity and Volume Needed toObtain Perforation Cleanup", J. Pet. Sc. & Engg., 17 (1997), pp19 - 28.

13. Devinder, S.A., & Sharma, M.M., "The Nature of theCompacted Zone Around Perforation Tunnels", SPE 58720,Presented at the SPE International Symposium on FormationDamage, Lafayette, LA, Feb. 23-24, 2000.

14. Underdown, D.R., & Conway, M.,"Minimze Formation DamagePotential with Rapid/Inexpensive Method of Completion andStimulation Fluid Selection", SPE 19432, Presented at the SPEFormation Damage Symposium, Lafayette, LA, Feb. 22-23,1990.

15. Miranda, R., & Underdown, D.R.,"LaboratoryMeasurement of Critical Rate; A Novel Approach forQuantifying Fines Migration Problems", SPE 25432,Presented at the Production Operations Symposium,Oklahoma City, OK, March 21-23, 1993.

16. Internal Chevron Report

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Figure 1 Perforating Test Fixture

SPE 64425 CONSIDERING THE RESERVOIR IN DETERMINING A PERFORATION STRATEGY 5

1. Confining chamber with confining fluid (Kerosene) 2. Simulated wellbore with wellbore fluid 3. Core sample with pore pressureand pore fluid 4. Pore fluid in accumulators 5. Gun with the shaped charge 6. Shooting leads 7. Five gallon accumulator 8. Micrometervalve 9. PCB fast gauges 10. Shooting Plate

Figure 2. CT Scan of Perforated Core

Figure 3. Epoxy Impregnated Core

6 D.R. UNDERDOWN, P. MARIOTTI, A. VENKITARAMAN SPE 64425

Figure 4. Fluid Sensitivity Tests

Figure 5. SEM of North Sea Formation Material

Figure 6. Thin Section of North Sea Formation Material

SPE 64425 CONSIDERING THE RESERVOIR IN DETERMINING A PERFORATION STRATEGY 7

Figure 7. Critical Rate to Gas, Non-Pulsed

Figure 8. Critical Rate to Brine

8 D.R. UNDERDOWN, P. MARIOTTI, A. VENKITARAMAN SPE 64425

Well # IntervalPerforated

(ft)

Underbalance(psi)

Skin

1 204 1100 -2.482 174 1230 -0.153 139 1200 -1.614 237 1260 +1.485 237 1080 -1.586 210 1235 -1.597 147 1235 -3.80*8 201 1537 -1.609 178 1537 +0.50

10 325 1488 +1.6011 237 1516 -0.4612 254 2137 -3.50*

* These particularly low skin values may be enhanced as aresult of phase behavior effect associated with gas condensatereservoirs

Table 1. Well Test Results