52 Oilfield Review Spring 2000

Modern perforating is inseparable from otherservices that improve well productivity, such asfracturing, acidizing and sand control or preven-tion.2 In addition to being conduits for oil and gasinflow, perforations provide uniform points ofinjection for water, gas, acid, proppant-ladengels for hydraulic fracture stimulations and fluidsthat place gravel to control sand in weak orunconsolidated formations.3 In other sand-man-agement applications, perforating provides therequired number, orientation and size of stableholes to prevent sand production.

Conveyance methods have also kept pacewith perforating technology and practices. In

the late 1970s and early 1980s, perforatingstrategies were limited to smaller

through-tubing or larger casing gunsconveyed primarily by wireline.

Charges for each gun size andtype were designed for

either maximum holesize or deep pen-

etration. By

the mid-1980s, conveyance choices were expand-ing. Since that time, tubing-conveyed perforating(TCP) grew from limited use in a small niche mar-ket to an essential element of many well comple-tions and an important perforating tool.4

In addition to coiled tubing, slickline andsnubbing units, systems are now available to runlong gun strings in live wells under pressure.These perforating and conveyance systems alsoperform other functions that fulfill completionneeds of varying complexity, such as releasingand dropping guns, setting packers and openingor closing valves. In the future, charges may beincorporated in and run directly with completionequipment during well construction.

This article reviews key aspects of perforat-ing, including basic physics, new charges andmanufacturing, perforation damage mitigation,optimized perforation parameters, perforatingpractices for natural, stimulated or sand-man-agement completions, safety and conveyancemethods. We also discuss the reasons for con-sidering specific formation, well and comple-tion requirements when selecting perforatingtechniques. Examples show how perforation

designs customized for specific reservoir andperforation interactions can maximize

well performance.

Establishing communication with oil and gas zones involves more than shooting

holes in steel casing by choosing guns and conveyance methods from a service

catalog. Perforating based on average formation properties and shaped-charge

performance is being replaced by a more tailored approach. Perforation design is

now an integral, often customized, element of completion planning that addresses

reservoir conditions, formation characteristics and specific well requirements.

Larry BehrmannJames E. BrooksSimon FarrantAlfredo FayardAdi VenkitaramanRosharon, Texas, USA

Andrew BrownCharlie MichelAlwyn NoordermeerBP AmocoSunbury on Thames, England

Phil Smith BP Amoco Houston, Texas

David Underdown Chevron Production & Technology CompanyHouston, Texas

For help in preparation of this article, thanks to JimAlmaguer, Bobby Carroll, John Corben, Janet Denney,Brenden Grove, Brad Hoffman, Manish Kothari, Jason Mai,Sam Musachia, Bob Parrott, Mark Vella, Ian Walton andWenbo Yang, Rosharon, Texas, USA; and Andy Martin,Aberdeen, Scotland.Bigshot, CIRP (Completion Insertion and Removal underPressure equipment), CleanSHOT, Enerjet, FIV (FormationIsolation Valve), GunStack, HSD (High Shot Density gunsystem), HyperJet, IRIS (Intelligent Remote ImplementationSystem), NODAL, PERFPAC, Pivot Gun, PowerFlow,PowerJet, QUANTUM, S.A.F.E. (Slapper-Actuated FiringEquipment), Secure, SPAN (Schlumberger PerforatingAnalysis), UltraJet, UltraPack and X-Tools are marks ofSchlumberger.

Modern perforating. Controlleddetonation of specially designedand manufactured explosiveshaped-charges creates path-ways from well to formationthrough steel casing, cement andreservoir rock so fluids can flowor be lifted to surface.

Perforated completions play a crucial role in hydro-carbon production. From well testing for reservoirevaluation to completion and remedial interven-tion, perforating is a key to successful exploration,economic oil and gas production, long-term wellproductivity and efficient hydrocarbon recovery.The perforating process instantaneously generatesholes—perforations—in steel casing, surroundingcement and the formation (next page).

Both well productivity and injectivity dependprimarily on near-wellbore pressure drop, com-monly referred to as skin, which is a function ofcompletion type, formation damage and perfora-tion parameters. In the past, perforations oftenwere characterized simply as holes in steel casingmade by mechanical cutters (before 1932), shoot-ing bullets (since 1932), pumping abrasives (since1958) or more commonly, by detonating specialshaped-charge explosives made specifically foroilfield perforators (since 1948).1 Far from simple,perforating is a complex element of well comple-tions brought into better focus by contemporaryresearch and an understanding of basic principles.

Deviation from symmetry reduces shaped-charged performance. In terms of penetrationand hole size, optimized designs and precisionmanufacturing are improving shaped charges.Strict quality-control and aggressive quality-assurance further ensure charge reliability. As aresult, perforating test results are more consis-tent and translatable to downhole conditions forperformance projections and productivity esti-mates.

Among the many advances in perforatingtechnology are new deep-penetrating charges thatincrease well productivity by shooting beyondinvasion, and big-hole charges for gravel packing.Increased performance per unit of explosivemakes these high-performance charges moreefficient. In the past two years, improved chargeshave yielded penetration depths and flow areasthat are many times greater than those achievedusing prior technology. Other developments con-trol debris, especially in high-angle or horizontalwells, by reducing debris size or retaining debrisinside charge carriers—guns.

Perforating is the only way to establish con-ductive tunnels that link oil and gas reservoirs tosteel-cased wellbores which lead to surface.However, perforating also damages formationpermeability around perforation tunnels. Thisdamage and perforation parameters—formationpenetration, hole size, number of shots and theangle between holes—have a significant impacton pressure drop near a well and, therefore, onproduction. Optimizing these parameters andmitigating induced damage are importantaspects of perforating. Ongoing research con-firms that underbalance—a wellbore pressurebefore perforating that is less than the formationpressure—is essential to partially or, in somecases, completely remove damage and debrisfrom perforations.

53

1. Behrmann L, Huber K, McDonald B, Couët B, Dees J,Folse R, Handren P, Schmidt J and Snider P: “Quo Vadis, Extreme Overbalance?” Oilfield Review 8,no. 3 (Autumn 1996): 18-33.

2. Martin A: “Choosing The Right Gun,” Petroleum EngineerInternational 71, no. 10 (October 1998): 59-72.

3. Naturally occurring or resin-coated sand and high-strength bauxite or ceramic synthetics, sized by screen-ing according to standard U.S. mesh sieves, are used as proppants. Gravel consists of extremely clean, round and carefullysized sand that is small enough to act as a filter andprevent production of formation particles, but largeenough to be held in place across productive intervalsby a slotted-screen assembly.

4. Cosad C: “Choosing a Perforation Strategy,” Oilfield Review 4, no. 4 (October 1992): 54-69.

Perforating Practices That Optimize Productivity

>

Shaped-Charge Dynamics Perforations are created in less than a second byshaped charges that use an explosive cavityeffect, which is based on military weapons tech-nology, with a metal liner to maximize penetra-tion (below). Perforating charges consist of aprimer, outer case, high explosive and conicalmetal liner connected to a detonating cord. Eachcomponent must be made to exact tolerances. Atthe Schlumberger Reservoir Completions Center(SRC) in Rosharon, Texas, USA, these charges aredesigned, manufactured and tested to meet strictquality standards.

A detonating cord initiates the primer anddetonates the main explosive. The liner collapsesto form a high-velocity jet of fluidized metal par-ticles that is propelled along the charge axis. Thishigh-energy jet consists of a faster tip and slowertail. The tip travels at about 4.4 miles/sec [7 km/sec], but the tail moves more slowly, lessthan 0.6 miles/sec [1 km/sec]. This velocity gradi-ent stretches the jet so that it penetrates casing,cement and formation. Perforating jets erodeuntil all energy is expended at the end of a per-foration tunnel.

Perforating jets act like high-velocity, rapidly-expanding rods. Rather than by blasting, burning,drilling or abrasive wearing, penetration isachieved by extremely high impact pressures—3 million psi [20 GPa] on casing and 300,000 psi[2 GPa] on formations. These enormous jetimpact pressures cause steel, cement, rock andpore fluids to flow plastically outward. Elasticrebound leaves shock-damaged rock, pulverizedformation grains and debris in the newly createdperforation tunnels.

Charge Design and Performance Shaped charges are designed to generate opti-mal combinations of hole size and penetrationusing a minimum of explosive material.Asymmetric, or crooked, jets reduce charge per-formance, so perforating jets must form exactlyaccording to design specifications. Consequently,shaped-charge effectiveness depends on chargesymmetry and jet characteristics. Penetrationdepends on consistently achieving long jets withoptimal velocity profiles. A velocity profile mustbe established from tip to tail, and perforatingjets need to travel as fast as possible. Incorrectvelocity profiles decrease penetration.

Hole size is related to jet shape. Initially, solid-metal liners, often copper, were used to generatehigh-density jets and big holes, but this createdundesirable metal slugs, or carrots, that plug per-forations. This plugging was believed to be offsetby large-diameter holes and the high permeabilityof formations where big-hole charges are used.Technology that eliminates slugs and maximizesarea open to flow (AOF) has revised this approach.Although solid copper liners are still used in somebig-hole charges, recent designs generate jetswithout a solid-metal slug.

54 Oilfield Review

5. Klotz JA, Krueger RF and Pye DS: “Effect of PerforationDamage on Well Productivity,” Journal of PetroleumTechnology 26 (November 1974): 1303.

6. On November 25, 1998, a gun loaded with new deep-penetrating PowerJet charges averaged 54.1 in. [137 cm]of penetration when fired into an API target.

7. Smith PS, Behrmann LA and Yang W: “Improvements inPerforating Performance in High Compressive StrengthRocks,” paper SPE 38141, presented at the SPEEuropean Formation Damage Conference, The Hague,The Netherlands, June 2-3, 1997.

> A fraction of a second. In a process that lasts microseconds, millions of dollars and months, if notyears, of preparation culminate when perforating clears a tunnel for hydrocarbons to flow into a well.Shaped charges, with a capability to instantaneously release energy in an explosive, use a cavityeffect and metal liner to maximize penetration (lower left). Shaped charges consist of four basic components—primer, main explosive, conical liner and case (top left). An explosive wave travels downthe detonating cord, initiating the primer and detonating the main explosive. A detonation advancesspherically, reaching pressures of 7.5 million psi [50 Gpa] before arriving at the liner apex. The chargecase expands and the liner collapses to form a high-velocity jet of fluidized metal particles that is pro-pelled along the charge axis (right).

Case

Conical liner

Detonating cord

Shaped charge

Explosive cavity effects

Charge detonation

Primer

Main explosive

Explosive Steel targetMetallic liner

Lined cavityeffect

Flat-end

Unlinedcavity effect

5 microseconds

25 microseconds

40 microseconds

50 microseconds

70 microseconds

Spring 2000 55

Deep penetration—Drilling and completionfluid invasion can range from several inches to afew feet. When formation damage is severe andperforations do not extend beyond the invadedzone, pressure drop, or skin, is high and produc-tivity is reduced.5 Perforations that reach beyondthe damage increase effective wellbore radiusand intersect more natural fractures if these arepresent. Deeper penetration also reduces thepressure drop across perforated intervals to pre-vent or reduce sand production. Designed andmanufactured to outperform other charges by atleast 20 to 30% in high-strength sandstonecores, PowerJet charges are the latest and mostefficient perforators available (left).

New liner designs—material and geometry—provide improved penetration performance(below left). Liners for PowerJet charges aremade of high-density powdered materials whichyield maximum jet length and impact pressuresthat maximize penetration.6

It is well known that high-density liners pro-duce deeper penetration, however, workingwith these materials is difficult. Improvementsin manufacturing capabilities now allow high-density liners to be produced consistently.Manufacturing improvements include strict andconsistent procedures, precision tooling andstringent quality control (see “Charge Manufac-turing and Testing,” page 62).

Charges are also test fired in different mate-rials—high-strength sandstone cores, standardconcrete and API Section 1 concrete—so thatperformance does not become optimized just forconcrete targets.

In high-strength rock, penetration is reducedby up to 75% compared to API Section I concretedata. However, charges can be customized forspecific formations.7 During PowerJet chargedevelopment, a project was initiated to optimizewell completion efficiency in hard sandstone for-mations of South America. The objective was toincrease perforation penetration in sandstoneswith 25,000-psi [172-MPa] compressive strengths.These high-permeability reservoirs have moder-ate porosity and corresponding large pore throatsthat contribute to fluid damage. A combination ofreduced penetration and deep invasion resultedin low productivity from perforations that did notextend beyond the damage.

