5

Charlie CosadSchlumberger Testing ServicesAberdeen, Scotland

Choosing a Perforation Strategy

COMPLETION/STIMULATION

The ultimate success of the

well—its productivity and life

expectancy—rests on making the

best possible connection between

the wellbore and formation. This

update reports on what we know

today about selecting a perfora-

tion strategy best suited to the

reservoir and the completion.

4

For their help in preparation of this article, thanks toLarry Behrmann, Klaus Huber, Tom Lebsack and TonyVovers, Schlumberger Perforating Center, Rosharon,Texas, USA; Bill Bell, Huntsville, Texas, USA; Dick Ellis,Pennzoil, Houston, Texas, USA; George King, AmocoResearch, Tulsa, Oklahoma, USA; Randy Saucier, Man-deville, Louisiana, USA; and Stephan Turnipseed, TriTechServices, Montgomery, Alabama, USA.In this article, Enerjet, HEGS (High-Efficiency Gun Sys-tem), HSD (High Shot Density gun system), S.A.F.E.(Slapper-Actuated Firing Equipment), Selectric, SPAN(Schlumberger Perforating Analysis), Pivot Gun, IMPACT(Integrated Mechanical Properties Analysis & Characteri-zation of Near-Wellbore Heterogeneity), MSRT (Multi-Sensor Recorder/Transmitter) and LINC (Latched Induc-tive Coupling) are marks of Schlumberger.1. Gravel is rounded particles of diameter typically

greater than 2 mm [0.8 in.].

The fate of a well hinges on years of explo-ration, months of well planning and weeksof drilling. But it ultimately depends on per-forming the optimal completion, whichbegins with the millisecond of perforation(above). Profitability is strongly influencedby this critical link between the reservoirand wellbore.

Perforations form conduits into the reser-voir that not only allow hydrocarbon recov-ery, but influence it. Each of the three maintypes of completions—natural, stimulatedand sand control—has different perforatingrequirements. In the natural completion (inwhich perforating is followed directly byproduction) many deep shots are most effec-tive. In stimulated completions—hydraulicfracturing and matrix acidizing—a smallangle between shots is critical to effectivelycreate hydraulic fractures and link perfora-tions with new pathways in the reservoir.And in gravel packing, many large-diameterperforations effectively filled with gravel1

are used to keep the typically unconsoli-dated formation from producing sand andcreating damage that would result in largepressure drops during production.

To meet the broad requirements of perfo-rating, there many perforating guns and gunconveyance systems. Optimizing perforatingrequires selection of hardware best suited tothe job. A good place to start, therefore, iswith the basics of perforating hardware.

The Language of PerforatingThere was a time when describing the perfo-ration operation defined the perforator: run-ning through-tubing guns, shooting casingguns or tubing-conveyed perforating (TCP)(next page). Not so with the present varietyof completion methods and gun systems.

Oilfield Review

55October 1992

nThree conveyance methods for perforating guns: through-casing and through-tubing, and tubing-conveyed systems.The through-tubing gun shown is held against the casing magnetically. The others hang free.

nUp in smoke. Sur-face detonation of astandard 4-in. gun,staged during themaking of a safetytraining video.Destruction of themannequin at left,positioned about 1foot [30 cm] fromthe end of the gun,shows the poten-tially devastatingeffect of a surfacedetonation, empha-sizing that safetyforms the essentialfoundation for per-foration operations.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaCasing

Casinggun

Through-casing perforation

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaThrough-tubing perforation

TubingPacker

Through-tubing gun

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaWorkstring

Firinghead

Guns

Packer

Flow entryports

Safety spacer

Tubing-conveyed perforation

5

nMajor geometrical parameters thatdetermine flow efficiency in a perforatedcompletion. Four key factors are shotdensity, phase angle, perforation penetra-tion into the formation and perforationdiameter. Productivity of a well alsodepends on the size of the crushed zone,whether the perforation extends beyondthe damaged zone and how effectivelythe crushed zone and charge debris areremoved from the tunnel.

Perforationvaries with

shotdensity

Phaseangle

Damaged zone

Perforationdiameter

Perforationpenetration

Crushedzone

Wirelinethrough-tubing

Wirelinethrough-casing

Tubingconveyed

Gun System

Application

Exposedgun

Strip

Pivot

x

x

The two broad categories of guns areexposed and hollow carrier guns (bottom).These can be used in two types of perforat-ing operations: through-tubing, in whichguns are run through a production or teststring into larger diameter casing; andthrough-casing, in which guns are largerdiameter and run directly into casing.

Exposed guns are run on wireline andhave individual shaped charges sealed incapsules and mounted on a strip, in a tubeor along wires. The detonator and detonat-ing cord are exposed to borehole fluids.These guns are used exclusively throughtubing and leave debris after firing. Theyinclude two designs, “expendable,” (chargesand mounting assembly become debris) and“semiexpendable” (mounting is recovered).For a given diameter, exposed guns carry alarger, deeper penetrating charge than a hol-low carrier gun. But exposed gun outerdiameter is generally not larger than about21/2 in. [6 cm], because above this size, thecasing, or hollow carrier design, becomesmore practical, allowing use of largercharges, optimal angle betweenshots—called phasing2 —and increasednumber of shots per linear foot—called shotdensity (above, right).

