Copyright 1999, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 1999 SPE European Formation DamageConference held in The Hague, The Netherlands, 31 May1 June 1999.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than 300words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

AbstractIn weak formations or in competent formations with potentialfor sanding problems from high in-situ stresses, screens andgravel packs often are not practical or economical. However,in such formations, the perforating program can significantlyaffect the production of sand. With knowledge of the in-situstress distribution around the target well, the technique ofoptimal phased and oriented perforating (OPOP) was appliedin the Eocene sands of the field in Lake Maracaibo,Venezuela, to minimize sand production. Results wereexcellent. In this paper, the OPOP technique is described andgeneral guidelines are given for its use. Field cases arepresented that compare results gained using this technique tothe performance of other wells in the same field.

IntroductionSand production has been a major problem in the Eocene Creservoir in Lake Maracaibo, Venezuela.1 The rock of thesereservoirs is competent and consolidated but under high in-situstresses caused by the complex tectonic environment. A stressstudy of the field has shown high contrast between themaximum and minimum horizontal stresses and similarmagnitude between the vertical and minimum horizontalstresses. Figure 1 shows the sand production history for thefield, indicating improvement in the last years. However, theaverage sand production of about 14 lbm/kbbl is stillconsidered high.

To minimize sanding, several techniques have been usedhydraulic fracturing, high-angle drilling and, in the last year,OPOP. The OPOP technique described in this paper has beenused in four wells with the best improvements in sandprevention.

In-Situ Stress DeterminationA detailed geomechanical study of the field was made tocharacterize the Eocene C reservoir.2,3 To control sandproduction and to extend the wells productive life, the criticaldrawdown pressure (CDP) was estimated from a 3Dsimulator.4 For a horizontal tunnel in the direction of themaximum horizontal stress, the CDP was estimated to be 40%of the initial reservoir pressure. This value was crossvalidatedwith a highly deviated pilot well.

The geomechanical properties in general can be determinedfrom laboratory measurements and from shear andcompressional well velocity logs. The critical rock mechanicalproperties needed to estimate the maximum horizontal stress

(H) were Youngs modulus (E), Poissons ratio (), Biotcoefficient () and the critical stress intensity factor (KIC).Table 1 shows the average value of the critical mechanicalproperties of the reservoir.

(psi)

KIC(psi*in.1/2)

UCS(psi)

3x106 0.3 0.9 1250 8000

Table 1. Average mechanical properties of the reservoir

The magnitude of the in-situ stresses were obtained as follows:

the vertical stress (v) was obtained by integrating the rock

SPE

Oriented Perforating for Sand PreventionA.L. Sulbaran, SPE, PDVSA, R.S. Carbonell, SPE, Intevep-PDVSA, J.E. Lpez-de-Crdenas, SPE, Schlumberger

0

10

20

30

40

50

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

San

d P

rod

uct

ion

(lb

m/k

bb

l)Figure 1 History of Sand Production in the field

2 A. SULBARAN, SPE, PDVSA, R. CARBONELL, SPE, INTEVEP-PDVSA, J.E. LPEZ-DE-CRDENAS, SPE, SCHLUMBERGER SPE

density from density logs; the minimum horizontal stress (h)was obtained from microfracture, minifracture, and leakoff

tests; the maximum horizontal stress (H) was estimated bymeans of the theory of poroelasticity and fracturemechanics.6,7,8 Table 2 shows a summary of the total in-situstress gradients of the field where Z is the vertical depth.

Table 2. In-situ stress gradients

An interesting feature of the field is that the maximum

horizontal stress H is the highest of the three; that is, H > v h. This is the result of field tectonics.

An observation in a more recent study of the magnitude of thestresses in the field, shows that the vertical wells in the areaswith larger contrast between the maximum and minimumhorizontal stresses have more sanding.9 This finding supportsthe recommendation of perforating to avoid the directions ofhigh contrast between horizontal and vertical stresses for thisfield.

To estimate the direction of H, a model was used at Petroleosde Venezuela S.A. (PDVSA) Intevep.10 The model usesborehole image log information and data from laboratory coremeasurements such as anelastic strain recovery, shear waveamplitude anisotropy and acoustic anisotropy analysis.

Figure 2 shows the map of the Eocene C reservoir with theestimated stress directions, where wide variations are basicallyinfluenced by the fault systems and the tectonic effects thatoccur in the area. Areas I and II in Fig. 2 show variation of the

stress direction, and Area III has more uniform stressdirection. This implies that hydraulic fracturing could develop

high tortuosity in I and II. Additionally, the close values of vand h suggest a possible fracture deviation duringpropagation.