1.15

1.0

0.85

0.7

0.55

0.44 8 12 16 20 24

Prod

uctiv

ity ra

tio, p

erfo

rate

d co

mpl

etio

nve

rsus

und

amag

ed o

penh

ole

Damaged-zone thickness, in.

PowerJet charges

UltraJet charges

HyperJet charges

> High-performance perforating. This graph shows the productivityratio of perforated completion versus undamaged openhole for various depths of formation invasion. For a damaged zone of 16 in.,perforating with a 33⁄8-in. HSD High Shot Density gun and PowerJetcharges results in more than twice the productivity of older HyperJet and UltraJet deep-penetrating charges.

Deep penetration. To ensure performance opti-mization for targets other than concrete, shapedcharges are now tested in different materials—high-strength sandstone, standard concrete and API Section 1 concrete. However, improveddesigns and materials provide most of theincrease in perforation penetration. Comparedwith previous deep-penetrating charges (top),new PowerJet high-density powdered liners andgeometry yield optimal jet velocity and length aswell as extremely high impact pressures (bottom).

>

To improve production, a three-phase approachwas used. Drilling fluids were reformulated toreduce invasion and damage, the number of per-forations was doubled and custom charges weredesigned to increase penetration. The firstredesign changed only the liner geometry, whichincreased penetration from 12.6 to 14 in. [32 to36 cm]. However, this was short of the 16-in. [40-cm] objective. Penetration was thenincreased to 15.9 in. by optimizing the explosivepellet design. In field trials, custom chargesimproved production and injection performance.In one case, a gas-injector perforated at fourholes per foot using optimized charges outper-formed other injectors with 12 holes per footmade by conventional charges.

In Australia, production from two wells with7-in. casing that were reperforated using 21⁄8-in.through-tubing guns with PowerJet chargesincreased from 300 to 780 BOPD [48 to 124 m3/d]and 470 to 1550 BOPD [75 to 246 m3/d]. Inanother example, an operator in Europe reperfo-rated wells with PowerJet charges to improveproductivity and reduce sand production. Prior toreperforating, more than 20 liters [2.7 gal] ofsand were produced each day at a wellhead

pressure of 2000 psi [13.8 MPa] and gas ratesabove 2 million m3/day [70.6 million scf/D]. Afterreperforating, sand-free gas rates of 2.5 millionm3/day at a surface pressure of 2700 psi [18.6MPa] were achieved. Efficiency is important notonly for producing wells, but also for injectors.Gas injectivity was improved nine fold, from 17.6to 159 million scf/D [500,000 to 4.5 million m3/d]by reperforating an injection well in the NorthSea Norwegian sector with PowerJet charges.

Big holes, less debris and optimized casingstrength—Proprietary liner geometry is also thebasis of PowerFlow slug-free big-hole shapedcharges, which generate large holes without asolid-metal slug (below). A large flow areaimproves gravel placement for sand control andreduces turbulent pressure-drop restrictions inhigh-rate wells, especially gas producers. In aunique packing arrangement patented bySchlumberger, PowerFlow shaped charges providethe largest area open to flow available, highestremaining casing strength and reduced debris.8

A hazard to well integrity and production, per-forating debris should be minimized. Gun andshaped-charge debris increase the risk of stuckpipe, collect at the bottom of vertical wells, may

not fall to bottom in deviated wells or may reachthe surface and damage production equipment.Two strategies are used to control debris.

The conventional approach uses zinc casesthat break up into small particles which are acidsoluble or can be circulated out. A possible short-coming of zinc is formation damage.9 Laboratorytests indicate that chloride-rich fluids and gaspercolating into an idle well may combine to pre-cipitate a solid from zinc debris that can stickguns. Another disadvantage is additional gunshocks from energy released when zinc is par-tially consumed during charge detonation.

Because of these disadvantages, operators aremoving away from charges with zinc cases thatproduce small debris. The Schlumberger patentedpacking method, which causes steel cases to frag-ment into large pieces that remain in the carrier, isbecoming the preferred option (next page, top).

Recent guns with increased AOF, optimizedperforated casing strength and reduced debrisare examples of customized solutions for perfo-rating high flow-rate and gravel-packed wells. In1998, Conoco requested a larger AOF than wascurrently available from any commercial guns forprojects around the world that require high pro-duction rates to ensure commercial viability. To address this need, Schlumberger developed a7-in. PowerFlow gun for 95⁄8-in. casing that pro-duces a 47% greater casing AOF than previousbig-hole guns and 31% more than that of thenearest competitor.

By ensuring adequate casing strength afterperforating, the newest PowerFlow guns alsoaddress an increasingly important aspect ofcompletion design—formation compaction asreservoir pressure depletes that can collapsecasing. Finite-element calculations for 95⁄8-in.casing perforated with the above record-breaking AOF 7-in. gun indicate that casingcollapse strength is 78% of the original value forcasing that is not perforated.

56 Oilfield Review

8. Brooks JE, Lands JF, Lendermon GM, Lopez de CardenasJE and Parrott RA: “Perforating Gun Including a UniqueHigh Shot Density Packing Arrangement,” U.S. PatentNo. 5,673,760 (October 7, 1997).On October 8, 1999, a 7-in. gun loaded with PowerFlowcharges at 18 shots per foot created 1.14-in. [2.89-cm]diameter holes and a world record 18.5 in.2/ft [391.6cm2/m] of casing area open to flow.

9. Javora PH, Ali SA and Miller M: “Controlled DebrisPerforating Systems: Prevention of an UnexpectedSource of Formation Damage,” paper SPE 58758, presented at the SPE International Symposium onFormation Damage Control, Lafayette, Louisiana, USA,February 23-24, 2000.

10. Behrmann LA, Pucknell JK, Bishop SR and Hsia T-Y:“Measurement of Additional Skin Resulting FromPerforation Damage,” paper SPE 22809, presented at the66th SPE Annual Technical Conference and Exhibition,Dallas, Texas, USA, October 6-9, 1991.

Solid metal slug

Fluidized particles

> Big holes. Previously, solid liners that generated residual slugs were used to produce big holes. Perforation plugging was believed to be offset by large-diameter holes and high formation permeability. Technology thateliminates solid slugs, or carrots, and maximizes hole size, or flow area, hasrevised this approach. Proprietary liners are the basis of these PowerFlowcharges. X-ray photography shows perforating jet formation in UltraPackbig-hole shaped charges (top) and PowerFlow (bottom) charges. The solidslug from an UltraPack charge is conspicuous. The PowerFlow charge generates only a fluidized jet of metal particles.

Spring 2000 57

as was believed previously. In addition to explo-sive by-products, another possible damagemechanism is transient injection of well fluidsthat may cause relative permeability problems.

In extremely hard rocks, microfractures cre-ated during perforating may serve as pathwaysthat are actually more permeable than theformation and bypass perforation damage. With3000-psi [20.7-MPa] underbalance, negativeskins equivalent to a stimulation treatment havebeen measured in some high-strength reservoirand outcrop rock cores.14 Shock-induced damage,however, most often contributes to total skin,restricts well performance and may offset pro-duction gains related to other perforation param-eters such as number of shots, hole size, anglebetween perforations and penetration.

The crushed zone can limit both productivityand injectivity. Fines and debris restrict injectivityand increase pump pressure, which decreasesinjection volumes and impairs placement or dis-tribution of gravel and proppants for sand controlor hydraulic fracture treatments.15 Erosion of thecrushed zone as well as removal of debris fromperforations by surge flow are essential to miti-gate perforating damage and ensure well suc-cess in all but the most prolific reservoirs.

> Controlling debris. A patented packingarrangement decreases the risk of debrisexiting the gun (top). Shaped charges areplaced in the closest possible arrangementfor a particular gun size and shot density sothat they cannot expand. Tight confinementcauses cases to break into large pieces thatremain in the gun (bottom). Small carrier exitholes also minimize the amount of debris thatcan escape.

Undamaged rock

Crushed-zone damage

Casing

Formation damage

Cement

Perforation tunnel

> Perforating damage. A zone of reduced permeability is created around perforation tunnels by shaped-charge jets.Shock-wave pressures pulverize adjacent rock, fracturematrix grains, break down intergranular cementation anddebond clay particles. Shattering of the formation around perforations damages in-situ permeability primarily by reducing pore-throat size. Photomicrographs show undamaged rock (top insert) compared to microfracturing in a perforation crushed zone (bottom insert).

11. Pucknell JK and Behrmann LA: “An Investigation of theDamaged Zone Created by Perforating,” paper SPE22811, presented at the 66th SPE Annual TechnicalConference and Exhibition, Dallas, Texas, USA, October 6-9, 1991.

12. Swift RP, Behrmann LA, Halleck P 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.

13. Behrmann LA, Li JL, Venkitaraman A and Li H: “Borehole Dynamics During Underbalanced Perforating,”paper SPE 38139, presented at the SPE EuropeanFormation Damage Control Conference, The Hague, The Netherlands, June 2-3, 1997.

14. Blosser WR: “An Assessment of Perforating Performancefor High Compressive Strength Non-HomogeneousSandstones,” paper SPE 30082, presented at the SPEEuropean Formation Damage Conference, The Hague,The Netherlands, May 15-16, 1995.

15. Behrmann LA and McDonald B: “Underbalance orExtreme Overbalance,” paper SPE 31083, presented atthe SPE International Symposium on Formation DamageControl, Lafayette, Louisiana, USA, February 14-15, 1996;also in SPE Production & Facilities (August 1999): 187-196.

Damaged Permeability An undesirable side effect of perforating is addi-tional damage in the form of a low-permeabilityzone around perforations. Single-shot flow andradial permeameter laboratory results confirmedand quantified this induced perforation skin.10

Perforating damage can consist of three ele-ments—a crushed zone, migration of fine forma-tion particles and debris inside perforationtunnels. Shock-wave pressures from the rock faceto perforation tips shatter adjacent rock and frac-ture matrix grains, which damages in-situ perme-ability primarily by reducing pore-throat size(below). Migration of small particles from grainfragmentation, clay debonding and charge debristhat block pore throats and further reduce perme-ability also has been observed in the laboratory.

Studies show that induced damage increasesfor larger explosive charges.11 The extent of per-foration damage is a function of lithology, rockstrength, porosity, pore fluid compressibility, claycontent, formation grain size and shaped-chargedesigns.12 Research in conjunction with numeri-cal modeling is providing a better understandingof permeability damage in perforated wells thatcan be used to improve completion designs.13

Crushed-zone porosity is generally unaffectedby perforating. At least in saturated rocks, den-sity and porosity around perforations are aboutthe same as in the undamaged matrix. Althoughperforating changes rock stresses and mechani-cal properties, it does not compact the formation

Mitigating Perforation Damage At one time, perforating was performed with mudor high-density fluids in wells—balanced or over-balanced conditions. Today, underbalance is morecommon to minimize or remove perforation dam-age. Underbalanced, balanced, overbalanced andextreme overbalance (EOB) describe the pressuredifferential between a wellbore and reservoirbefore perforating. An underbalance exists whenpressure inside a well is less than the formationpressure. Balanced conditions occur when thesepressures are equal. An overbalance occurs whenwell pressure is greater than reservoir pressure.Extreme overbalance means that well pressuregreatly exceeds rock strength—fracture initia-tion, or breakdown, pressure. Both EOB and frac-turing attempt to bypass damage.16

The potential of underbalance perforatingwas recognized in the 1960s. Wells perforatedwith underbalance tended to show productionincreases. In the 1970s and early 1980s,researchers recognized that the flow efficiency ofperforated completions increased when higherunderbalance pressures were used. They con-cluded that post-shot flow was responsible forperforation cleanup and recommended generalunderbalance criteria.17 Since then, variousaspects of perforating have been investigatedusing field and laboratory data. These studiesconsistently reinforce the advantages of an initialsurge to erode perforation crushed zones andflush out perforating debris.

A 1985 Amoco study evaluated 90 wells thatwere acidized after being perforated with tubing-conveyed guns in underbalance conditions andcorrelated productivity with permeability toestablish minimum underbalance criteria.18

Results did not suggest that there was no perfo-ration damage, only that acid was not needed oras effective if underbalance was sufficient. Thisstudy was the main source of field data forcorrelating underbalance with reservoir perme-ability and perforation performance.