Hollow carrier guns have shaped chargespositioned inside pressure-tight steel tubes.This design is available for most tubing andcasing sizes. It is used through tubing whendebris is unacceptable and in hostile condi-tions that preclude exposed guns. There arefour main types of hollow carrier guns: •Scallop guns, so-called because charges

shoot through dished out areas in the car-rier, which is recovered and junked. Scal-lop guns are wireline-conveyed and shot

6

Hollowcarrier gun

Scallop

Port plug

Highefficiency

High shotdensity

x

x

x

2. The nomenclature of phasing may be a source of con-fusion. A 60° phasing means one shot every 60°azimuthally; a 180° phasing means one shot every180°. Phasing of 0° has all shots in one line, meaningthe angle between shots is actually 360°. Speaking of“reduced phasing” or “reduced phase angle” meansthe angle between shots is smaller. A 45° phasing istherefore “reduced” compared to a 90° phasing.

nA taxonomy of perforating

only through tubing. They are usedmainly in hostile environments or wheredebris is unacceptable.

•Port plug guns, in which charges shootthrough replaceable plugs in a reusablecarrier. These are wireline conveyedmainly for deep penetration and where 4shot-per-foot (spf) density is acceptable.

•High shot density guns, which aredesigned for each casing size to optimizeshot density, hole size, penetration andphasing. The majority of sand controlcompletions use high shot density gunsloaded with charges designed to providelarge entrance holes. All TCP is performedwith high shot density guns.

•The HEGS High-Efficiency Gun System,which is a wireline-conveyed alternativeto port plug guns, with longer carriers thatare faster to load and run. The HEGS sys-tem is available in 31/8- in. and 4-in. outerdiameter. It is rated to 210°F [99°C] and4000 psi, making it useful in many shal-low wells. A big hole charge is availablefor the 4-in. size.

To determine the type of perforation andgun system best suited to the well, a practi-cal first step is to consider the general inter-action of the perforation and reservoir. Asecond step is to look at how perforationdesigns vary for each of the three maintypes of completions: natural, stimulatedand sand control.

Oilfield Review

xx

x

x

guns and systems.

nProgression of shaped-charge detonation. The

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaPrimer

Liner

Detonatingcord

Case Explosive

0 µsec

4 µsec

9.4 µsec

16.6 µsec

A Perforation Glossary

Big hole charge: A shaped charge that gives prior-ity to entrance hole over depth of penetration, usedexclusively in sand control completions. A “bighole” has an entrance diameter of 0.5 to 1.2 in. [13to 30 mm], usually about twice that of a deep pene-trator charge of similar size. Conventional deeppenetrators have an entrance hole diameter of 0.3to 0.5 in. [8 to 13 mm].

Booster: A secondary explosive attached to the endof the detonating cord, used to assure passage ofinitiation between the detonator and detonatingcord or between detonating cords.

Carrier: In hollow carrier guns, a steel tube thatcarries a loading tube and protects it from the well-bore environment. The loading tube secures andaligns the detonating cord and shaped charges. Thedetonator is housed in a firing head attached to thecarrier.

Completion: Work required to make a well ready toproduce oil or gas. It generally includes—not nec-essarily in this order—running and cementing cas-ing, perforating, stimulating the well, running tub-ing and installing control and flow valves. In apermanent completion, the well is not killed afterperforating underbalance and is ready for immedi-ate production. In TCP, the guns remain downholeafter firing. In a temporary completion, the well iskilled after perforating and the workstring retrievedbefore installing the permanent completion.

Deep penetrating charge: A charge design thatgives priority to penetration depth instead ofentrance hole diameter.

Detonating cord: A secondary explosive containedin a protective flexible outer sheath. The detonatoris connected to the detonating cord, which trans-mits the detonation to each shaped charge. It mayalso pass detonation along to another gun via abooster.

schematic at 0 µsec shows the charge components.The volume of explosive is greatest at the apex of theliner and least near its open end. This means that asthe detonation front advances, it activates less explo-sive, resulting in a lower collapse speed near theliner base. The subsequent drawings show the casedeforming as the detonation front advances, thrust-ing the liner into a jet along the shaped-charge axis.The fully formed jet, at 16.6 µsec, is moving at about21,300 feet/sec [6500 m/sec].

October 1992

Detonator: A primary explosive that initiates thedetonating cord. Detonators can be fired electricallyor by impact.

Drillstem test (DST): A temporary completion inwhich a downhole shut-in valve, controlled fromsurface, is incorporated in the workstring, usuallywith a retrievable packer. The well can then beflowed in a test program, either recording data indownhole memory or conveying them to surface inreal time to analyze reservoir properties such aspermeability and reservoir boundaries.

