Copyright 2000, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 2000 SPE International Symposium onFormation Damage Control held in Lafayette, Louisiana, 2324 February 2000.

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AbstractSand prevention implies an acceptable risk of sand productionover the producing life of the well with no sand controlmechanisms implemented. This paper reviews availablemethods to optimize the choice of perforation parameters(phasing, shot density and charge type) for sand prevention.Prior work has shown that sand production is preceded byfailure of the perforation tunnels. In order to have successfulsand prevention it is necessary to have stable perforationtunnels through rate (drawdown) changes, depletion, andwater-cut. Available methods to determine the ability ofperforation tunnels to produce sand free can be classified intotheoretical models, experimental methods and historicaltechniques.

Deep penetrating charges are recommended as theyproduce smaller diameter perforation tunnels that are morestable than larger diameter tunnels produced by big holecharges. Optimum phasing technique relies on themaximization of distance between adjacent perforations in 3-dimensional space for a given wellbore radius and shotdensity. This is advantageous in avoiding inter-linking offailed zones around adjacent perforations. Data from theMagnus field in the North Sea supports the use of these twotechniques in minimizing sand production. Where there aresignificant stress contrasts in the formation and the directionsare known, oriented perforating can be used to increase thestability of perforation tunnels (especially when increasingdrawdown and when depleting the reservoir).

It is shown how these three main techniques can be used toperforate for sand prevention. In addition, the paper alsoprovides guidelines on how to avoid sand production at thetime of perforation, selective perforating where there is a

contrast in formation strength with depth and the use ofexperimental techniques to determine perforation stability dueto rate (drawdown) changes, depletion and water-cut.

IntroductionIn most unconsolidated and weakly consolidated wells aroundthe world, traditional approach has been to use sand controltechniques whenever there was a risk of sand production. Thiswas driven mainly by safety (erosion of surface hardware) andeconomic concerns. However many wells where sand controlmechanisms are installed have proven to be costly in terms ofproductivity impairment. There has been a two-fold approachto tackling this problem: a) determine the sources ofimpairment to sand control methods and find out how tominimize them1, and b) prudent use of sand preventiontechniques as opposed to total sand exclusion.

The essence of sand management is the quantification ofthe risk of sand production that helps decide if/how/when sandexclusion (control) or sand prevention should be implemented.Sand prevention incorporates methods to minimize the amountof sand produced and also methods to minimize the impact ofsand produced. The objective of this paper is to outline bestperforating practices for minimizing the amount of sandproduced over the producing life of cased and perforatedwells. Three main events are responsible for sand production:rate or drawdown changes, depletion (effective stress) andwater cut.

Sand production is a two-part decoupled phenomenon:Sand must be separated from the perforation tunnel (failure),and the flowing fluid must transport the failed sand. Stress,controlled by drawdown and depletion does the first, and rate,also controlled by drawdown does the second2. Using thistheory sand production is dictated by the stability ofperforation tunnels. Prior to perforating for sand prevention itis necessary to determine whether the tunnels would be stableover the producing life of the well.

Perforation Tunnel Stability DeterminationFor successful sand prevention, a good understanding of thestability of the tunnels over the producing life of the well isneeded before completion. Three different approaches areused by the industry to accomplish this.

SPE 58788

Perforating Requirements for Sand PreventionA. Venkitaraman, SPE, L.A. Behrmann, SPE, Schlumberger Reservoir Completions, A. H. Noordermeer, SPE, BP AmocoExploration

2 A. VENKITARAMAN, L.A. BEHRMANN, A. H. NOORDERMEER SPE 58788

Theoretical Models: The models originally developed forborehole stability are extended to perforations. Three steps areused, determination of rock mechanical properties (using logdata, core samples), determination of in-situ stress conditions,and determination of failure (conditions) using a particularmodel3. Theoretical models are effective in predictingperforation stability with change in stress conditions(drawdown and depletion). Two distinct approaches have beendeveloped: the tensile failure model and the shear failuremodel.