From these data, minimum and maximumunderbalance pressures based on potential sandproduction were calculated from sonic velocitiesfor gas wells in 1989.19 The original Amoco studyas well as new data were reanalyzed.20 Toaccount for permeability, fluid viscosity and fluiddensity, equations for minimum underbalancewere based on fluid velocity and turbulent flowthrough perforations. The disadvantage was thatthis model required knowledge of damage-zonethickness, tunnel diameter in rock and fluid vis-cosity. In addition, recent test results do not sup-port the viscosity dependence of underbalance.

These models imply that flow after early-tran-sient surge, including pseudosteady-state flow orsurging wells after perforating, is less critical forperforation cleanup. However, post-shot flow maysweep some fines into the well and further cleanup perforations.21 In some cases, this accounts forlimited sand production when wells come on line.

Magnitude and duration of an initial pressuresurge are believed to dominate cleanup of

crushed-zone damage. Instantaneous flow mini-mizes fluid invasion, loosens damaged rock andsweeps away rock debris in perforation tunnels(above). The degree to which material is loosenedis primarily a function of underbalance pressuredifferential. The high-velocity surge is followed bypseudosteady-state flow, which is less effectivebecause rates and associated drag forces are lessthan those generated during an initial transientsurge. Fluid volume and flow that occur later arebelieved to be secondary.

The underbalance pressures required to effec-tively clean perforations and reduce permeabilitydamage have been measured in single-shot perfo-rate and flow tests that provide a basic understand-ing of damage mitigation.22 Immediately afterperforating in underbalanced conditions, there isinstant decompression of reservoir fluids around aperforation. The dynamic forces—pressure differ-ential and drag—that mitigate permeability dam-age by eroding and removing fractured formationgrains from tunnel walls are highest at this time.

58 Oilfield Review

Casing Undamaged formation Balanced perforating

Formationdamage

Cement Perforation debris

Crushed and compactedlow-permeability zone

Casing Undamaged formation 3000-psi underbalance perforating

Cement

Low-permeability zone andperforation debris expelled by surge of formation fluid

Formationdamage

> Underbalanced perforating. In an overbalanced or balanced perforation without cleanup andbefore flow, the tunnel is plugged by shattered rock and debris (top). Production flow may removesome debris, but much of the low-permeability crushed zone remains. The initial surge flow gener-ated by using an adequate underbalance during perforating helps remove debris and erode thecrushed zone (bottom).

21. Hsia T-Y and Behrmann LA: “Perforating Skins as aFunction of Rock Permeability and Underbalance,” paper SPE 22810, presented at the 66th SPE AnnualTechnical Conference and Exhibition, Dallas, Texas, USA,October 6-9, 1991.

22. Behrmann et al, reference 10.Hsia and Behrmann, reference 21.Pucknell and Behrmann, reference 11.Behrmann LA, Pucknell JK and Bishop SR: “Effects of Underbalance and Effective Stress on PerforationDamage in Weak Sandstone: Initial Results,” paper SPE 24770, presented at the 67th Annual TechnicalConference and Exhibition, Washington DC, USA,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 and Exhibition, Pittsburgh, Pennsylvania, USA, November 2-4, 1993. Also in Journal of Petroleum Science andEngineering 17 (1997): 19-28.

16. Behrmann et al, reference 1.17. Bell WT: “Perforating Underbalanced—Evolving

Techniques,” Journal of Petroleum Technology 36(October 1984): 1653-1652.

18. 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 60th SPE Annual TechnicalConference and Exhibition, Las Vegas, Nevada, USA,September 22-25, 1985.

19. Crawford HR: “Underbalanced Perforating Design,”paper SPE 19749, presented at the 64th SPE AnnualTechnical Conference and Exhibition, San Antonio,Texas, USA, October 8-11, 1989.

20. Tariq SM: “New, Generalized Criteria for Determining theLevel of Underbalance for Obtaining Clean Perforations,”paper SPE 20636, presented at the 65th SPE AnnualTechnical Conference and Exhibition, New Orleans,Louisiana, USA, September 23-26, 1990.

23. Behrmann et al, reference 10.Mason JN, Dees JM and Kessler N: “Block Tests Modelthe Near-Wellbore in a Perforated Sandstone, paper SPE 28554, presented at the 69th SPE Annual TechnicalConference and Exhibition, New Orleans, Louisiana,USA, September 25-28, 1994.

24. Behrmann LA: “Underbalance Criteria for MinimumPerforation Damage,” paper SPE 30081, presented at the 1995 SPE European Formation Damage Conference,The Hague, The Netherlands, May 15-16, 1995; also inSPE Drilling & Completion (September 1996): 173-177.

Spring 2000 59

Transient surge-flow velocities are dependenton underbalance and formation permeability. Thepressure differential required to create clean, effec-tive perforations is a function of permeability,porosity and rock strength in addition to charge typeand size. For example, deep-penetrating chargesare less damaging than big-hole charges. Less thanoptimal underbalance results in variable perforationdamage and flow rate per perforation, and mostdata suggest that higher underbalance pressuresthan those often used in the field are needed tominimize or eliminate perforating damage.23

Although turbulent flow does occur at earlytimes with low-viscosity fluids, test results indi-cate that turbulence is not required for perfora-tion cleanup. Instead cleanup of permeabilitydamage around a perforation has now beenrelated to viscous drag.24 The key factors arepressure differential and subsequent transient,slightly compressible radial flow, either laminaror turbulent, which was the starting point forobtaining semi-empirical underbalance and skinequations with historic data sets.

The resulting combined theoretical andempirical equations provide a way to calculateoptimal underbalance for zero perforation dam-age or perforation skin if less than optimal under-balance is used. Single-perforation skin can beused in flow simulators to obtain total perfora-tion skin and evaluate or compare perforatingoptions. Now the most widely accepted criteriafor estimating underbalance to obtain zero-skinperforations, this methodology was the result ofmore than a decade of research on optimizing

10,000

1000

10010,0001000100

Permeability, mD

Optimum underbalance versus permeability

101

Optim

um u

nder

bala

nce,

psi

Behrmann (1995)King (1985)

1000-psi underbalance

1500-psi underbalanceUnderbalance criteria. Underbalanceis widely accepted as the most efficientmethod to obtain clean perforations.Optimal underbalance pressure criteria have increased substantiallyover the past decade as a result ofhundreds of laboratory tests. Fieldobservations by King et al developedcriteria based on the efficiency ofsandstone acidizing. Behrmann correlated laboratory data with the viscous drag force to remove fine particles (left). Laboratory tests confirmthat higher underbalance is needed toclean perforations (right).

Advanced flow laboratoryfor core perforation-flow studies

Simulated reservoir core samples

Shootingleads

Wellbore-pore

Wellbore pressure

Micrometer valve

Fast quartz gauges

Confining chamber

30-gallon accumulator

Shooting plate simulatingcasing and cement

5-gallon accumulatorconnected to wellbore

Simulated wellbore

Gun with shaped charge

Core sample

Conf

inin

g fa

st d

ata

Wel

lbor

e fa

st d

ata

pressure differential

Single-perforation flow tests. The advancedflow laboratory at SRC includes two vessels forinvestigating perforation flow under downholeoverburden, pore and wellbore pressure condi-tions (top). One vessel is for cores up to 7-in.diameter and 18 in. long; the other accommo-dates cores as large as 11.5-in. diameter and 24 in. long. This setup allows flow tests throughoutcrop or reservoir cores that can be orientedfrom horizontal to vertical (bottom).

perforation cleanup. Underbalance requirementscalculated using this method are two to fourtimes greater than previous criteria (above).

Because underbalance impacts perforationperformance and well productivity, it is essentialto understand the fluid dynamics involved.Knowledge about perforating shocks, pressuresand fluid flow is helpful in selecting an optimalunderbalance and designing downhole tools. Theadvanced flow laboratory at SRC includes twotest vessels for investigating perforation flow

and other completion operations under downholeconditions with overburden stress as well aspore and wellbore pressure (below).

This setup allows researchers to shoot andflow through a single perforation in outcrop orreservoir cores oriented from horizontal to verti-cal with any perforating system. Oil and watertwo-phase flow and dry-gas flow can be evalu-ated at constant rates with a continuous recordof absolute and differential pressure measure-ments. Perforations can be examined with a color

>>

video probe during flow through the core whileunder hydrostatic stress (above). Other opera-tions, like gravel injection and acidizing also canbe evaluated. Wellbore dynamics can be simu-lated to measure transient pressures, surge flowand perforating shocks.

Surge-flow rate and duration are controlled byinitial underbalance pressure, formation perme-ability, perforation damage, depth of near-well-bore formation damage, and the nature ofwellbore and reservoir fluids. Fast transient data,not acquired previously due to the cost anddifficulty of obtaining these measurements, arehelping researchers understand underbalancedperforating (below).25 Wellbore pressure, reser-voir-wellbore pressure differential and surge-flowdata recorded at millisecond resolutions indicate

a short period of injection into the perforationassociated with a transient overbalance due toinjection of detonation gases from the gun. Themagnitude of the pressure differential driving thisfluid injection depends on charge size and rock-sample permeability.

Underbalance perforating has evolved as theresult of research that concentrates on predictingthe pressure differential to minimize perforationskin. However, the likelihood of sand production,casing collapse, gun movement and stuck toolsmust be weighed against potential benefits.Design guidelines include minimum underbal-ance pressure for perforation cleanup, maximumunderbalance pressure to avoid sanding, andfluid cushions—a gas or liquid column—ormechanical anchors to minimize tool movement.

Optimizing Perforation ParametersDamage removal and perforation cleanup areimportant elements of perforating design and jobexecution, but consideration must also be givento tunnel diameter and length in the formation,shot density, or number of holes specified inshots per foot (spf), perforation orientation, orphasing—angle between holes—and entrance-hole size in the casing and cement (next page,bottom left). Pressure drop from perforating dam-age, or total perforation skin, is a function ofthese key perforation parameters, formation per-meability and crushed-zone thickness.

Well completions have different perforatingrequirements. Some wells produce commercialvolumes naturally after perforating and do notrequire stimulation or sand management duringcompletion. These natural completions are asso-ciated with permeable, high-porosity, high-strength sandstones and carbonates with littleformation damage and adequate matrix conduc-tivity. Perforation length and shot density are thedominant perforating parameters that dictateproductivity in these applications. Perforationsmust overcome drilling-induced damage andfluid invasion. As a rule of thumb, deep penetra-tion, at least 50% beyond damage, is needed toeffectively connect with undamaged rock.

Shot density and phasing also play importantroles. Increasing shot density reduces perforationskin, and wells produce at lower pressures. If for-mations are laminated or have high anisotropy—significantly different vertical and horizontalpermeabilities—shot density needs to be high.As skin approaches zero, shot density is impor-tant. Phased charges reduce pressure drop near awell by providing flow conduits on all sides of awell. For naturally fractured formations, multiplephasing of deep-penetrating charges helps inter-sect more fractures. If the natural fractures areparallel, oriented perforations are best.

60 Oilfield Review

> Flow lab video. Perforation flow can be examined visually with a color video probe while cores are under hydrostatic stress. A perforation filled with pulverized formation material and surrounded by fragmented quartz grains is shown on the left. A perforation without fragmentation is shown in the middle, but pulverized material remains along the bottom of the tunnel. A clean perforation with no fill is shown on the right.

6000

4500

3000

1500

0

-1500

-30000.001 0.01 0.1 1 10 100 1000

80

60

40

20

0

-20

-40

Flow

rate

, cc/

sec

Pres

sure

, psi

Time, sec

Wellbore pressure Underbalance pressure Surge-flow rate

> Typical underbalance perforating pressure responses and flow rates versus time. Data wereobtained at 2000 samples/sec in single-perforation flow tests under simulated downhole conditions of effective stress, well and reservoir pressure. After detonation, well pressure (red) increases and underbalance (blue) declines, allowing some flow (green) into the perforation. As detonation gases gointo solution and the empty gun fills with fluid, wellbore pressure again falls causing transient surgeflow into a well. This initial flow is believed to mitigate damage and permeability reduction in thecrushed zone. High-velocity transient surge flow is followed by pseudosteady-state surge flow, which may sweep loose rock and charge debris into the well and clean perforations. Surge flow continues until well and reservoir pressures equalize—zero underbalance, or balance. These sameresponses occur in balanced and overbalanced pressures, except there is no surge flow in balancedperforating and flow is from well to formation in overbalanced conditions.