Explosive: There are two types used in well perfo-rating, primary and secondary explosives. The maindifference is in their sensitivity. A primary explo-sive, used in the detonator, detonates from heat(applied by electric power) or impact (from a dropbar or a pressure-driven firing pin). A secondaryexplosive, used in detonating cord, shaped chargesand boosters, is detonated only by another detona-tion, from either a primary explosive or electricallygenerated shock, such as from the S.A.F.E. system.

Limited entry perforating: Varying the number ofperforations in each layer, depending on layerthickness and stress state, to achieve the desiredfracture geometry. Fewer perforations in the layertaking the most fluid restrict flow and divert it intoother layers.

Primer: A small amount of higher sensitivity sec-ondary explosive at the base of the shaped charge,which ensures correct initiation of the charge bythe detonating cord.

Proppant: Material pumped into a hydraulic frac-ture to prevent closure and provide a conduit forproduction once pressure is released. The mostcommon proppant is sand. High-strength prop-pants, like sintered bauxite and zirconium oxideparticles, are used where fracture closure stresswould crush sand.

Shaped charge: A precisely engineered cone ofpressed metal powder, or drawn solid metal, sur-rounded by a secondary explosive and case, andinitiated by detonating cord. Detonation collapsesthe cone into a jet that penetrates the completionand formation (right).

Strip gun: An expendable gun in which individualcharges in capsules are secured and aligned alonga strip of metal.

57

Perforation-Reservoir Interactions— Get-ting StartedFlow efficiency of a perforated completionand stimulation success are determinedmainly by how well the perforation programtakes advantage of the reservoir properties.The program includes determination of twomain factors:•The proper differential between reservoir

and wellbore pressure (The usual prefer-ence is for underbalance, meaning well-bore pressure is less than reservoirpressure at time of perforating).

•Gun selection, which determines penetra-tion tunnel length, shot phasing, shot den-sity and perforation entrance hole diame-ter. The relative importance of thedifferent components of shot geometryvaries with the completion type (below).

The main reservoir property that affects flowefficiency is permeability anisotropy fromwhatever cause—in sandstone, typicallyfrom alignment of grains related to theirdeposition; in carbonates, typically fromfractures or stylolites.3 Shale laminations,natural fractures and wellbore damage,which can cause permeability anisotropy,

are considered separately because they areso common. In most formations, verticalpermeability is lower than horizontal. In allthese cases, productivity is improved by useof guns with high shot densities.

Natural fractures are common in manyreservoirs and may provide high effectivepermeability even when matrix permeabilityis low. However, productivity of perforatedcompletions in fractured reservoirs requiresgood hydraulic communication betweenthe perforations and fracture network. Tomaximize the chances of intersecting a frac-ture, penetration length is the highest prior-ity, with phase angle second. Shot density isless important because fractures form planesand increasing density does not increasecontact with a fracture system. In fracturedformations, a popular gun configurationuses 60° phasing with 5 spf. A Schlumber-ger version of this gun has a large chargethat penetrates 30 in. [76 cm] into the stan-dard API test target.4

An important geometric consideration ofa perforation is how deeply it penetrates—whether it reaches beyond the zone dam-aged during drilling or connects with exist-

Sand Control

Consolidated

Stimulated

PerforationGeometry

AnisotropyIsotropic

PermeabilityPerforationGeometry

NaturalFractures

WellboreDamageOf Any

CauseLaminar

Shale

Natural

Unconsolidated

Completion Type

Shot density

Perforationdiameter

Perforationphasing

Perforationlength

2

3

1

4

2

1

3

4

1 or 2

3 or 4

3 or 4

1 or 2

Shot density

Perforation diameter

Perforationphasing

Perforationlength

1

3

4

2

2

4

3

1

1

4

3

2

3

4

2

1

2

4

3

1

nRelative impor-tance of four maingeometrical factorsin the three com-pletion types,where 1 is greatestand 4 is least. Theoptimum perfora-tion design estab-lishes the propertradeoff of thesefactors. The lowerpart of the figureshows commonconsiderations forperforating naturalcompletions. Whennatural fracturesare present, phas-ing becomes moreimportant thandensity to improvecommunicationbetween fracturesand perforations.

58

ing fractures. The penetration of variousshaped charges is documented in surfacetests and in tests under stress with API targets.Penetration in surface tests is different thanunder stress in the well.5 Unconfined com-pressive strength of test targets is a minimumof 3300 psi, representing only low-strengthreservoir rock (reservoir rock strength rangesfrom 0 to 25,000 psi). To estimate depth ofpenetration into a rock of arbitrary strengthunder a given stress, data measured atunstressed surface conditions have to betransformed. Because rock penetration dataexist for only a few combinations of charges,rock strengths and stresses, a semiempiricalapproach is used that combines experimentaldata with penetration theory.6

Schlumberger calculates penetrationchange caused by formation stress usingexperimental data for three generic chargedesigns after first calculating the change dueto formation strength at zero stress. Thesedata provide transforms implemented in theSPAN Schlumberger Perforating Analysisprogram. The SPAN program consists of twomodules: penetration length calculation andproductivity calculation. In the penetration

3. Stylolites, common in carbonates, function like shalelayers in sandstones, inhibiting vertical migration ofhydrocarbons. They are interlocking wave- or tooth-like seams that often parallel bedding, and containconcentrations of insoluble rock constituents, such asclay and iron oxides. They are thought to be causedby pressure solution, a process that increases contactarea between grains and reduces pore space.