Perforation Tunnel StabilityThe stability of a single perforation tunnel was evaluated usingan elastic-plastic model, a finite element analysis and Mohr-Coulomb failure criterion from laboratory testing. As theperforation tunnel is rotated about the axis of the borehole,there is a critical angle measured from the direction of themaximum stress where the tunnel is still stable. We call thisangle , the allowable perforation angle. This angle is used toselect the phasing and orientation of the gun. Figures 3 and 4show the meshing and strain field for this analysis with adrawdown pressure of 1000 psi. Figure 3 shows the equivalentplastic strain contour plot of a perforated tunnel oriented in the

direction of H.11According to the triaxial test laboratory data, failure takes

place after the equivalent plastic strain has exceeded the valueof 6.5 x 10-3.

Figure 4 shows the initiation of failure of the perforationtunnel rotated 25 from the position of Fig. 3; that is, 25

Stress Field (HH))

Area I

Area II

Area III

Figure 2 Estimated direction of the maximumhorizontal stress

v / Z(psi/ft)

H / Z(psi/ft)

h / (psi/ft)

1.10 1.351.40 1.051.10

Figure 4 Equivalent plastic strain contour plot of aperforation at an azimuth of +/- 25 from the direction of H

6.2E-03

Figure 3 Equivalent plastic strain contour plot of aperforating cavity in the direction of the maximum in-situstress (H)

3.8E-05

SPE ORIENTED PERFORATING FOR SAND PREVENTION 3

about the wells axis.12,13 From this analysis, we concludedthat, for this field, if stays within +/-25 the perforationtunnel is expected to be stable.

Recommendations: Perforating for Sand PreventionBased on experiments at the Schlumberger Perforating andTesting Center in Rosharon, Texas,14 and the geomechanicalstudies at PDVSA Intevep in Los Teques, Venezuela, thefollowing steps were followed in perforating the Venezuelafield and are listed here as recommendations:

Determine the magnitude and direction of the in-situstresses around the well location.

Define the zone around the wellbore where theperforation tunnel is expected to be stable.

Select appropriate deep-penetrating charges. Use a shot density for sufficient productivity (6 to 8

shots per foot in this case). Select a shot phasing that offers sufficient

perforation-to-perforation distance to avoid rockfailure.

Orient the guns to avoid shooting in the directionswhere the perforation tunnels are less stable (for thisfield, areas having the largest contrast between thehorizontal and vertical stresses).

Perforate with sufficient underbalance.

Figure 5 shows a top view of the plane of the perforations ofthe especially loaded tubing-conveyed perforating (TCP) gunscompared with the perforations of a standard gun.

ChargesDeep-penetrating charges, as opposed to big-hole charges, arerecommended for sand prevention in competent rocks for thefollowing reasons:

1) Deep-penetrating charges produce perforations of smallerdiameter than big-hole charges. Perforating tunnels ofsmaller diameter in rocks are more stable than large-diameter holes.14,15,16

2) The thickness of the crushed zone around the perforationtunnel increases with the diameter of the perforationtunnel.

3) Deep-penetrating charges have more chances ofperforating beyond the borehole-damaged zone andprovide a larger effective wellbore radius.

If the formation is not consolidated, or if a stimulation such ashydraulic fracturing is planned, then big-hole charges aregenerally recommended.

Shot DensityPerforating with high shot density is commonly desiredbecause of the expected higher productivity and lower flowvelocity in the perforations. However, increasing the shotdensity could reduce the perforation-to-perforation distance.The mechanical failure between adjacent perforationsobserved in the laboratory suggested that if the distance is notsufficient a failure may occur.14 The objective was then to findthe minimum distance to avoid rock failure and to havesufficient shot density within for adequate productivity.

Minimum Perforation-to-Perforation DistanceOperational restrictions allowed a minimum distance of aboutthree times the estimated diameter of the crushed zone 3,where is the diameter of the outer boundary of theperforation crushed zone at the borehole face. To verifyperforation tunnel stability under this condition, the followinganalysis was conducted.

To avoid possible rock failure resulting from the interactionbetween perforations, the minimum distance min (see Fig. 6)was validated based on two scenarios: 1) the expected stressprofile around one perforation tunnel, and 2) the superpositionof stresses from consecutive perforation tunnels. Theassumptions used were an elastic, homogeneous media withtwo parallel cylindrical tunnels under plane strain conditions.

h

H

H

v

H

h

Perfs in direction of largecontrast between horizontaland vertical stress could fail

Conventional gun Oriented gun

No perfs in areas of high stress contrast

Figure 5 Top view of perforation

Figure 6 Minimum perf-to-perf distance

4 A. SULBARAN, SPE, PDVSA, R. CARBONELL, SPE, INTEVEP-PDVSA, J.E. LPEZ-DE-CRDENAS, SPE, SCHLUMBERGER SPE

The expected stress field around one cylindrical perforationtunnel in an homogeneous isotropic elastic media indicatesthat the magnitude of the tangential stress around the tunnel

drops to about 1.1v 1.1h at a distance of about 1.