Spring 2000 61

Although useful for estimating well produc-tivity and assessing trade-offs between differentguns, computer analysis sometimes obscures theinteraction and relative importance of competingparameters. Grouping parameters together revealsunderlying dependencies. This type of analysishelped develop a simple method to estimate theproductivity of perforated natural completions.26

Combining perforation and formation parametersin a single dimensionless group gives quick pro-ductivity estimates over a range of variables thatagree with the established analytical estimatesof commercially available computer programs.

Applicable for perforations that extendbeyond formation damage in a spiral phasingpattern, this method assumes that perforationlength, shot density, perforation tunnel diameter,wellbore diameter, local formation damagearound a well, perforating-induced permeabilitydamage and permeability anisotropy are the pri-mary variables governing productivity. The theo-

retical maximum well productivity ratio is definedby an ideal gun with infinite shot density thatenlarges the wellbore radius by a distance equalto the perforation penetration (below, top right).This establishes the theoretical productivity thatcan be obtained for a perforated natural comple-tion and defines a maximum productivity effi-ciency for perforating systems in terms of adimensionless factor. Practical application of thismethod lies in determining trade-offs betweenperforation parameters, underbalance, productivityimprovement and economics.

Penetration and shot density clearly areimportant for natural completions. Penetrationhas an increasing proportional effect as perfora-tions extend farther beyond formation damage.Shot density has a 1.5 exponential power effect.In addition, because perforation damage isinversely proportional to the dimensionless fac-tor, it should be minimized by perforating with anappropriate underbalance pressure differential.

High shot density is particularly effective ifdeep penetration is not possible. In natural com-pletions, tunnel diameter in the formation is theleast important of the perforation parametersand increasing hole size usually occurs at theexpense of penetration. A 10% increase in diam-eter sacrifices about 20% of the penetration and reduces the dimensionless factor by 15%.Another reason not to emphasize hole size whenselecting guns for natural completions is thatperforating jets that make big holes may alsocause additional damage.

Reduced flow from high anisotropy, perforat-ing damage or formation damage can be partiallyovercome by selecting a gun with the highestdimensionless factor, whether by deep penetra-tion, high shot density, underbalance damagemitigation or a combination of these factors. Thebest perforating strategies are defined as thosethat provide productivity efficiencies close to100% (bottom right).

25. Behrmann et al, reference 13.26. Brooks JE: “A Simple Method for Estimating Well

Productivity,” paper SPE 38148, presented at the SPEEuropean Formation Damage Conference, The Hague,The Netherlands, June 2-3, 1997.

Openhole diameter

Phase angle

Perforationtunneldiameter

Perforationlength

Crushed-zonediameter

Perforationspacing

(dependent onshot density)

Damaged-zone diameter

> Perforation parameters. To be effective, perforations must overcomedrilling-induced damage and fluid invasion around a well. Shaped-charge performance is defined by casing entrance-hole size and tunnellength. Well productivity, however, is governed by formation damage,perforation length, shot density, perforation damage that remains afterunderbalance surge and the ratio of horizontal to vertical permeability—anisotropy. Shot density is the number of holes specified in shots perfoot (spf). Phasing is the angle between holes.

N=4

P

N=8 N= ∞

2P + D

Ideal perforating gun

D

> A simple method to estimate well productivity. Maximum well productivityis defined by an ideal perforator with infinite shot density, which enlargesthe effective wellbore diameter (D) by the perforation length, or depth ofpenetration (P). In natural completions, this theoretical flow limit is used todefine perforating system efficiency for perforations that extend beyond formation damage in a spiral pattern.

0

20

40

60

80

100

1000100

Dimensionless factor, β0 = PN3/2d1/2α-5/8

1010.1

Prod

uctiv

ity e

ffici

ency

, %

P = penetrationN = shot densityd = perforation diameterα = anisotropy ratio

> Productivity efficiency versus dimensionless perforating factor.

(continued on page 64)

Mixtures of metal powders, corrosion inhibitorsand lubricants that help the powders flow havereplaced solid liners in most Schlumbergercharges. At the Schlumberger Reservoir Comple-tions Center (SRC) in Rosharon, Texas, linersand charges are produced in a series of pressingoperations (below). Powdered components areshaped into a cone using a mechanical punch.Copper, tungsten, tin, zinc and lead powders arecommonly used to produce required jet densityand velocity, properties critical to perforatingperformance. The main explosive is poured intoa case, levelled and pressed to optimal densityunder a high load. A liner is then pressed intothe explosive to complete the charge.

Although conceptually simple, shaped-chargemanufacturing requires great precision. Chargecomponents—case, primer, explosive and liner—

must meet strict quality standards and be fabri-cated to exact tolerances to ensure that perfo-rating jets form exactly according to designspecifications. A nonuniform liner collapse willcreate heterogeneous jet densities, shapes andvelocity profiles that adversely affect hole sizeand shape, and drastically reduce performance.To maintain proper tolerances, precision manu-facturing tools are built and maintained in-houseusing a state-of-the-art machine shop (right).Computerized pressing operations ensure highquality and minimize variations.

Charge manufacturing is computer-controlled,but there is human intervention to handle linersand check for cracks, make visual inspectionsand clean die tools. Technicians manufactureand package millions of charges each year. A team approach with functions located in

a single area facilitates efficient manufactur-ing and helps optimize charge performance.Multiple-bay work areas speed manufacturingand provide flexibility to meet changing wellcompletion requirements (next page, top).Manufacturing parameters are displayed in realtime to detect process deviations.

Quality control is maintained on all materialsused to manufacture charges, from cases andpowdered-liner metals to explosives. A databasewith serial numbers, history cards, associateddrawings and historical information tracks allcharges (next page, bottom left). These recordsallow day-to-day oversight of shaped-charge pro-duction quality and highlight manufacturingimprovements that impact charge performance.For example, procedures that were initiatedwhile developing new deep-penetrating chargeswere implemented for other charges, resultingin further performance improvement.

Perforating systems are tested according tothe American Petroleum Institute (API) RP 43,5th Edition, Section 1.1 New RP 19B proceduresare compatible with RP 43, except for a majorrevision to prevent target inconsistencies.2 The

Charge Manufacturing and Testing

62 Oilfield Review

Linerpunch

Linerdiebody

Linerpowder

Linerejector

Pressing force

2Liner pressed with

high force

1Liner powder placed

in ejector

3Completed liner ejected

from diebody

Linerdiebody Liner

Linerejector

Completedliner

Pressing force

6Form explosive powder

into conical shape

Finalform

punchExplosivepellet

5Preform explosivepowder for density

Pressing force

Preformpunch

Explosivepellet

4Explosive powder

placed in charge case

Explosivepowder

Caseejector

Loadingdiebody

Case

Pressing force

7Liner inserted and pressed

into explosive

Linerinsertion

punchLiner

8Completed charge ejected

from diebody

Completedcharge

Liner fabrication

Explosive loading

Shaped-charge manufacturing. Today, most linersare mixtures of metal powders, corrosion inhibitorsand lubricants that help the powders flow (top). In a series of pressing operations, these powdersare shaped into a cone using a mechanical punchand die (middle—steps 1-3). Assembling a shaped-charge involves placing a primer at the base of acase and pouring in the main explosive (middle—step 4). The main explosive is then levelled andpressed to optimal density under high loads (bottom—steps 5 and 6). A charge is completed by pressing a liner onto the explosive (bottom—steps 7 and 8).

> Manufacturing tools. To maintain proper tolerances, Schlumberger produces and maintainsprecision mechanical dies, punches and equipmentusing an in-house, state-of-the-art tool shop.

>

Spring 2000 63

sand used in concrete targets is specified as16/30 U.S. mesh. This change, which wasrecently approved to address discrepancies inpenetration-depth tests that result from largevariations in the sand grain sizes used to makeconcrete targets, is being implemented.3

Schlumberger API tests are performed inlarge concrete targets at SRC (right). Testsinclude certification of new charges as well asperiodic recertification to ensure that publisheddata represent charges currently being pro-duced. The API test site is also used for specialclient tests involving API Section 1-type targets.Of particular interest are custom tests involvingmultiple casing or completion geometry otherthan the standard API RP 43 configuration.

At the beginning of a new production run, a minimum of two charges is shot in targetsbuilt to Schlumberger standards using actual guncarriers in a water standoff that simulates down-hole conditions. These concrete targets have a minimum compressive strength of 5000 psi[34.5 MPa]. Expected penetration in thesequality-control targets is calculated based onAPI Section I, and a minimum penetrationrequirement for manufacturing is set. Full pro-duction begins once test results indicate thatminimum requirements have been surpassed.Repeated measurements of total target penetra-tion and minimum and maximum entrance-holesize are used to check charge quality.

During a manufacturing run, periodic testsare performed to confirm compliance with estab-lished performance specifications for penetrationand hole-size standards. Samples are tested every240 charges for large runs, and every 120 chargesfor the small runs associated with high-tempera-ture charges. Case and liner integrity are verifiedby a shock, or drop, test, and ballistic transfersensitivity is checked. For random batches ofcharges, detailed measurements are made on allcomponents. A few charges from each manufac-turing run are stored for audit purposes. Duringthis period, charges are pulled from storage

bunkers and test fired at regular intervals tocheck for aging effects. Internal audits also verifyproper charge performance.

Test facilities at SRC, while used extensivelyto evaluate new charges and qualify perforatingequipment, are also available for oil companyuse in completion planning and analysis of diffi-cult well conditions. In addition to improvingperforating performance, standardized andcustom testing helps researchers and clientsaddress confidence in perforating practices andoperations by verifying that perforating systemsperform consistently at rated temperatures andpressures for the duration of operations.

1. The American Petroleum Industry (API) consults with theoil and gas industry, considers advice and input from ser-vice companies, operators and scientific organizations,and recommends procedures that balance industryneeds, technology and service-provider opinions.

2. API RP 19R, 1st Edition is a revised version of RP 19B inwhich tests are scheduled and registered with the API,and can be witnessed by third parties. The advantages ofRP 19R are that manufacturing companies make a com-mitment to schedule and register tests, which carrygreater credibility than those under RP 43.

3. Brooks JE, Yang W and Behrmann LA: “Effect of Sand-Grain Size on Perforator Performance,” paper SPE 39457,presented at the SPE International Symposium onFormation Damage Control, Lafayette, Louisiana, USA,February 18-19, 1998.

Water

Test briquette

Steel culvert

CasingGun

28-day concrete

> Shaped-charge testing. Schlumberger API testsare performed in large concrete targets at SRC(top). Tests include certification of new charges as well as periodic charge recertification. The APItest site is used for special client tests involving APISection 1-type targets and testing that involves mul-tiple casing or well-completion configurations otherthan a standard API RP 43 configuration (bottom).Oil companies routinely use the API test site andother facilities at SRC for customized testing.

> Manufacturing functions. Teams of trained techni-cians assemble and package millions of chargeseach year. To facilitate high-quality, efficient fabri-cation and optimal charge performance, liner-press-ing operations and charge loading are located in asingle area (top). Multiple-bay work areas provideflexibility and the capability to respond quickly tochanging perforating needs. A special weighingroom is used to carefully control the explosive con-tent of shaped charges (bottom).

> Quality assurance. Control is maintained on allmaterials from steel cases and metal powders toexplosives and the mechanical tools used to fabri-cate charges. A real-time display helps techniciansidentify manufacturing deviations quickly and adatabase tracks each shaped charge. Theserecords are used to oversee daily operations andhelp quantify process improvements so that newprocedures that impact perforating performancecan be implemented across the manufacturingprocesses of other charges.

Stimulated CompletionsFracture and acid treatments, alone and in com-bination, stimulate well productivity.27 Effectivewell stimulation requires communication throughas many perforations as possible. This objectiveis achieved by perforating with optimal underbal-ance, limited-entry techniques or by using ballsealers or straddle packers that mechanicallydivert stimulation fluids to ensure that perfora-tions are open.28 Rather than create longhydraulic fractures in a formation, EOB is also an option to enhance communication betweenperforations and reservoir. Extreme overbalanceperforating can be used before a fracture stimu-lation to reduce breakdown pressure.29

Because hydraulic fracturing is often per-formed in low-permeability zones, minimumunderbalance to remove perforation damage canbe extremely high. Maximum underbalance isrequired to ensure removal of perforation damageand debris. If damage is not removed, residualdebris may form a filter cake in perforations thatlimits injectivity. Inflow is often not affected, butthe restriction may create high pressures duringinjection. An acid job may be needed when perfo-ration damage is not removed before fracturing.