4. The American Petroleum Institute (API) publishes rec-ommendations for testing shaped charges in a docu-ment, API RP-43. Section 1 specifies the length andentrance hole diameter produced by a gun system(charges and carrier) in a steel and concrete target.Section 2 gives this information for single shots into astressed Berea sandstone target. As of this writing, theavailability of target material for Section 2 is underreview by the API.

5. Halleck PM, Saucier RJ, Behrmann LA and Ahrens TJ:“Reduction of Jet Perforator Penetration in RockUnder Stress,” paper SPE 18242, presented at the 63rdSPE Annual Technical Conference and Exhibition,Houston, Texas, USA, October 2-5, 1988.

6. Behrmann LA and Halleck PM: “Effect of Concreteand Berea Strengths on Perforator Performance andResulting Impact on the New API RP-43,” paper SPE18242, presented at the 63rd SPE Annual TechnicalConference and Exhibition, Houston, Texas, USA,October 2-5, 1988.Halleck PM and Behrmann LA: “Penetration ofShaped Charges in Stressed Rock,” in Hustrulid WAand Johnson GA (eds.): Rock Mechanics Contribu-tions and Challenges: Proceedings of the 31st USSymposium. Rotterdam, The Netherlands: A.A.Balkema (1990): 629-636.

7. Karakas M and Tariq S: “Semianalytical ProductivityModels for Perforated Completions,” paper SPE18271, presented at the 63rd SPE Annual TechnicalConference and Exhibition, Houston, Texas, USA,October 2-5, 1988.Economides MJ and Nolte KG (eds): Reservoir Stimu-lation, 2nd ed. Englewood Cliffs, New Jersey, USA:Prentice Hall (1989): 1-17.

8. Pucknell JK and Behrmann LA: “An Investigation ofthe Damaged Zone Created by Perforating,” paperSPE 22811, presented at the 66th SPE Annual Techni-cal Conference and Exhibition, Dallas, Texas, USA,October 6-9, 1991.

module, perforation length and diameterestimates are calculated under downholeconditions for any combination of gun,charge and casing size. It can also calculatepenetration in multiple casing strings. Theseparameters are used in the productivitymodule to evaluate the anticipated produc-tivity of the perforated completion.

Another influence on flow efficiency isformation damage, usually considered inthe context of skin, an index of flow effi-ciency related to properties of the reservoirand completion. Skin comprises a variety ofinfluences: flow convergence, wellboredamage, perforation damage, partial pene-tration (perforation of less than the totalheight of the reservoir) and the anglebetween the perforation and bedding plane.The goal is to design perforations that mini-mize skin and therefore maximize flow effi-ciency (top).

Formation damage is caused by invasionof mud filtrate and cement fluid loss into theformation, creating a zone of lower effectivepermeability around the wellbore (above,right). Extending the perforation beyond thedamaged zone may reduce this skin signifi-cantly, enhancing productivity.7 But even forperforations that do penetrate farther, thewellbore damage zone reduces the effectivetunnel length.

During perforating, a “crushed zone” ofreduced permeability is created around theperforation. In laboratory experiments, thethickness and permeability damage of the

nRelationship of perforation phasing and depth to productivity (left) and to wellbore skin (right). Curveson the left are for undamaged conditions. Damage would reduce their absolute values, but they wouldmaintain the same position relative to each other. For 0° phasing perforation, skin is higher at thewellbore because flow follows a less direct path to the perforation than for the 90° phasing case.Perforations with lower skin distribute the pressure drop over a greater distance from the wellbore,yielding a higher production rate for a given wellbore pressure. The left figure shows the increase inproductivity with perforation length. In the theoretical case of no damage, a 9-in. [23-cm] perforationat 0° phasing has the same productivity as a 3-in. [8-cm] perforation of 90° phasing.

nHow a damaged zone near a perforatedcompletion affects productivity, for a 9-in.perforation with 0° phasing and 4 shotsper foot. The influence of loweredeffective permeability in the damagedzone can be combated by perforationsthat extend into the virgin formation. Inthis example, there is no crushed zone, socrushed zone permeability, kcz, equalsvirgin formation permeability, k. Butpermeability of the damaged zone, kdz, is60% lower than that of the virgin zone.