For consecutive perforation tunnels, the stress concentration inthe vicinity of one tunnel was first estimated. Then, based onthe Mohr-Coulomb criteria, min between the two cylindricalcavities at which no failure is expected was found to be lessthan 3 , supporting that a value of min = 3 is satisfactory.

Therefore, the shot density and phasing for the perforating gunmust satisfy two conditions: 1) to perforate within theallowable perforation angle = +/-25 measured from thedirection of H, and 2) to maintain a distance min. Beforedeciding the final shot density, maximization of theproductivity of the well should be considered.

Expected Productivity as Function of Shot DensityFigures 7 and 8 show the productivity and skin calculations asa function of the shot density for this field as calculated with aperforating and productivity numerical analysis15. For thiscase, shot densities below 6 shots per foot (spf) show a rapidreduction in productivity and a rapid increase in skin, andabove 8 spf there is no significant increase in productivity butthe risk of sanding may increase. Thus, to satisfy all therequirements for a borehole diameter of 7 in., a shot densityrange between 6 to 8 spf was selected.

The first three wells were perforated at 6 spf usingconventional guns but with shaped charges loaded in a patternto satisfy the above conditions. The last well was perforatedwith a custom-designed gun that allows 8 spf, still satisfies theoriginal requirements of the minimum perforation-to-perforation distance, and offers a more uniform distribution ofthe perforations within the allowable perforation angle .

OPOP OperationThree special components were used in the completion string:

1) 4-in. TCP guns with the special phasing discussedabove

2) orienting sub, placed on top of the guns, with an internalkey aligned with the guns that worked as a guide for agyroscope

3) hydraulic packer to avoid rotation to set the packer.

After positioning the guns on depth, a gyroscope was run onwireline through the string. With the gyroscope sitting on theorienting sub, the string was rotated until the desiredorientation was achieved. The hydraulic packer was set andthe orientation of the guns was verified with the gyroscope.Finally, after removing the gyroscope, the guns were firedunderbalanced.

Field CasesFour jobs have been performed using the OPOP technique forsand control. The improvements are evident when productionis compared with the average sand and oil production of otherwells in this field. The average production for the field is 14.3lbm/kbbl of sand and 1500 BOPD. In all four cases, the sandproduction was reduced within or below 3 lbm/kbbl.

At the time this paper was prepared, the first job had the mostcomplete production data, which showed sand productionbelow 0.5 lbm/kbbl with an initial oil production of 4400BOPD. The second well produced about 2200 BOPD (stillabove average) and 3.0 lbm/1000 bbl of sand. The sandproduction of the third job was reported at 3.0 lbm/kbbl withan initial oil production of 682 BOPD. The fourth job had asand production of 0.4 lbm/kbbl with an initial oil productionof 1085 BOPD. Table 3 compares the average oil and sandproduction of these four jobs with the fields average. In allfour cases the objective of sand prevention was successfullyachieved.

2

46

9 12

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15

Shot Density (spf)

Pro

du

ctiv

ity

Rat

io

Figure 7 SPAN Computation of the Productivity Ratio

2

4

6

912

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 5 10 15

Shot Density (spf)

Ski

n

Figure 8 SPAN Computation of Skin

SPE ORIENTED PERFORATING FOR SAND PREVENTION 5

FieldAverage

Job 1

Job2

Job3

Job4

Initial Oil(BOPD)

1500 4427 2189 682 1085

AverageSand(lbm/kbbl)

14.3 0.5 3.0 3.0 0.4

Table 3. Oil and sand production comparison

Figure 9 shows the oil production from the first job during thefirst nine months, and Fig. 10 shows the corresponding sandproduction. The initial oil production of 4400 BOPD is aboutthree times more than the average production of the wells inthe field. Sand production of 0.5 lbm/kbbl is only 3.5% of theaverage well in the field.

In all cases, the wells are producing with an approximate 1500psi of drawdown pressure. This suggests the model used topredict the collapse of the perforation tunnel as it is rotatedhorizontally away from the direction of maximum horizontalstress was adequate and conservative.

Future WorkThe assumptions and model used for this analysis seem toprovide adequate and conservative ranges of the controlling

parameter. However, a more detailed modeling would beuseful to determine the optimum values of the controllingparameters. Optimization of these parameters could allowshooting at higher shot densities and within a wider allowableperforation angle. A 3D finite element analysis could be veryuseful in this process.

ConclusionsThe presented methodology seems to be effective for sandprevention in competent rocks under high in-situ stressmagnitudes and contrast.