Trade-offs between penetration and hole sizehave to be balanced when selecting shapedcharges for fracturing applications. While perfo-rations that penetrate more than six inches into aformation may not be necessary, adequate sizeholes are needed to avoid screenout—proppantbridging—in or near perforations. Prematurescreenout limits the fracture length and proppantvolumes that can be placed. At moderate to highproppant concentrations, perforation diametermust be at least six times the average particlediameter to prevent screenout. A perforationdiameter of 8 to 10 times the average particlediameter is preferred to allow for variations incharge performance and gun position.

Perforations are the point where pressurecontacts a formation and fractures initiate.Except for limited-entry and diversion tech-niques, it is important to design perforations thatminimize pressure drop across the perforationsduring pumping and subsequent production,including perforation friction, microannulus pinchpoints and tortuosity caused by curved fracturesand multiple competing fractures.

Fluid injection rates directly affect surfacepump and fracture initiation pressures. Highrates and pressures promote fracture initiation atsingle sites. At low rates, injection pressure isreduced and multiple fractures may initiate fromperforations and discrete points around a well.Shot density is calculated during fracture design.Minimum shot density depends on required injec-tion rate per perforation, surface pressure limita-tions, fluid properties, completion tubular sizes,acceptable perforation friction pressure andentrance-hole diameter (above).

A microannulus is often present after cement-ing, casing pressure-integrity testing, displacingdrilling or completion fluids, establishing anunderbalance, or after perforating and pumpingoperations that weaken the hydraulic bondbetween cement and formation (right). Becauseof the resulting pinch points, or flow restrictions,a microannulus should be avoided.

If a microannulus is present or might beinduced by perforating, various factors need to beconsidered.30 To minimize pinch points and reduceflow-path tortuosity, wells with inclinations lessthan 30° should be perforated with 180°-phasedcarrier guns oriented within 10° of the preferredfracture plane (PFP). The PFP direction can beinferred from local geology or well logs.31

64 Oilfield Review

27. In hydraulic fracturing, fluid is injected at pressuresabove the formation breakdown stress to create a crack,or fracture, extending in opposite directions from a well.These fracture wings propagate perpendicular to theleast rock stress in a preferred fracture plane (PFP).Held open by a proppant, usually sand, these conductivepathways increase effective well radius, allowing linearflow into a fracture and to the well. In matrix treatments,acid is injected below fracturing pressures to dissolvenatural or induced damage that plugs pore throats. Acidfracturing, most often without proppants, establishesconductivity by differentially etching uneven surfaces in carbonates that keep fractures open.

28. Limited entry involves low shot densities—1 spf or less—across one or more zones with different strengths andpermeability to ensure uniform acid or proppant place-ment by limiting the pressure differentials between per-forated intervals. The objective is to maximize stimulationresults. Rubber ball sealers can be used to seal openperforations and isolate intervals once they are stimu-lated so that the next interval can be treated. Becauseperforations must seal completely, hole diameter anduniformity are important.

29. Behrmann et al, reference 1.

10

9

8

7

6

5

4

1

00 0.2 0.4 0.6 0.8 1

Perforation diameter, in.

Inje

ctio

n ra

te p

er p

erfo

ratio

n, b

bl/m

ln

2

3

psi pressure droppsi pressure droppsi pressure droppsi pressure drop

25-50-

100-200-

> Injection rate versus perforation diameter for a water-based fractur-ing fluid. The minimum hole size and shot density for fracture stimula-tion designs are a function of required injection rate per perforation,surface pressure limitations, fluid properties, completion tubular sizes,acceptable perforation friction loss and entrance-hole diameter.

>30°

Preferredfracture plane

Pinch point

Microannulus

(PFP)

> Pinch points. A microannulus is caused byweakening of the hydraulic bond between cementand formation. Because of accompanying tortu-osity, flow restriction and increased pressure, amicroannulus and associated pinch point shouldbe avoided. If the angle between perforations andthe PFP is greater than 30°, a fracture initiatesfrom the sandface.

Spring 2000 65

Full-scale laboratory tests on fracture initia-tion through actual perforations show genericfracture initiation sites at the base of perfora-tions and the PFP intersection with a borehole.32

The fracture initiation site depends on perfora-tion orientation in relation to the PFP. Typically, ifthis angle is greater than 30°, fractures occurwhere no perforation exists. If a fracture does notinitiate at the perforations, fluid and proppantmust travel around the cement-sandface inter-face to communicate with a fracture, which

30. Behrmann LA and Nolte KG: “Perforating Requirementsfor Fracture Stimulations,” paper SPE 39453, presentedat the SPE International Symposium on FormationDamage Control, Lafayette, Louisiana, USA, February 18-19, 1998.

31. Brie A, Endo T, Hoyle D, Codazzi D, Esmersoy C, Hsu K,Denoo S, Mueller MC, Plona T, Shenoy R and Sinha B:“New Directions in Sonic Logging,” Oilfield Review 10,no. 1 (Spring 1998): 40-55.

32. Behrmann LA and Elbel JL: “Effect of Perforations onFracture Initiation,” paper SPE 20661, presented at the65th SPE Annual Technical Conference and Exhibition,New Orleans, Louisiana, USA, September 23-26, 1990.

33. Romero J, Mack MG and Elbel JL: “Theoretical Modeland Numerical Investigation of Near-Wellbore Effectsin Hydraulic Fracturing,” paper SPE 30506, presentedat the 70th SPE Annual Conference and Exhibition,Dallas, Texas, USA, October 22-25, 1995.

PerforationsMaximum

stress

Minimumstress

60°

Fracture

PFP

Effectiveperforations

Fracturing vertical and high-angle wells.For vertical intervals and wellbore inclina-tions less than 30°, guns with 180° phasingoriented within 10° of the preferred fractureplane (PFP) are recommended (top left). IfPFP direction is not known, 60° phasing athigher shot densities should be used (bottomleft). If well inclination is greater than 30°and the wellbore lies in or near the PFP, gunswith 180° phasing oriented to shoot up anddown should be used (top right). As well-bores turn away from the PFP, perforatedintervals should be decreased and 60° orlower phasing may be more effective than180° (bottom right). Perforations should beclustered over short intervals of a few feetwith maximum shot density and phasing tooptimize communication with one dominantfracture per interval.

When well inclination is greater than 30° anda wellbore lies in or near the PFP, the recommen-dation is to use guns with 180° phasing orientedto shoot up and down. The Wireline OrientedPerforating Tool (WOPT) may be used to orientwireline-conveyed guns in vertical and nonverti-cal wells. Several methods are also available toorient TCP guns. As wellbores turn away from thePFP, perforated intervals should be decreased,and 60° rather than 180° phasing may be moreeffective (below).

For high-angle and horizontal wells where theangle between wellbore and PFP is greater thanabout 75°, perforations should be clustered overa few feet at maximum shot density and withphasing angles that optimize communicationwith one dominant fracture per interval.

results in higher treating pressures, prematurescreenout and the possibility of multiple or asym-metric fractures.

Perforation phasing and orientation also areimportant in fracturing. Tortuosity from a curvedfracture path results from misalignment betweengun phasing and the PFP. Phased perforationstend to create multiple competing fractures. Boththese factors increase fracturing pressures.33

Vertical wells with inclinations less than 30°should be perforated with 180°-phased carrierguns oriented within 10° of the PFP to increasethe number of perforations open to a fracture,maximize fracture width near the well andreduce fracture initiation, or breakdown, pres-sure. If PFP direction is not known or orientation isnot possible, 60 or 120° phasing is recommended.

>

Sand Management: Control or Prevention?Depending on formation strength, perforationstresses, flow rate and fluid type, sand may beproduced with oil, gas and water when flow issufficiently high, and there are unconsolidated orloose formation grains in and around the perfora-tions. Changes in flow rate related to pressuredrawdown, increasing effective stress due todepletion and increasing water production withtime are the main factors in sand production.

Sand control utilizes mechanical methods toexclude sand from produced fluids. Sand pre-vention incorporates techniques to minimize oreliminate the amount of sand produced and also to reduce the impact of produced sand with-out mechanical exclusion methods. Choosingbetween these options is a function of perfora-tion and formation stability and whether per-foration failure can be predicted. The essence ofsand management is quantification of sand pro-duction risk, which helps operators decide if,how and when sand control or sand preventionshould be implemented (above).

Several methods help predict perforation tun-nel stability over the life of a well. Theoreticalborehole stability models adapted to perforationsare useful in predicting perforation stability asstress conditions change due to pressure draw-down and depletion.34 Experimental methodsinvolve testing reservoir cores or outcrop rockswith similar properties.35 Sand-prediction criteriabased on production history, by far the mostwidely used technique, rely on experience fromother wells and correlation of rock strength tocalibrate theoretical models and help choosebetween sand control and sand prevention.36

Perforating for sand control assumes that theproduction of sand is unavoidable and gravel pack-ing, fracture packs or other mechanical techniquesthat exclude sand from production flow areneeded. Perforating must address adequateunderbalance to minimize pressure drop, or skin,and remove loose sand to clean out perforationtunnels for optimal gravel placement and efficientgravel packing. In sand prevention, perforationsare designed to avoid sand production over the lifeof a well. Making the right decision impacts initialcosts, production rate and ultimate recovery.

Sand-Control RequirementsIn weak, unconsolidated formations, the conven-tional belief is that there are no open perfora-tions in the formation. The only opening forplacing gravel is the hole through casing andcement. This general theory proposes that if for-mations are incompetent and sand is producedwith hydrocarbons, there is little chance thatopen tunnels exist. Single- and multiple-shot per-forating tests have not shown this to be true inall cases. Instead, research indicates that perfo-ration definition in weak sands depends primarilyon rock strength, but also on other factors,including effective stress, underbalance, distancebetween adjacent perforations and fluids in thepore spaces and wellbore.

When perforation tunnels are not defined, theobjective of perforating for conventional gravel-pack operations is to minimize pressure dropacross the gravel-filled hole in casing and cement.This pressure drop is dictated by total AOF—thearea of individual holes multiplied by the total

66 Oilfield Review

Sand management(cased and perforated wells)

Quantification ofsand-production risk

Sand control(exclusion methods)

Increasing sand strength

Identify and minimize sources ofproductivity impairment

Sand prevention

Perforating methods to minimizesand-production risk

Increasing cost

Acceptable riskUnacceptable risk

> Sand-management decision tree.

> Perforating for sand control. Perforation tunnelsare assumed to be undefined and have little or no opening in weak formations (top). An ideal perforation cleaned out by hand in the laboratoryhas no perforating-induced rock debris and thereis little intermingling of debris and placed gravelas shown in scanning electron microscope (SEM)images (middle). In an actual single-shot test, perforation debris mixes with gravel and plugs the pack (bottom).

Spring 2000 67

number of shots—gravel permeability and flowrate per perforation. Tests on core samples showthat when tunnels are defined, perforating debrisand formation fines can impair gravel permeability(previous page, right). The objective is to minimizeinduced damage and gravel-pack impairment.

Perforation damage, formation fines andcharge debris should be removed before gravelpacking. Underbalanced perforating and flowbefore gravel packing are the best methods toachieve this objective. The maximum underbal-ance pressure must be selected to avoid perfora-tion collapse and catastrophic sand productionduring perforating. Perforating with the surfacechoke open ensures post-shot flow to transportdebris into the wellbore. Provisions need to bemade to handle transient, finite sand productionat surface until the perforations are clean. Whenpressure drop and flow rate per perforation arelow, deep-penetrating charges can be used.Deep-penetrating charges cause less localizeddamage and debris, and provide a larger effec-tive wellbore radius that reduces pressure drop.As in fracturing applications, perforation diame-ter needs be 8 to 10 times the gravel diameter.