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaa a aIncreasing pressurePro

duct

ivity

rat

io

Wel

lbor

e

Incr

easi

ng p

rodu

ctiv

ity

Incr

easi

ng s

kin

Perforation length, in. Distance from wellbore

Reservoirpressure

Wellborepressure

Damagedzone

90° 4 spf

0° 4 spfA

B

A

B

1.1

1.0

0.9

0.8

0.7

0.60 3 6 9 12 15

59

Pro

duct

ivity

rat

io

Open hole

Effect for 9-in.perforation

Damaged zone thickness, in.

1.0

0.9

0.8

0.7

0.6

0.50 3 6 9 12 15

No crushed zonekcz/k=1kdz/k=0.4

nPhotomicrographs of rock thin sections, showing the effect of perforation. The leftimage is from rock near the perforation tunnel, showing microfracturing. The right thinsection is undamaged rock. (From Pucknell and Behrmann, reference 8.)

crushed zone are influenced by all vari-ables to varying degrees: the type of shapedcharge, formation type and stress, underbal-ance and cleanup conditions. Pucknell andBehrmann found that permeability near theperforation is reduced because microfrac-turing replaces larger pores with smallerones (above). The current rule of thumb isto assume a crushed zone 1/2 in. [13 mm]

October 1992

thick with permeability reduced by 80% to90%. Recent experimental data, however,cast some doubt on this assumption, withcrushed zone thickness a function of chargesize, pore fluid type, and the preservation ofpermeability when perforating underbal-ance.8

1 mm

The Natural Completion—Perf and ProduceThe natural completion is often defined asthat in which little or no stimulation isrequired for production. This approach isusually chosen for reservoirs that are lessprone to damage, have good transmissibil-ity, and are mechanically stable.

Of primary importance in selecting theperforating gun are its depth of penetrationand effective shot density (see “NaturalCompletion,” next page). Depth is importantbecause the deeper the perforation, thegreater the effective wellbore radius; alsoflow is less likely to be influenced by forma-tion damaged during drilling. In the contextof well productivity, a deep penetratorshoots to a depth 1.5 times that of the well-bore damage.

Shot density also ranks high becausemore holes mean more places for hydrocar-bon to enter the wellbore and a greater like-lihood that perforations will intersect pro-ductive intervals of an anisotropic reservoir.After shot density and depth of penetration,

most important is phasing because, whenproperly chosen, it provides hydrocarbonswith the most direct path to the wellbore(below, left). Under typical flow conditions,perforation diameter does not adverselyaffect flow once it exceeds 0.25 in. [6 mm],which today is provided by nearly all gunsused in natural completions.

A key consideration in perforation designof natural completions is the selection ofoverbalance versus underbalance perforat-ing. Overbalance means the pressure ofwellbore fluids exceeds reservoir pressure atthe time of perforating. Under this condi-tion, wellbore fluids immediately invade theperforation. For this reason, clean fluidswithout solids are preferred to prevent plug-ging of perforations. Cleanup can occuronly when production begins.

Increasingly, wells that have sufficientreservoir pressure to flow to surface unas-sisted are completed in underbalance con-ditions. Underbalance is the trend becauseof wider recognition that it provides cleanerperforations—therefore better produc-

tion—and because of greater availability ofgun systems that allow it. Underbalanceperforating can provide large gains in reser-voir productivity. The question is, howmuch underbalance is appropriate? Exces-sive underbalance risks mechanical damageto the completion or test string by collapsedcasing or a packer that becomes damaged,stuck or unseated. It can also encouragemigration of fines within the reservoir,reducing its permeability. Insufficient under-balance, however, doesn’t effectively cleanthe perforations. Production may thereforebe hindered, mainly by lack of removal ofthe crushed zone and, secondarily, by lackof removal of debris. The crushed zone isthe damaged rock in and around the perfo-ration tunnel; debris is mainly the linermaterial of the spent shaped charge, plusfragments of cement and rock (below).

The optimal underbalance, whichremoves both debris and the crushed zoneand does not damage the formation, accom-plishes virtually all cleanup during the por-tion of initial production that is dominated

60 Oilfield Review

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aDamagedzone

Virgin formation

Chargedebris

Cement

CasingCrushed (low-permeability)zone still exists

Overbalanced perforating before flowing

Part of low-permeabilityzone still exists

Perforation partially pluggedwith charge debris

Overbalanced perforating after flowing

Crushed zone and chargedebris expelled by surgeimmediately after perforating

Ideal underbalanced perforating

nThree idealizedconditions in aperforation tunnel:overbalance perfo-ration before flow-ing, overbalanceperforation afterflowing andunderbalance per-foration. The topfigure indicatesthat withoutcleanup, the perfo-ration tunnel isplugged bycrushed rock andcharge debris. Inthe second case,flow has removedmost chargedebris, but some ofthe low-permeabil-ity crushed zonecreated by the jetremains. In thethird figure, suffi-cient underbal-ance during perfo-rating removeddamage—bothcharge debris andcrushed rock.nThree views of perforating with a

135°/45° phased gun: the gun fired incasing, phasing viewed from the top, andwith the perforated casing unrolled andlaid flat. The 135°/45° designation meansthe angle between successive shots is135°, resulting in an overall phasing of45°. There is 1 vertical inch [2.5 cm]between shots, making 12 shots per foot.In the natural completion, this phasingprovides hydrocarbons with the mostdirect path to the wellbore.