According to the tensile failure criterion the fluid flow intoa cavity at high production rates will induce a tensile stressnear the cavity resulting in formation failure (sand grainsbeing pulled away from the tunnel) and subsequent sandproduction4. This model is seldom used as numerical studiesand experiments indicated that this criterion predictsunrealistically high production rates to initiate sand productionin weak but consolidated sandstone. Also, some sandproduction experiments showed stress-induced shear failure toprecede sand production5. Shear failure models can beclassified according to the assumed material behavior: linearelastic/brittle, elasto-plastic. The models can also be classifiedaccording to the assumed geometry (simple 1D to 3D). Thematerial property requirements and the complexity increase inthe more sophisticated geometry and material behaviorconditions. Mohr-Coulomb criterion is most widely used forshear failure assessment.Experimental Methods: Experimental methods involvetesting of available reservoir core samples or outcrop rocksamples (with similar mechanical properties). There are twodifferent types of test: drilled hole tests and single-shotperforation and flow tests.

In a typical drilled hole test, a cylindrical cavity of uniformdiameter is drilled in a core sample. The drilled sample is thenplaced inside a rubber sleeve and isotropic confining pressureis applied on the outside of the core6. The stress on the sampleis increased until the yield point is reached. According toelastic theory when the circumferential stress on the inner wallof the hole reaches the (apparent) strength of the material thehole will fail. The main drawback is that the sample size/holesize ratio of the hollow cylinder can influence the resultobtained7.

Though not widely used, available core sample from thewell is perforated and flowed at different rate, depletion andwater-cut conditions8. The test parameters can be chosenbased on the expected conditions during the producing life ofthe well. This method can be used to augment analyses fromtheoretical models and to check for sand production duringwater-cut. The tests can also help determine (the stability ofperforation tunnel or) sand production at the time ofunderbalance perforating. The drawbacks to this method arethe discrete nature of data (core sample from specific depths)and availability of samples.Historical: Historical sand production prediction criteria relyon production experiences (rate, drawdown, percentage water-cut) on other wells in the same reservoir to arrive at a choicebetween sand control and sand prevention. In some cases

reservoir strength data is used as the yardstick to compare andpredict potential for sanding across different reservoirs. This isby far the most widely used technique. The best use of thisapproach utilizes available data to calibrate theoretical modelsfor future sand production prediction.

Perforating for StabilityCharge Type, Shot density and Phasing: For maximumsingle perforation stability use deep penetrating charges.Smaller holes (deep penetrating charge) are more stable thanlarger holes (big hole charge). In addition to single perforationstability one has to consider inter-linking of failed zonesaround adjacent perforation tunnels. This can lead to collapseof structure inducing massive sand production. Besides thestability of individual perforation tunnels this is dictated by theperforation spacing in the wellbore. The perforation spacing isdictated by the shot density and phasing. Though the shotdensity can be decreased to increase perforation spacing thiswill have the undesirable effect of increasing the flow rate perperforation (which can enhance transport of any failedmaterial leading to sand production). Optimum phasing usingSandFreeX guns will allow the maximization of perforationspacing for a given wellbore radius and shot density. Figure 1demonstrates the principle behind optimum phasing (notdrawn to scale), showing a gun with charges at 60 degreephasing, and the perforations in the formation sandface (intwo-dimensional form, with the corresponding angles markedin the bottom). The idea behind optimum phasing is tomaximize the perforation-to-perforation spacing for a givenshot density to preserve as much as possible, the interveningformation material. In the figure the three distances betweenadjacent perforations are marked L1, L2, and L3. By adjustingthe phasing for a given wellbore radius (R) and shot density,the distances can be optimized (with the ideal of obtaining L1=L2=L3) to avoid interaction between adjacent perforations.

Figure 2 shows optimum phasing (or optimum perforationspacing) calculation for different total wellbore radius(wellbore radius multiplied by shot density). In reality, it is notpossible to have L1=L2=L3 for a spiral phased gun. Theoptimum solution (maximum perforation-to-perforationspacing for a given shot density) occurs when any two of theabove perforation spacings are equal. The example shown inFigure 1 show the critical perforation spacings between thefirst two spiral wraps. For R*spf > 42, an additionalperforation spacing, L4, between the first and third wrap mustbe considered and will control the minimum perforationspacing. Simple algebraic equations are used to calculate theseperforation-to-perforation spacings. The discontinuities inFigure 2 are a result of different pairs of Ls being equal. Forsimplicity, the wellbore radius (R) is used in Figure 2;however this is only true for a centralized gun. The exactdefinition of R is the distance from the perforator jet virtualorigin (a mathematical term for the location within theperforator where the jet is assumed to originate) to thesandface. For practical purposes, the distance from the center X Mark of Schlumberger

SPE 58788 PERFORATING REQUIREMENTS FOR SAND PREVENTION 3

of the perforating gun to the sand face can be used to definethe distance (R). For an eccentered or a non-oriented gun, theminimum distance from the center of the gun to the sandfacewill determine the optimum phasing. Thus, the minimumperforation-to-perforation spacing will be on the low side ofthe well.