Exposing formations to damaging completionfluids or lost circulation material (LCM) andchemicals during hydrostatic well-control opera-tions should be avoided. Damage to open perfo-rations was observed in tests on Bereasandstone blocks that were perforated, openedto flow, plugged by LCM and then reopened toflow.37 If a well must be killed, nondamagingbrines or mutual solvents are best.

For conventional gravel packing inside casing,three steps are necessary: set a bottom packer,perforate and circulate gravel behind gravel-packscreens. Disadvantages include long duration ofoperations, and potential formation damage fromfluid loss or LCM. Perforating guns and gravel-pack hardware can now be run in one step. ThePERFPAC system is a single-trip sand-controlmethod that limits fluid loss, reduces formationdamage and saves time (above right).

In addition to internal gravel packs, perforat-ing plays an important role in external sand-con-trol applications like fracture packing andscreenless gravel packs.38 Perforating require-ments for fracture packing are the same as forinternal gravel packs because it is more importantto minimize pressure drop through the pack andcontrol sand production than to create long frac-tures. However, efficient proppant placement isrequired to create an external pack. Big holeswith high shot density—12, 16, 18 or 21 spf—

and 60 or 45° phasing maximize flow area andprevent proppant screenout, or bridging, in theperforations.

In screenless gravel packs, the formation isconsolidated with resin and then fractured.Proppant injected in the fracture prevents the pro-duction of formation sand. Because proppant doesnot fill the perforations, perforating requirements

are more like conventional hydraulic fracturingstimulations. The length of perforated intervalshould be limited. Perforations that do not commu-nicate with the fracture may produce sand andneed to be eliminated or minimized. Hole diameterneeds to be 8 to 10 times greater than the proppantdiameter and perforations with 0 or 180° phasingshould be oriented to within 30° of the PFP.

36481, presented at the 71st SPE Annual TechnicalConference and Exhibition, Denver, Colorado, USA,October 6-9, 1996.

35. Behrman L, Willson SM, de Bree P and Presles C: “Field Implications from Full-Scale Sand ProductionExperiments,” paper SPE 38639, presented at the 72ndSPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 5-8, 1997.Presles C and Cruesot M: “A Sand Failure Test Can CutBoth Completion Costs and the Number of DevelopmentWells,” paper SPE 38186, presented at the SPE EuropeanFormation Damage Conference, The Hague, The Netherlands, June 2-3, 1997.

36. Venkitaraman A, Li H, Leonard AJ and Bowden PR:“Experimental Investigation of Sanding Propensity forthe Andrew Completion,” paper SPE 50387, presented at the SPE International Conference on Horizontal Well Technology, Calgary, Alberta, Canada, November 1-4, 1998.

37. Mason et al, reference 23.38. Behrmann and Nolte, reference 30.

34. Bruce S: “A Mechanical Stability Log,” paper SPE 19942,presented at the 1990 IADC/SPE Drilling Conference,Houston, Texas, USA, February 27-March 2, 1990. Weingarten J and Perkins T: “Prediction of SandProduction in Gas Wells: Methods and Gulf of MexicoCast Studies,” paper SPE 24797, presented at the 67thSPE Annual Technical Conference and Exhibition,Washington, DC, USA, October 4-7, 1992. van den Hoek PJ, Hertogh GMM, Kooijman AP, de Bree P,Kenter CJ and Papamichos E: “A New Concept of Sand Production Prediction: Theory and LaboratoryExperiments,” paper SPE 36418, presented at the 71stSPE Annual Technical Conference and Exhibition,Denver, Colorado, USA, October 6-9, 1996.Kooijman AP, van den Hoek PJ, de Bree P, Kenter CJ,Zheng Z and Khodaverdian M: “Horizontal WellboreStability and Sand Production in Weakly ConsolidatedSandstones,” paper SPE 36419, presented at the 71stSPE Annual Technical Conference and Exhibition,Denver, Colorado, USA, October 6-9, 1996.Blok RHJ, Welling RWF, Behrmann LA and VenkitaramanA: “Experimental Investigation of the Influence ofPerforating on Gravel-Pack Impairment,” paper SPE

> Single-trip gravel packing. A typical PERFPAC assembly includes a TCP gun with an automaticexplosive release, a bottom packer, sand-control screens, a gravel-pack packer with a flappervalve, pressure gauges and recorders, firing head and a dual-drillstring test valve. The TCP guns are positioned, fired, released and dropped (left). The assembly is then repositioned so that thescreens are across the perforated interval (right). The upper QUANTUM gravel-pack packer is setand gravel is injected behind the screen. The workstring is then disengaged, leaving the packedscreens in place. Operations take place in a controlled environment so formations are not exposedto overpressure, LCM or damaging fluids.

Preventing Sand Production Sand production in unconsolidated and some weakconsolidated formations results from tunnel col-lapse or formation failure between perforations. Toavoid subsequent problems that adversely affectproductivity and profitability, and limit well-inter-vention options, sand prevention must addresschanges in producing rates, formation stress andwater production. Once formation stability and per-foration failure thresholds are determined by mod-eling, laboratory testing or analysis of historicaldata, perforating methods are available to mini-mize sand production.39 Prevention implies anacceptably low risk of sand production.

More powerful big-hole charges, phase angleand excessive underbalance contribute to perfo-rating damage and potential interperforation fail-ure. To prevent sand production, perforationdesigns should minimize hole size in the forma-tion, pressure drawdown across perforated inter-vals and flow rate per perforation. Perforationsalso should be as far apart as possible. When alarge stress contrast exists in the formation andstress directions are known, oriented perforatingusing various systems can increase tunnel stabil-ity by taking advantage of minimum stress direc-tions.40 Selective perforating can avoid weakzones or formations altogether.

Because small-diameter perforations aremore stable than those created by big-holecharges, deep-penetrating charges are recom-mended for sand prevention. This also minimizesperforation damage, provides more stabilityduring drawdown and depletion, and increasesthe distance between perforations. Higher shotdensities keep drawdown, flow rate and dragforces through each perforation below a criticalvalue and minimize formation erosion.

Optimal underbalance perforating reducesperforation damage and avoids sanding fromcatastrophic tunnel failure that could stick guns.Perforation stability models help determineunderbalance limits that keep pressure draw-

down below the critical level of formation failure.Single-shot perforation and flow tests on corescan confirm underbalance values that preventsand transport, quantify the impact of increasingwater production and generally verify formationand perforation stability (above right).

In addition to single-perforation instability,interlinking of failure zones around adjacent per-forations, which is dictated by the distancebetween perforations, leads to formation collapseand sand production. Smaller holes and decreasedshot density increase perforation spacing, but thishas the undesirable effect of increasing flow rateand pressure drop per perforation, which exacer-

bate transport of failed formation material andmay lead to sand production.

A method for designing guns with optimalphasing and maximum distance between holeswas developed to further reduce the risk of for-mation collapse between perforations (below).41

By adjusting phase angle for a given wellboreradius and shot density, the distance betweenperforations can be increased to avoid interac-tion between adjacent perforations. Optimizedphasing minimizes interference and interlinkingof adjacent damaged zones, which reduces therisk of formation failure without compromisingflow rate per perforation.

68 Oilfield Review

39. Venkitaraman A, Behrmann LA and Noordermeer AH:“Perforating Requirements for Sand Prevention,” paper SPE 58788, presented at the SPE InternationalSymposium on Formation Damage Control, Lafayette,Louisiana, USA, February 23-24, 2000.

40. Sulbaran AL, Carbonell RS and López-de-Cárdenas JE:“Oriented Perforating for Sand Prevention,” paper SPE57954, presented at the 1999 SPE European FormationConference, The Hague, The Netherlands, May 31-June 1, 1999.

41. Behrmann LA: “Apparatus and Method for Determiningan Optimum Phase Angle for Phased Charges in aPerforating Gun to Maximize Distances BetweenPerforations in a Formation,” U.S. Patent No. 5,392,857(February 28, 1995).

4000

3000

2000

1000

0 2000Reservoir pressure, psi

Safedrawdown

1000

Wel

lbor

e pr

essu

re, p

si

400030000

Formation failure

Perforation stability. For sandprevention, stability analysiscan determine a safe operatingenvelope for pressure draw-down during production thatwill prevent perforation failureand the interlinking of failurezones around adjacent perforations.

0° 300° 360°240°180°120°60°

L1

L2

L3

> Optimal phasing for sand prevention. The actual perforation phasing in the formation depends on wellbore radius and shot density. A new methoddeveloped and patented by Schlumberger helps design guns with a phaseangle that maximizes the distances (L1, L2 and L3) between holes. The goal for a given shot density is to preserve the intervening formation as much as possible without compromising flow rate per perforation.

>

Spring 2000 69

The effectiveness of optimal phasing wasdemonstrated in the BP Amoco Magnus field inthe North Sea. The original perforating strategyused guns with 6 spf at 60˚ phasing (below left).In 1997, this was changed to 99˚ optimal phasingwhile maintaining the same shot density andcharge type. Wells perforated with the new gunshad fewer sand-related production problems. Theincrease in perforation spacing for an optimumgun phasing can be substantial compared withstandard gun phasing. For Magnus field, assum-ing a centralized gun, minimum perforation spac-ing was increased from 4.88 to 7.61 in. [12.4 to19.4 cm], a 56% increase, by changing from 60 to99˚ phasing.

Optimal underbalance and phasing inconjunction with deep-penetrating charges arepreferred in sand-prevention applications.Ultrahigh-shot density guns with deep penetra-tion also have been used to prevent sanding inweak, but consolidated rocks. However, evenwith perforating techniques for sand prevention,production flow may transport limited volumes ofdebris from perforation crushed zones and tun-nels. As in the case of sand control, transientsand production at surface needs to be dealt withuntil perforations are completely cleaned up.

An Overall Perforating Strategy Operated by Chevron and Conoco, the North SeaBritannia field is a gas reservoir (above). Beforethe wells were completed, potential sand produc-tion—perforation stability—and optimal under-balance pressure during perforating to minimize oreliminate perforation skin were major concerns.Theoretical models were used to predict optimalunderbalance conditions based on log-derived for-

mation properties. With detailed log permeabilitydata, numerous simulations were carried out toevaluate guns, charges, shot densities and perfo-rating strategies. Based on these simulations, finalcompletion designs included specific chargedesigns and shot densities for various formationsections instead of using average properties todetermine perforating parameters.42

In general, four key aspects of perforatinghave a major impact on productivity and play animportant role in determining well completionsuccess—perforation dimensions (length anddiameter), shot density, phasing angles anddegree of perforation damage. The choice of gunsystem parameters to optimize a completion wascarried out using theoretical analysis of comple-tion efficiency using inflow, or NODAL, analysisprograms. For the Britannia study, lithology varia-tions also were taken into account. Log and coredata were used to determine the productivity ofvarious individual layers based on conductivityand formation damage. For each layer, numericalproductivity simulations were carried out to deter-mine the optimal perforation parameters of shotdensity, penetration and underbalance conditions(below). An acceptable gun phasing was fixed.

Increasingstress

Formationfailure

60° phasing 99° phasing

> Optimal phasing. Optimal phasing was used successfully in the BP Amoco Magnus field in the North Sea to prevent failure of the formationbetween perforations. The original perforatingstrategy used 33⁄8-in. guns with 6 spf at 60˚ phasing (left). In 1997, this was changed to 99˚ optimal phasing while maintaining the same shot density (right). Wells perforated with the newguns had fewer sand-related production problems.

N

BraePiperClaymore

BuchanBeatrice

Montrose

Britannia

Forties

Fulmar

Aberdeen Erskine

Lomond

U K

> Britannia field location.

X300

X200

X100

X000

1000Underbalance, psi

Dept

h, ft

Permeability, mD10,000 0 300

Zone

BC

Formationthickness, ft

10.510

Permeability,mD

98.5620.3

Unconfinedstress, psi

89289346

Porosity, %

15.7713.54

Drawdown, psi (rate, MMscf/D)Rate 1 Rate 2 Rate 3 Rate 4227 (20)259 (5)

822 (40)643 (10)

1739 (60)1181 (15)

3401 (80)1935 (20)

Thickness of near-wellbore damage,

in.246810

not applicable4.914.384.033.68

not applicable4.994.403.713.12

0.7960.6460.5730.526

not applicable

0.8970.7110.6190.527

not applicable

Productivity index, MMscf/D/100psiZone B (98.56 mD)

5 spf, charge A 12 spf, charge X 5 spf, charge A 12 spf, charge XZone C (20.3 mD)

> Optimizing perforating strategies. For the Britannia field, lithology variations were taken intoaccount instead of just using average reservoir properties. Zone B had greater formation damagedepth than zone C. Charge A at 5 spf was used for zone B, resulting in about a 15% productivityincrease. Charge X at 12 spf was used for zone C, resulting in about a 10% productivity increase.