Gun incasing

Phasing from top

Casing unrolled (7 in.)

45°

135°1

5

4

7

28

3

6

0 45 90 135 180 225 270 315 360

1

4

7

2

5

8

6

1

4

72

6

1

2

5

8

3

6

3

6

8

61October 1992

Natural CompletionPerforation Technique Selection

No

No

No

No

No

No

No

No

Casing guns(wireline conveyed only)

Run guns and packeron wireline.

Stab guns throughpermanent packer.Select guns, firingsystem and TCPhardware.(Tubing conveyed)

Run guns and packeron tubing.

Do packer/gun assembly weight and welldeviation allow setting by wireline?

Pivot Gun (perforatesmaximum of 15 feet[5 m] per run)

Is deeper penetration important?or

Is selectivity not required?

Conventional strip guns

Production Test

Exposed gunsHollow carrier guns(scallop or HSD guns,depending on tubingsize)

Run guns belowproduction packer.Select guns, firingsystem and TCPhardware.

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

TCP (high shot density guns)

Does improvement in well cleanup from underbalance perforating justify added operational complexity?

Is the well ≤ 4000 psi, < 210°F [99°C]?

•Is there sufficient rathole for dropping guns, if subsequent remedial operations are required?

andDo any of the following apply?•Shot density > 6 shots per foot•Deviation ≥ 60°•Simultaneous perforation needed for a single, long zone or multiple zones, exceeding what can be perforated in one wireline run

•Two or fewer zones to be shot selectively for a multizone test.

Through-tubing guns(wireline conveyed)

•Will gun or charge debris be a problem for downhole equipment?

or•Are more than two zones to be perforated selectively?

Does completion benefit from high shot density or reduced phase angle?

Port plug guns

Select correctdiameter high shotdensity guncompatible withdownholerestrictions; selectphasing and shotdensity.

HEGS (wireline

conveyed only)

Perforate underbalance. Perforate overbalance with high shot density or casing guns.

Select TCP firingsystem andaccessories tointegrate withDST string.

Is the option of gun retrieval required?or

Are guns with diameter less than the maximumallowed by casing size acceptable?

Drillstem test

Boxes in red outline denote final decision points.

by surge of reservoir fluids into the perfora-tions. Cleanup after this point is negligiblebecause hydrocarbon follows the alreadycleaned paths of least resistance. Duringproduction, pressure drop across damagedareas is insufficient for further cleanup.Recent experiments have shown that if asuboptimal underbalance is used, somecleanup will take place during production,but productivity never reaches that achievedwith optimal underbalance.9

When well testing is planned, underbal-ance perforating has become the standard,particularly when a drillstem test (DST) isincluded. Underbalance perforating is ide-ally suited because a DST includes hard-ware that allows establishing underbalanceand running high shot density guns. Thissetup provides excellent well control andoften saves time because the perforatingguns are run below the test string. Pressuremeasurements can be recorded eitherdownhole or in real time at surface, and areavailable for decision-making during thetest. The MSRT MultiSensor Recorder/Trans-mitter and LINC Latched Inductive Couplingequipment allow real-time measurementand surface readout of downhole pressure.The main advantage of this system is theadded mechanical and safety reliability ofmeasuring pressure below the DST shut-invalve. In addition, memorized data can beread out at surface when LINC equipment isrun, eliminating the need for the cable inthe test string while the well is flowing.

From an operations viewpoint, underbal-ance perforating by wireline-conveyed gunscauses a surge that lifts cable and guns. Thehigh flow rate or liquid slugs associatedwith this surge can blow the guns and cableup the well. A common limit on underbal-ance when perforating via wireline is 700psi, although this is often higher in tightreservoirs, which are not capable of deliver-ing a substantial surge.

The choice of underbalance may bebased on data collected since the early1980s from laboratory and field studies andfrom increasing use of underbalance com-pletions (primarily tubing-conveyed perfo-rating).10 More recently, computer programshave been developed. The IMPACT Inte-grated Mechanical Properties Analysis &Characterization of Near-Wellbore Hetero-geneity interpretation program computes avalue of safe underbalance based on themechanical properties of the formation esti-mated from sonic and density logs. Local

nA family of through-tubing, wireline-conveyed guns. From left, the 0° phased Enerjet(a semiexpendable strip gun); the phased Enerjet, with two rows of charges at 90° (anexpendable strip gun); and the 60° phased scallop gun (a retrievable gun). Unlike theEnerjet, the scallop gun has negligible debris and can be run in hostile environments.