The increase in perforation-to-perforation spacing for anoptimum phased gun can be substantial when compared withthe standard phased guns. For example, for R*spf=25.5, theminimum perforation spacing increased from 4.88 inches to7.61 inches, a 56% increase by changing the phasing from 60degrees to 99 degrees.Field Data (Magnus): Evidence of the effectiveness ofoptimum phased perforations in minimizing sand productioncomes from field data evaluation of BP Magnus in the NorthSea. The Magnus reservoir (age: Upper Jurassic) is split up intwo main selections: Magnus main sand MSM (top sands) andLKCF (lower sands). There is communication between thelayers but only in some parts of the reservoir. MSM, Magnusmain sand is high permeability (500 md), high porosity, thicksands with high net to gross ratio whereas LKCF is mediumpermeability (200 md), medium porosity, thin sands with lownet to gross ratio. Initially the reservoir pressure was 6653 psiat 240F (3050m TVD) but this dropped quickly to 3000 psidue to insufficient water injection in some parts of thereservoir. Increased and more efficient water injectionreversed this process and the reservoir pressure is back up(MSM 3000 to 6000 psi & LKCF 3000 to 4000 psi). The oil is39 API with 0.30 cp viscosity at reservoir conditions and totalproduction of 90,000 reservoir barrels a day. Watercut variesfrom 0 to 95 %. The wells are completed with 5-inch tubingwith 5-inch or 7-inch liner. Most wells are completed withgaslift. Typical well depths are 4000 to 5000m MD (2900 to3300m TVD) with deviations from 40 to 60 degrees.

The original perforating strategy used 3 3/8 inch guns at 60degree phasing (6 spf). This was later changed during 1997 to99 degree phasing (optimum phasing) while keeping the sameshot density. Comparison of wells perforated with the twodifferent phasings showed a decline in sand productionevidenced by the decrease in problems associated with sandproduction.Theoretical Validation: A 2-D plane strain model of the twodifferent perforating strategies was used to simulateperforation tunnel failure due to increasing effective stress.For comparative purpose normalized values of effective stresswere used in both cases assuming elasto-plastic behavior ofthe rock material. For increasing effective stress (depletion ordrawdown) it can be seen that the inter-linking of failed zonesaround adjacent perforation tunnels happen at a 33% lowereffective stress for the 60 degree phased perforation tunnelsthan the 99 degree phased perforation tunnels (Figure 3).

Oriented PerforatingIn regions where there is a large contrast between the vertical,maximum and minimum horizontal stresses, perforationsshould be oriented in the direction of maximum stability9. Inthese cases, if the rates per perforation are not too high, 0/180

degree phased perforating guns can be used. If the rate perperforation is a concern: For vertical wells, shoot in directionof maximum perforation tunnel stability at a +/- angle of phi(see Figure 4) and for horizontal wells shoot up/down at a +/-angle of phi. Phi is dependent on the in situ stresses and willtypically be between 15 and 25 degrees. Phi can also beoptimized using plain strain simulation mentioned in theprevious section if in-situ stress conditions and reservoirproperties are known.

The concept of optimum phasing for an oriented gun(using SandFreeX guns) is similar to that of a continuousphased non-oriented gun: to have a maximum shot density fora given perforation-to-perforation spacing. The currentpractice is to use 0/180 degree phased guns shot in thedirection of maximum perforation stability. As an example,assume a minimum perforation-to-perforation spacing of 4inches is required. This allows only 6 spf for a 0/180 degreephased gun. Because sand production requires both loose sand(failed perforation tunnel) and sufficient flow velocity totransport the sand, one would like to increase the shot densityto minimize the transport of any failed sand. Since theperforation stability changes slowly with modest angularmisalignment between the preferred and proposed perforationorientation, a substantial increase in shot density is possible byshooting an angle of +/- phi to the preferred direction. Usingthe above example, for a wellbore radius of 4.25 inches,shooting at +/- 22.5 degrees from the preferred direction givesan 81% increase in shot density from 6 to 10.9. In thisexample, each shot is in a different axial plane.