The controlling parameters, allowable perforation angle andthe minimum perforation-to-perforation distance min, arecharacteristic of a given reservoir (i.e., in-situ conditions andgeomechanical properties) and can be obtained from theborehole stability analysis of a perforation tunnel.

An optimum gun design to prevent sand production is afunction of the formation damage, reservoir properties,completion scheme and constrains given by and min.

Using deep-penetrating charges with oriented guns andshooting underbalanced have yield good well productivitywhile reducing sanding significantly.

The recommendations from laboratory observations byBehrmann14 appear to be consistent with the field results.

The presented methodology can also be used to identify thepreferencial fracture plane and orient the perforationsaccordingly for hydraulic fracturing operations.

AcknowledgmentsThe authors thank PDVSA and Schlumberger for their supporton this work.

References1. Urdaneta, I., Sulbaran, A., Hernandez, T.: Development ofEocene Complex Reservoir in Ceuta Field Lake MaracaiboVenezuela, paper SPE 53994, presented at the 1999 SPELatin America and Caribbean Petroleum EngineeringConference, Caracas, Venezuela, April 21-23, 1999.

2. Franquet, J., Gonzalez, H., Linares, J., Natera, J. y Quenza,R.: "Anlisis y Caracterzacin Geomecnica, Area-2 Sur,Campo Ceuta", Informe Tcnico, INT-3539, PDVSA Intevep,Los Teques, Venezuela, 1997.

3. Vasquez, A.: Correlaciones Preliminares para definirDrawdown en el Arenamiento de Pozos enel Lago deMaracaibo, VVA Consultores, Caracas, Venezuela, Octubre1996.

Oil Production - case Study #1

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Figure 9 Oil production of first OPOP job

Sand Production History - Case Study #1

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Figure 10 Sand production of first OPOP job

6 A. SULBARAN, SPE, PDVSA, R. CARBONELL, SPE, INTEVEP-PDVSA, J.E. LPEZ-DE-CRDENAS, SPE, SCHLUMBERGER SPE

4. Morita, N.: Sand-3D Finite Element Sand Production andWell Mechanic Analysis System, User Manual, 1994.

5. Love, A.E.H.: A Treatise on the Mathematical Theory ofElasticity, 4th ed., Dover, New York, 1927.

6. M. J. Economides and K.G.Nolte, Reservoir Stimulation,second edition, Prentice-Hall Inc., NJ (1989) 2-1.

7. Detournay, E. and Carbonell R.: Analysis of theBreakdown Process in Minifrac or Leakoff Test, paper SPE28076, presented at the 2nd SPE/ISRM Rock Mechanica inPetroleoum Engineering (EUROCK 94) InternationalConference, Delft, Netherlands Aug 29-31, 1997.

8. Carbonell R. S. and Detournay E.: Modeling FractureInitiation and Propagation from a Pressurized Hole: ADislocation Based Approach, proceedings of the 35th U.S.Symposium on Rock Mechanics, Tahoe, Balkema, Rotterdan,1995.

9. Faustino, M., Franquet, J., Sanchez, M., DeterminacionesAnalticas de Tendencias de arenaminto en el rea 2 Sur,INT 03424, PDVSA Intevep, Los Teques, Venezuela, 1997

10. PDVSA Intevep, GEOSTRESS, Version 3.2, Diciembre,1997

11. Hibbitt, Karlsson & Sorensen, Inc.: ABAQUS, UsersManual, Volume I, Version 5.6, USA, 1996

12. Fuentes, J. y Carbonell, R.: "Estabilidad de CavidadesCaoneadas Orientadas," Informe Tcnico, en publicacin,PDVSA Intevep, Los Teques, Venezuela, 1999.

13. Carbonell, R., Fuentes, J. y Heath L.: "Estudio deEstabilidad de Cavidades, Ceuta Area 2 Sur", InformeTcnico, INT-4701, PDVSA Intevep, Los Teques, Venezuela,1998.

14. Behrmann, L.A., Willson, S.M., de Bree, PH and Presles,C.: Field Implications from Full-Scale Sand productionExperiments, paper SPE 38639, presented at the 1997 SPEAnnual Technical Conference and Exhibition, San Antonio,Texas, October 5-8, 1997.

15. Papanastasiou, P. and Vardoulakis I.: Numericaltreatment of progressive localization in relation to boreholestability, Int. J. Num. Anal. Meth. Geomech., Vol. 16, (1992)398.

16. Papanastasiou, P. and Zervos, A.: Stability ofPerforations Near Hydraulic Fractures, Int. J. of Rock Mech.& Min. Sci. Paper No. 91, 1998.

17. Schlumberger Perforating Analysis, SPAN/Win95, Version

5.1, January, 1998.