62 Oilfield Review

60° phasedscallop gun

± 45° phasedEnerjet

0° phasedEnerjet

1.69 in. 3.79 in.

Deploymenthead

9 shotsper foot

Run in Hole Deployed

nThe Pivot Gun system in the run-in anddeployed positions. Charge performancein surface tests exceeds that of most cas-ing guns—25-in. [64-cm] penetration and0.33-in. [8-mm] entrance hole diameter inan API RP-43 section 1 target (see footnote4). Shot density is fixed at 4 shots/ft with180° phasing. The Pivot Gun system givesthe deepest possible penetration whenperforating through tubing. The mainlimitation is the maximum gun length of10 feet [3 m]. It is rated to 330°F [165°C]and 12,000 psi.

9. Berhmann LA, Pucknell JK, Bishop SR and Hsia T-Y:“Measurement of Additional Skin Resulting fromPerforation Damage,” paper SPE 22809, presentedat the 66th SPE Annual Technical Conference andExhibition, Dallas, Texas, USA, October 6-9, 1991.Hsia T-Y and Behrmann LA: “Perforating Skin 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.

10. Bell WT: “Perforating Underbalance—Evolving Tech-niques,” Journal of Petroleum Technology 36 (Octo-ber 1984): 1653-1662.King GE, Anderson A and Bingham M: “A FieldStudy of Underbalance Pressures Necessary toObtain Clean Perforations Using Tubing-ConveyedPerforating,” paper SPE 14321, presented at the 60thSPE Annual Technical Conference and Exhibition,Las Vegas, Nevada, USA, September 22-25, 1985.

11. King et al, reference 10.

experience also helps guide the selection ofoptimal underbalance.

Overbalance perforating still has a role,however. Often significant are its speed forshort intervals and the availability of larger,high shot density guns compared to thosefor through-tubing underbalance perfora-tion. The selection of overbalance versusunderbalance rests on weighting economicversus production variables.

A long-recognized disadvantage ofthrough-tubing gun systems is their trade-offbetween phasing and depth of penetra-tion—either 0° phasing with good penetra-tion, or improved phasing with less penetra-tion because of smaller shaped charges(previous page). A recent innovation thataddresses this problem is the Phased Enerjetgun, which provides two rows of charges at

October 1992

90° phasing. A second is the Pivot Gun sys-tem, which delivers casing gun performancewith 180° phasing but can be run throughdiameters as small as 1.78 in. To do this, thegun is inserted into the tubing with thecharges aligned along the axis of the gun.Once in casing, a deployment head is usedto rotate charges 90° to the firing position.The charges then reach the full 3.79-in.outer diameter (left ). In case of a misrun,each pivot charge assembly is designed tobe broken, returning the gun to its original1.69-in. diameter. This allows retrieval ofthe gun with deployed charges. Only thedeployment head is recovered after success-ful perforation; the carrier and fired chargesbecome debris that settles to the bottom ofthe well.

The Stimulated Completion—Getting Morefrom LessStimulated completions fall into two cate-gories, acidizing and hydraulic fracturing(see “Stimulated Completion,” next page).Occasionally, the two are combined in anacid-frac, which improves productivity byusing acid to etch surfaces of hydraulicallyinduced fractures, preventing full closure.

Success of stimulation depends largely onhow well the perforation allows delivery oftreatment fluids and frac pressures into thereservoir. Because these fluids and pressure-induced fractures are intended to movebeyond the perforation, shot phasing, den-sity and hole diameter are of higher prioritythan depth of penetration. Underbalanceperforating is often used because cleanerperforation tunnels give fluids more directpaths to the reservoir. In some cases, such asTCP with high shot density guns, underbal-ance can be increased to where stimulationis not required to improve productivity.11

However, stimulated reservoirs are usuallyof low permeability, greatly limiting thesurge available to clean the perforations.Further increases in underbalance mayachieve no improvement in cleaning.

When stimulating long intervals—oftenconsidered more than 40 or 50 feet [12 to15 m]—or multiple zones, the perforationstrategy may change. Delivering treatmentfluid to all perforations may be difficult.Once fluid enters a zone of higher perme-ability, a path is established that preventsstimulation of zones of lower permeability.Here, limited entry perforating can help. Bymaking a lower number of perforationsthroughout the zone, stimulation can beapplied more uniformly across zones ofvarying permeability. High-permeabilityzones may take more fluid than low-perme-ability zones, but because there are fewer

holes, a high enough pressure can be main-tained to encourage treatment of low-per-meability zones. After stimulation, perfora-tions are often added to optimally producethe zone.

Uniformity of perforation diameter isessential to accurately determine the cumu-lative area of the casing entrance holes.Knowing this area and pumping pressureallows calculation of flow rate into the for-mation, needed to monitor progress of thestimulation. Uniformity and smoothness ofperforation diameter also provide consis-tently sized seats for ball sealers. These areballs of nylon or hard rubber pumped totemporarily block perforations with highfluid intake, thereby diverting injection.