For larger wellbores, the minimum perforation-to-perforation spacing changes from adjacent perforationsseparated by an angle of 2*phi to adjacent perforations at thesame azimuth. In this configuration, there are two shots peraxial plane and the maximum shot density is 48/L, where L isthe minimum perforation-to-perforation spacing.

Evidence of the effectiveness of oriented perforating forsand prevention comes from two earlier publishedapplications. The technique of optimal phased orientedperforating was applied to the Eocene C reservoir in LakeMaracaibo, Venezuela10. Another case is the Andrew field inthe North Sea where the perforations were oriented at an angle+/- 25 degrees in the topside in horizontal wells 11. In bothcases the perforating strategy used deep penetrating charges.

Other ConsiderationsUnderbalance Perforating: One of the main reasons forperforating underbalance is to reduce the extent ofpermeability damage in the crushed zone (extent of damagedzone around the perforation tunnel walls). If this material isnot removed at the time of perforation, it will result in a largerpressure drop at the perforations that can contribute to tensilefailure12. This may or may not constitute a sand productionproblem (depending on whether the failure occursimmediately or at later stages when the drawdown isincreased, or reservoir depletes, or during water-cut and alsodepending on whether this material is transported). Perforatingat underbalance allows us to produce the sand during the

4 A. VENKITARAMAN, L.A. BEHRMANN, A. H. NOORDERMEER SPE 58788

initial stages and thus avoid having to manage transient sandproduction during later stages of well production13. Theunderbalance value must be chosen to avoid catastrophicfailure of the formation (sanding in the guns) at the time ofperforation. The limit on the underbalance can be chosenbased on values obtained from perforation stability model(keeping the underbalance value below the critical drawdownvalue). Single-shot perforation and flow experiments can beused to confirm the underbalance value chosen.Selective Perforating: In formations where the strength variesdrastically with depth, by avoiding perforating in sections thatare weaker, one can maintain sand-free production throughoutthe reservoir life11. Both productivity analysis using nodalanalysis programs (to study the impact of partial penetrationon productivity) and strength analysis (using methodsmentioned in previous section) need to be carried out prior tomaking this choice.

Summary1. Use deep penetrating charges to minimize perforation

damage, for tunnel stability through depletion anddrawdown, and to have good perforation spacing.

2. Use optimum phasing to minimize inter-linking of failedzones around adjacent perforations (minimize risk ofcollapse of structure) without compromisingrate/perforation.

3. Use maximum shot density to keep rate/perforation belowa critical value to minimize transport of sand.

4. Perforate at optimum underbalance to minimizeperforation damage. Keep underbalance below a criticalvalue to minimize perforation failure at the time ofperforation.

5. Orient perforations for maximum perforation stability incases where there is a large stress contrast.

6. Manage initial transient sand production.7. Core Studies will help obtain maximum drawdown/rate to

prevent sand production through failure/transport and alsoknow the impact of water-cut.

Nomenclaturespf =Shots per foot (shot density)

MD =Measured DepthTVD =True Vertical Depth

R = Radius of Wellbore (Sandface), inchesL =Minimum Perforation Spacing, inches

AcknowledgementsThe authors thank the BP-Amoco and Schlumberger ReservoirCompletions organizations for permission to publish thepaper.

References1. Blok, R.H.J., Welling, R.W.F., Behrmann, L.A.,

Venkitaraman.A.: "Experimental Investigation on the Influenceof Perforation Induced Gravel-Pack Impairment", SPE 36481,

presented at the 1997 SPE Annual Technical Conference andExhibition, Denver, Colorado, Oct 6-9.

2. Kooijman, A.P., van Den Hoek, P.J., Ph. de Bree, Kenter, C.J.,Zheng, Z., Khodaverdian, M.: Horizontal Wellbore Stabilityand Sand Production in Weakly Consolidated Sandstones, SPE36419, presented at the 1997 SPE Annual Technical Conferenceand Exhibition, Denver, Colorado, Oct 6-9.