42. Underdown DR, Jenkins WH, Pitts A, Venkitaraman Aand Li H: “Optimizing Perforating Strategy in WellCompletions to Maximize Productivity,” paper SPE 58772,presented at the SPE International Symposium onFormation Damage Control, Lafayette, Louisiana, USA,February 23-24, 2000.

Current underbalance guidelines lead to largepressure differential requirements in high-strength, low-permeability zones. This issue wasaddressed during the Britannia study in single-shot perforate and flow tests on reservoir andoutcrop rocks conducted in the advanced flowlaboratory at SRC in Rosharon, Texas. Anotherconcern during underbalance perforating ispotential sand production from perforation col-lapse, which was also addressed in the single-shot studies that simulated downhole stress andflowing conditions.

Laboratory tests confirmed theoretical under-balance predictions and perforation stability.Reservoir and outcrop cores were perforatedusing simulated downhole conditions and under-balance pressures determined from simulations.The perforation strategy for this field wasselected based on results from this study. Flowperformance of perforated reservoir cores veri-fied earlier conclusions about formation sensitiv-ity to aqueous wellbore fluids—brine—andconfirmed perforation stability at high underbal-ance cleanup conditions. A 1000-psi [6.9-MPa]underbalance in outcrop sample tests resulted inlow perforation skin. Analysis of performanceafter completion indicated low to negative skin in12 wells. In addition to determining the best per-forating design for each completion application,this approach emphasized the need to study opti-mal underbalance, especially in gas formations,to optimize overall completion strategies.

Gun and Conveyance ChoicesShaped charges are placed in guns and conveyeddownhole to the correct depth by wireline, slick-line, tubing or drillpipe, and coiled tubing. Thereare two types of guns, capsule and carrier (below).Capsule guns, like the Enerjet and Pivot Gun sys-tems, are used in through-tubing electric wirelineand slickline perforating. Charges in capsule gunsare exposed to well conditions and must be encap-sulated in separate pressure-proof containers.Debris from these expendable guns is left in a wellafter firing. Carrier guns are conveyed on wirelineor slickline, tubing or drillpipe run by drilling andworkover rigs or snubbing units, and on coiled tub-ing with or without an electric line. In these guns,

charges and most of the debris are contained inhollow steel carriers that are retrieved or releasedand dropped to bottom after perforating.

Casing and through-tubing guns, both capsuleand carrier, were initially run on wireline; tubing-conveyed perforating (TCP) with HSD High ShotDensity guns became popular in the early 1980s.Through-tubing guns, including casing and HSDguns, are limited in gun size and length by wellcompletion design and surface pressure controlequipment. The use of underbalance is also lim-ited when guns are run on electric line. Gunsdeployed on tubing offer a wide variety ofchoices and allow for simultaneous underbal-ance perforating of long intervals.43

70 Oilfield Review

RetrievableEnerjet

StandardEnerjet

ExpendableEnerjet

Capsule guns

Carrier guns

111/16 -in.OD running

3.79-in.OD deployed

1.56-in. HSD gun4 spf zero phasing

2.0-in. HSD gun6 spf, 60˚ spiral

phasing

2.25-in. HSD gun6 spf, 60˚ spiral

phasing

5.85-in. Bigshot18 spf, 120˚/60˚

phasing

Patented chargepacking

6 5/8-in. Bigshot18 spf, 120˚/60˚

phasing

Pivot Gun

Tubing

Casing

Gun types. Perforating guns are classified ascapsule or carrier. A few examples are shown atright. Capsule guns are conveyed by wireline orslickline in through-tubing operations. Detonatingcords are exposed to downhole conditions, sothe charges are encapsulated in pressure-proofcontainers. Expendable through-tubing capsuleguns generate debris, which remains in a wellafter perforating. Carrier, or casing, guns areconveyed by wireline, tubing and coiled tubingand can be designed to retain debris inside thecarrier. Detonation occurs inside the carrierunder atmospheric pressure.

>

Spring 2000 71

Today, perforating often encompasses morethan traditional running and firing of guns.Perforating systems are an integral part of wellcompletion equipment and completion opera-tions that are designed to perform multiple oper-ations in permanent completions, such as settingpackers, pressure testing, perforating one ormore intervals and initiating tool functions, all ina single operation. The timing of perforatingevents, such as charge detonation, resultingshocks and gun release, are used to help ensurethat perforating TCP guns release and drop, evenin high-angle wells (right). Guns have beenreleased and dropped successfully in well pro-files up to about 84°.

Downhole operations—A family of X-Toolsperforating gun-actuated completion tools—wireline/coiled tubing explosive-type automaticrelease (WXAR), superfast explosive-type auto-matic gun release (SXAR), monobore anchor withexplosive-type release (MAXR), superfast explo-sive-type production valve (SXPV) and superfastexplosive-type vertical shock absorber (SXVA)—are designed to perform specific functions likefast release and dropping of gun strings afterperforating and opening valves. These functionsare initiated by an explosive on the same ballis-tic chain as the perforating guns. Actuation ofthese explosive devices after guns are firedgreatly increases the versatility of perforatingcompletion operations.

Gun length and perforating without killingwells—Total weight of long gun strings andrunning or retrieving guns under pressure restrictwireline, coiled tubing and tubing-conveyedperforating. However, these limitations areovercome by permanent completion perforating(PCP) systems.

The GunStack stackable perforating gun sys-tem, also known as Completions DownholeAssembly and Disconnect (CDAD), allows down-hole assembly of multiple gun sections to anylength with or without a rig. This equipment

allows underbalanced perforating of long inter-vals in one descent. The system can be deployedand retrieved by slickline, electric wireline orcoiled tubing. When necessary, gun sections canbe retrieved without killing the well. This systemcan be used to perforate wells without interrupt-ing production. In combination with techniqueslike WXAR or MAXR, the GunStack, or CDAD,system also allows guns to be run in sectionsaccording to available lubricator length andweight capacity of the conveyance method.

The first gun section is run and latched onto adownhole anchor, bridge plug or packer set bywireline for precise depth control. The gun stringalso can be landed against the bottom of a well.In this configuration, the string is not anchored.Consecutive sections are assembled and con-nected on top of each other until the required gunlength is achieved. Rather than simply stackingor latching, the connectors solidly connect eachgun section to the next. Guns can be discon-nected mechanically at any time. The connectorsdisconnect automatically after a delay that fol-lows gun detonation. This prevents gun sectionsfrom moving uphole during detonation and under-balance surge flow, and allows wells to be perfo-rated with maximum underbalance.

The CIRP Completion Insertion and Retrievalunder Pressure perforating system was designedso gun strings could be assembled at surface,inserted in wells, extracted and disassembledwithout killing the wells. The CIRP system facili-tates running long guns in and out of wells underpressure using wireline or coiled tubing. Thisallows an entire interval to be perforated at onetime with an appropriate underbalance.Retrieving and disassembling guns under pres-sure eliminate the need to drill deeper to allow

for dropping guns or the need to kill wells afterperforating. The CIRP system is used with gundiameters from 2 to 4.5 in. Gun lengths of 2000 ft[610 m] with up to 60 connectors have been run.

The completion FIV Formation Isolation Valvetool, integrated into the permanent completiondesign, allows long strings of perforating guns tobe run in and out of wells without hydrostaticoverbalance control. A fullbore completion valvethat is normally run below a permanent packer,the FIV tool acts as a downhole lubricator valveand isolates perforated intervals from the pro-duction string above. The gun length per run islimited only by weight restrictions of the con-veyance method used.

After perforating, guns are pulled above theFIV tool, which is closed by a shifting tool on theend of the gun string. Well pressure is bled offand the guns are retrieved. The FIV tool then isopened for production by applying a predeter-mined sequence of pressure cycles. The FIV toolalso can be opened and closed an indefinite num-ber of times with a mechanical shifting tool. Thisvalve system was developed for the BP AmocoAndrew field in the North Sea.44

Success of the FIV tool was the basis fordesign of a liner top isolation valve (LTIV) thatoperates on the same principles. The LTIV is afullbore ball valve that isolates formations fromcompletion fluid after a zone is completed withan uncemented liner. The LTIV tool is run directlybelow a liner-hanger packer and can be openedand closed as many times as required. Once theball is closed, the formation is isolated from com-pletion fluid until the well is ready for production.The valve holds pressure from above and below,which makes it suitable as a long-term barrier.

43. In June 1999, the longest gun to date, a special taperedHSD gun, was successfully fired in Well M-16 at the BP Amoco Wytch Farm field in southern England. Thisworld record gun string was 8583 ft [2616 m] long fromtop to bottom and shot with more than 25,000 CleanSHOTdeep-penetrating charges.

44. Patel D, Kusaka, Mason J and Gomersall S: “The Develop-ment and Application of the Formation Isolation Valve,”presented at the Offshore Mediterranean Conferenceand Exhibition, Ravenna, Italy, March 19-21, 1997.Kusaka K, Patel D, Gomersall S, Mason J and Doughty P:“Underbalance Perforation in Long Horizontal Well in theAndrew Field,” paper OTC 8532, presented at the 1997Offshore Technology Conference, Houston, Texas, USA,May 5-6, 1997. Mason J and Gomersall D: “Andrew/Cyrus HorizontalWell Completions,” paper SPE 38183, presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, June 2-3, 1997.

Jet in

teract

ion w

ith w

ellbo

re flu

id

Maximum

annu

lus pr

essure

from gu

n swell

Jet ta

il exit

s gun

End o

f pen

etrati

on

Gun op

en to

well

bore

fluid

Reservo

ir rea

ction

X-Tool

valve

open

Jet ta

il form

s

Fluid column

Timing of perforating eventsafter charge detonation

10 100 1000microseconds milliseconds

Time

1 10 100 1000

response

Reservoirresponse

> The timing of perforating events. Today’s perforating systems do more than just deploy and fire gun strings. These systems often set packers, initiate pressure tests, perforate more than one interval and initiate downhole tool functions, all in a single operation. For example, the timing of charge detonation, resulting shocks, reservoir response and tool functionsare coordinated to ensure that guns drop to the bottom of wellbores.

High-angle wells—In high-angle and horizon-tal wells, wireline may not allow guns to descendunless a tractor is used. Coiled tubing is the pre-ferred conveyance method, unless a horizontalsection is so long that helical buckling occursbefore the perforating interval is reached.Tractors have also been used successfully toextend the maximum reach of coiled tubing. Inmany of today’s high-angle and extended-reachwells, there may be no alternative to TCP or PCP.

If mechanical pulling or pushing force mustbe exerted on a gun system, TCP, snubbing,coiled tubing and tractors offer more versatilitythan electric line and slickline. For long guns likethose used in horizontal wells, gun-string designmust consider tensile strength. High-strengthadapters and tapered gun strings have beenused successfully. Gun bending must also bemodeled and addressed.

Perforating-deployment technology hasevolved from early electric line and tubing-, ordrillstring-, conveyed guns, and now includescoiled tubing with or without electric line, snub-bing units, slickline and downhole tractors onwireline and coiled tubing. Each conveyancemethod has advantages and disadvantagesrelated to performing downhole operations, gunlength and pressure control, perforating withoutkilling wells, mechanical strength and wellboreangle, depth correlation, rigless intervention andgun type. To optimize perforation designs, thesepros and cons must be weighed for all gunsystems being considered for a specific comple-tion (above right). Other considerations includeunderbalanced perforating and timing or durationof operations.

Underbalance—Options for perforating withunderbalance have reached a high degree ofsophistication as a result of hardware for TCP orPCP and wireline anchoring devices. Whateverthe conveyance method, it is usually possible toperforate with sufficient underbalance. Practicalexceptions when optimal underbalance cannotbe achieved are depleted reservoirs, shallowwells or wells with existing open perforations.

For certain conditions, a high underbalance isneeded to clean out perforations and generatepost-shot flow. With wireline-conveyed guns, thisis possible only if anchoring devices are usedwhile shooting to prevent guns from being blownuphole. Anchoring devices are also recom-mended when the level of underbalance isunknown and guns are exposed to a sudden fluidinflux, as for example, when perforating newintervals in formations with differentially depletedproducing intervals.