3. Bruce, S.: "A Mechanical Stability Log", SPE 19942, presented atthe 1990 IADC/SPE Conference, Houston, Texas, Feb 27 - Mar2.

4. Weingarten, J., Perkins, T.: "Prediction of Sand Production in GasWells: Methods and Gulf of Mexico Case Studies", SPE 24797,presented at the 1992 SPE Annual Technical Conference andExhibition, Washington DC, Oct 4-7.

5. Behrmann, L.A., Willson, S.M., Ph. De Bree, Presles, C.: "FieldImplications from Full Scale Sand Production Experiments",SPE 38639, presented at the 1997 SPE Annual TechnicalConference and Exhibition, San Antonio, Texas, Oct 5-8.

6. Presles, C., Cruesot, M.: "A sand Failure Test can cut bothCompletions Costs and the Number of Development Wells,SPE 38186, presented at the 1997 SPE European FormationDamage Conference, The Hague, The Netherlands, Jun 2-3.

7. Papamichos, E., van Den Hoek, P.: "Size dependency ofCastlegate and Berea Sandstone Hollow Cylinder Strength onthe Basis of Bifurcation Theory", Proceedings, 35th USSymposium on Rock Mechanics (1995).

8. Venkitaraman, A., Li, H., Leonard, A.J., Bowden, P.R.:"Experimental Investigation of Sanding Propensity for theAndrew Completion", SPE 50387, presented at the 1998 SPEInternational Conference on Horizontal Well Technology,Calgary, Alberta, Canada, Nov 1-4.

9. Santarelli, F.J., Ouadfel, H., Zundel, J.P.: "Optimizing theCompletion Procedure To Minimize Sand Production Risk",SPE 22797, presented at the 1991 SPE Annual TechnicalConference and Exhibition, Oct 6-9.

10. Sulbarban, A.L., Carbonell, R.S., Lopez-de-Cardenas, J.E.:"Oriented Perforating for Sand Prevention", SPE 57954,presented at the 1999 SPE European Formation DamageConference, The Hague, The Netherlands, May 31 - Jun 1.

11. Mason, J.N.E., Gomersall, S.D.: Andrew/Cyrus Horizontal WellCompletions, SPE 38183, presented at the 1997 SPE EuropeanFormation Damage Conference, The Hague, The Netherlands,Jun 2-3.

12. Morita, N., Burton, R.C., Davis, E.: "Fracturing, Frac Packing,and Formation Failure Control: Can Screenless CompletionsPrevent Sand Production?", SPE 51187, SPE Drilling &Completion, Sep 1998, pp 157-162.

13. Behrmann, L.A., Li, J., Venkitaraman, A., Li, H.: "BoreholeDynamics During Underbalanced Perforating", SPE 38139,presented at the 1997 SPE European Formation DamageConference, The Hague, The Netherlands, Jun 2-3.

SI Metric Conversion Factorsin. 2.54* E+00 = cmpsi 6.894757 E+00 = kPamd 9.869233 E-12 = cm2

* Conversion factor is exact

SPE 58788 PERFORATING REQUIREMENTS FOR SAND PREVENTION 5

L1

L 2

L3

0 60 120 180 240 300 360

Figure 1 Perforations at the wellbore sandface are shown in 2-Dgeometry along with the critical distances between adjacentperforations

80

100

120

140

160

180

200

0 30 60 90 120 150

Distance * SPF

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Phase Angle

Perf-spacing/Distance

Figure 2 Optimum phasing for a given wellbore radius at thesandface and shot density (Distance = R, the wellbore radius atsandface)

Figure 3 Results of 2-D plane strain elasto-plastic simulation ofinter-linking between failed zones around adjacent perforations(60 degree phasing and 99 degree phasing). The effective stressis increased (depletion) as one moves down the column. The lefthand column shows the 60 degree phased adjacent perforationsand the right hand column shows the 99 degree phasedperforations. For similar inter-linking to occur for 99 degreephased perforations the effective stress would have to be a factorof 1.3 times the stress at which inter-linking occurred for the 60degree phased perforations

L

0 60 120 180 240 300 360

2 phi

Figure 4 Oriented perforating shown in 2-D

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