A Wireline Perforator Anchoring Tool (WPAT)device was developed to anchor guns in slimholemonobore completions and prevent guns frommoving after detonation. The WPAT device, nowavailable in two sizes, one for 2-in. guns in 27⁄8-in. tubulars and another for 21⁄4-in. or 21⁄2-in. gunsin 31⁄2-in. completions, counteracts potentiallylarge forces generated by flowing fluids that canforce guns uphole with disastrous consequences.

The main application of the WPAT anchor is toperforate with extremely high underbalance.Another application is to protect cable weakpoints from high-tensile loads.

The tool has positive anchoring and releasingmechanisms. Mechanical slips are designed to benondamaging and can be retracted by jarringupward if guns become stuck after perforating.

A calibrated orifice that meters oil at a spe-cific rate provides the holding period, which canbe set for up to an hour. This allows sufficienttime to establish an underbalance, perforate andconduct a pressure drawdown test. The toolreleases automatically after the programmedtime elapses. The tool may be configured in twoways; one operates on well pressure and theother, for a dry hole, operates on pressure suppliedby a gas bottle that is part of the system.

Duration of operations—The timing of opera-tions varies for each well. If intervals are verticaland short—less than 40 ft [12 m]—and perfo-rated in balanced or overbalanced conditions,wireline perforating usually can be performed ina few hours and may be the most efficientmethod. If the interval is longer or has multiplesections, wireline operations require more thanone trip, which prevents use of underbalanceduring subsequent gun runs. As well deviationincreases, operating time increases, especially ifthe gun-string weight is low and surface pres-sure-control equipment is used. When well devi-ation exceeds about 65°, other conveyancemethods like TCP and PCP that require a longerrunning-in time must be used. If perforating inter-vals become significantly longer, the overall dura-tion of TCP is shorter than wireline operationsand the entire interval can be perforated withunderbalance for optimal perforation cleanup.

72 Oilfield Review

45. Huber KB and Pease JM: “Safe Perforating Unaffectedby Radio and Electric Power,” paper SPE 20635, pre-sented at the 65th Annual SPE Technical Conference and Exhibition, New Orleans, Louisiana, USA, September 23-26, 1990.Huber et al: “Method and Apparatus for Safe TransportHandling Arming and Firing of Perforating Guns Using aBubble Activated Detonator,” U.S. Patent No. 5,088,413(February 18, 1992).Lerche et al: “Firing System for a Perforating GunIncluding an Exploading Foil Initiator and an OuterHousing for Conducting Wireline Current and EFICurrent,” U.S. Patent No. 5,347,929 (September 20, 1994).

Through-tubing

SXAR

MAXR

WXAR

FIV

Wireline CIRP

Coiled Tubing CIRP

GunStack (CDAD)

Reservoir

1 Rig required for installation, but not for perforating 3

Guns are in place impeding cleanup2 Best in monobores 4

Requires suitable conveyor

TechniqueEconomics

1

3

3

3

3 44

1

1

2

4

Never s

hut in

well

High-an

gle w

ells

New w

ellBest

gun

_ size,

spf, p

enetr

ation

, hole

size

Horizon

tal w

ells

Workove

rs

No rig

requir

ed to

perfo

rate

No add

itiona

l drill

ing (ra

thole)

requ

ired

Reperf

orate

while p

roduci

ng

Reperf

orate

withou

t killin

g the

well

Shoot

pay z

one u

nderb

alance

d in o

ne ru

n

Optimum

perfo

ration

clean

up

Fast g

un re

moval to

surge

all p

erfora

tions

Remove

guns

withou

t con

trollin

g (kill

ing) w

ell

( )

Advantage

Limitations

> Conveyance choices. To optimize perforating operations, the advantages, disadvantages and limita-tions of all gun systems that are considered for a specific completion must be weighed. This table listsreservoir, economic and technical benefits of equipment that is used to perforate without killing a well.

Spring 2000 73

SafetyTwo types of detonators are used in perforatingguns: electrical detonators, or blasting caps, andpercussion detonators. Conventional electricaldetonators are susceptible to accidental applica-tion of power from electric potential differences(EPD), which constitutes a safety hazard.Percussion detonators that are used in TCP sys-tems actuate mechanically when a firing pinstrikes a pressure-sealed membrane and deto-nates a primary high explosive.

The S.A.F.E. Slapper-Actuated Firing Equip-ment system was developed to be immune frompotential differences created by radio-frequency(RF) radiation, impressed current from corrosioncathodic protection, electric welding, high-tension power lines and induction motors such astopdrives on drilling rigs. This system eliminatesthe need to shut down vital radio communicationsand equipment during perforating operations.45

The detonating mechanism in the S.A.F.E. sys-tem is an Exploding Foil Initiator (EFI) rather thana primary high explosive. To fire a gun, a capaci-tor in the downhole electronic cartridge ischarged and then allowed to discharge abruptly.The heat produced by this discharge vaporizes asection of metal foil, which slaps an adjacentexplosive pellet with sufficient energy to deto-nate it. This detonation shears a small aluminumflyer that impacts a booster that fires the gun. Amajor advantage of S.A.F.E. equipment is thatwellsite assembly is quicker than for conven-tional electrical detonators. Disadvantages of theS.A.F.E. detonator are cost and size, which takesup lubricator space.

The Secure detonator is a third-generationS.A.F.E.-type device that also uses an EFI. It doesnot contain primary high explosives or a down-hole electronic cartridge. A microcircuit performsthe same functions as the electronic cartridge andEFI together in a package that is similiar in size toa conventional electric detonator. The Secure sys-tem has all the technical advantages of S.A.F.E.detonators, but is more reliable, fully expendableand smaller so that gun strings can be shorter.

Perforation Design and AnalysisPerforated completions can be designed usingthe SPAN Schlumberger Perforating Analysissoftware, which predicts perforating efficiencyunder downhole conditions.46 The programcombines modules that estimate downhole pen-etration, calculate productivity and determineoptimal underbalance. In the first module, pene-tration depth and hole size are estimated for usein a second module, which calculates well pro-ductivity. Optimal underbalance for zero-skinperforations is determined by using algorithms inthe third module for currently accepted underbal-ance criteria.47 When calculations cannot bemade algorithmically, as in the case of correctingsurface-test penetration for in-situ environmen-tal conditions like rock strength and formationstress, an extensive database of perforatingperformance in Berea sandstone cores or slabs,API data and other test results is used.

In design mode, this software helps selectgun systems based on specific well parame-ters—completion geometry, fluids and under-balance (above). When actual underbalance isless than the minimum required for zero damage,perforation skin due to residual damage is calcu-lated to show how productivity is reduced.

The SPAN program also can be used toanalyze production after wells are completed orrecompleted. If actual production data matchSPAN program calculations, a perforated comple-tion is considered successful. When productionobjectives are not realized, the reasons—deepformation invasion, incomplete damage removalor incorrect assumptions—need to be deter-mined. Because the SPAN program incorporatesgeological aspects, it is helpful for integratingreservoir descriptions in perforation designs.48

Gun12345

Phase spfDescription4 1/2-in. HSD UltraJet4 1/2-in. HSD PowerJet1 11/16-in. Enerjet2 1/8-in. Power Enerjet4 1/2-in. HSD UltraJet

Prod

uctiv

ity ra

tio, p

erfo

rate

d co

mpe

tion

vers

us u

ndam

aged

ope

nhol

e

Crushed-zone versus formation permeability, Kc/K

Gun 2Gun 5Gun 1

Gun 4

Gun 3

00 0.2 0.4 0.6 0.8 1

0.3

0.6

0.9

1.2

1.5

Anisotropic ratio: 10Damaged-zone thickness: 4 in.

Ratio of damaged-zone versusformation permeability, Kd/K: 0.5Crushed-zone thickness: 1 in.

SPAN Version 6.0© Copyright 1999 Schlumberger

135°72°

0°0°

72°

125465

> Perforation design and analysis. The SPAN Schlumberger Perfo-rating Analysis program is used to predict completion efficiencyand select the best gun system. Underbalance calculations arebased on the most current criteria. If the actual pressure differen-tial is less than the minimum underbalance for zero damage, skindue to residual damage is calculated to show how productivity isreduced. Here well productivity is calculated for five gun types atdifferent shot densities and phase angles.

46. Carnegie A: “Application of Computer Models toOptimise Perforating Efficiency,” paper SPE 38042,presented at the SPE Asia Pacific Oil and Gas Conference, Kuala Lumpur, Malaysia, April 14-16, 1997.

47. Behrmann and Elbel, reference 32.48. de Araujo PF and Coelho de Souza Padilha TC:

“Integrating Geology and Perforating,” World Oil 218, no. 2 (February 1997): 128-131.

Smart PerforatingEvery cased well must have perforations toproduce hydrocarbons, but different reservoir and completion combinations have different per-forating requirements. Because perforating issuch a critical element of well productivity, therequirements of each well should be optimizedbased on specific formation properties. The bestway to achieve this is to understand how reser-voirs respond to natural, stimulated and sand-management completions. Factors that need tobe taken into account include formation com-pressive strength and stress, reservoir pressureand temperature, zone thickness and lithology,porosity, permeability, anisotropy, damage andfluid type—gas or oil.

Hard—high-strength—formations and reser-voirs damaged by drilling fluids benefit the mostfrom deep-penetrating perforations that extendbeyond the formation damage and increase theeffective wellbore radius. Low-permeabilityreservoirs that need hydraulic-fracture stimula-tion to produce economically require appro-priately spaced and oriented perforations.Unconsolidated formations that may producesand need big holes which reduce pressure drop

and can be packed with gravel to keep the for-mation particles out of the perforation and thewellbore. Perforations also can be designed toprevent tunnel and formation failure associatedwith sand production.

In the past, integrating formation and perfo-rating considerations, including underbalance,was an exception rather than a rule. Theory andsoftware were available to analyze perforationperformance, but completion decisions wereoften based on average formation properties orperforating limitations unrelated to productivity.Today, thinking in terms of what’s best for areservoir is the predominant approach. Operatorsconsider what a particular field developmentrequires and then select the best completiontechniques and hardware that are available.

Standard “off-the-shelf” equipment and ser-vices sometimes do not meet those needs. Newtools, procedures and services—shaped charges,completion equipment, conveyance alternativesand applications for underbalance, overbalanceor extreme overbalance—often need to be devel-oped. As a result, significant Schlumbergerresearch and engineering resources are dedi-

cated to developing customized solutions. Manyof these new developments eventually becomestandard products and services that extend the range of options available to operators. Thebest perforation designs are based on specificwell requirements to optimize production. Thistotal-systems approach—smart perforating—emphasizes practices that maximize well produc-tivity and helps operators realize the most benefitfrom the perforating solutions that are availableto overcome dilemmas associated with perfo-rated well completions (above).

By adapting perforation designs to specificreservoirs, perforating technology can be inte-grated with geology, formation evaluation andcompletion techniques to determine the rightequipment, shaped charge, carrier system, con-veyance method and pressure condition for per-forming efficient and effective perforatingoperations. Computer simulations can be used tocompare performance versus design expecta-tions. Existing tools and methods can then beimproved and used more effectively. The ultimategoal is to design custom perforating solutions foreach well to maximize productivity. —MET

74 Oilfield Review

ResearchReservoirconsiderations

StandardequipmentlimitationsSmart

perforating

Gun-activatedtools

Wirelineanchor

Formationrequirements

Hard rocks

WellrequirementsNo-kill

perforating

Sandcontrol

Capsule guns Carrier gunsTCP and

PCPconveyance

Hydraulicfracture

stimulation

Orientation Optionalphasing

Single-tripgravel

packing

UnderbalancePhaseangle

High-ratewells

High-anglewells

High shotdensity

Naturalfractures

Low-debrisguns

Formationdamage Big-hole

charges

Sandprevention

Wellproductivity

Perforating-induceddamage

Customsolutions

Deep-penetrating

charges

Shotdensity

Hole size

> Fitting together the pieces of the puzzle. The many perforating options and a myriadof well-completion factors exponentially increase the number of decisions that must bemade before perforating. A smart perforating systems approach helps operators realizemore benefit from perforating solutions that are available to overcome the technicaldilemmas associated with perforated completions.