Limited entry perforating is usually donevia wireline. The Selectric system isdesigned specifically for this application. Itconsists of any number of short (1-foot [30-cm]) single-shot guns fired selectively fromthe bottom up, providing uniform entryholes. Unlike other systems, in which a mis-fire terminates the operation, this system haselectrical switches, rather than mechanicalswitches, between guns. These allow firingthe next gun even when there is misfire.

Perforation plays a key role in the successof hydraulic fracturing. Hydraulic fracturinghas two main steps: fracture creation byapplication of pressure, and injection offluid carrying proppant, which holds openthe fractures to allow production (see“Cracking Rock: Progress in Fracture Treat-ment Design,” page 4). Once the fracture iscreated, perforations provide the entrance tothe fracture for the proppant. Perforationdiameter must be sufficient to prevent“bridging,” accumulation of proppant thatblocks the entrance hole, preventing furthertreatment. To quantify causes of bridging,Gruesbeck and Collins performed experi-

63

64 Oilfield Review

Stimulated CompletionPerforation Technique Selection

No

No

No

Are uniform and circularentrance holes a high priority?

andIs 12-in. API section 3penetration acceptable?

Is limited entry perforatingrequired?

Is the well ≤ 4000 psi, < 210°F [99°C]?

Port plug guns

Port plug gunsSelectricsystem

HEGS

Yes

Yes

Yes

Yes

Select correctdiameter of scallopor high shot densitygun compatiblewith downholerestrictions; selectphasing and shotdensity.

It is undetermined whether the well needs stimulation. Could underbalance perforating eliminate the need for stimulation?or

Stimulation is required. Is any added operational complexity of underbalance perforating justified by the likely improvement in well cleanup and stimulation?

No

No

Exposed guns

Through-tubing guns(wireline conveyed)

•Will gun or charge debris be a problem for downhole equipment?

or•Are more than two zones to be perforated selectively?

Will perforation be performed through workstring?

Perforate overbalance.Perforate underbalance(see underbalance perforatingin Natural Completionflowchart).

NoYes

NoYes

No

Select correct diameter high shotdensity gun compatible withdownhole restrictions; selectphasing and shot density.

Casing guns(wireline conveyed)

Wirelineconveyed

Tubingconveyed

Yes

Yes

Will stimulation benefit from high shot density or reduced phase angle?

Is well deviation ≥ 60° or is theinterval long enough to justifyrunning on tubing?

Boxes in red outline denote final decision points.

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaa a aa aaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaCasingWellbore

Hydraulic fracturenormal to least

stress

Area offlow restriction

60° phasing-never > 30° fromfracture

Channel to fracture wings

0° phasingperforatiion

nThe importance of shot phase angle to maximizing communi-cation between perforations and stimulated fractures. Studies offracture and perforation orientations show that for optimumwell productivity, the two lie within 30°, preferably 10°. Thisminimizes fracture initiation pressure and the length of thechannel between the perforation and fracture wings, andincreases the likelihood the fracture will initiate along a perfora-tion. Perforating guns with small phase angle and high shotdensity achieve this optimum angle most effectively. The figureshows that a 0° phasing could place the perforation far from thefracture, which initiates along the plane normal to the leaststress. But in reality, wells to be fractured are often perforated

ments to determine the minimum allowableratio of perforation diameter to proppantdiameter for varying proppant concentra-tions12 (below). They found that the perfora-tion must always be at least twice the prop-pant diameter. When perforation diameter isat least six times proppant diameter, prop-pant concentrations can increase withoutrisk of bridging.

A number of studies have investigated therelationship between perforation phasingand the development of hydraulic fractures.In general, hydraulic fractures propagatenormal to the minimum stress in the portionof the reservoir undisturbed by the presenceof the wellbore. The general conclusion isthat for an ideal fracture job, perforationsare aligned with the maximum stress direc-tion, so fractures extending from the perfo-ration will lie in the plane that has the leastresistance to opening. Methods for align-ment of perforations with hydraulic fracturesare still under investigation. A method indeviated wells was reported by Pearson and

12. Gruesbeck C and Collins RE: “Particle TransportThrough Perforations,” paper SPE 8006, presented atthe 3rd Symposium on Formation Damage Controlof the SPE of AIME, Lafayette, Louisiana, USA,February 15-16, 1978.

13. Pearson CM, Bond AJ, Eck ME and Schmidt JH:“Results of Stress-Oriented and Aligned Perforatingin Fracturing Deviated Wells,” Journal of PetroleumTechnology 44 (January 1992): 10-18.

14. Daneshy AA: “Experimental Investigations ofHydraulic Fracturing Through Perforations,” Journalof Petroleum Technology 25 (October 1973): 1201-1206.

15. Warpinski NR: “Investigation of the Accuracy andReliability of In-Situ Stress Measurements UsingHydraulic Fracturing in Perforated Cased Holes,”Proceedings—Symposium on Rock Mechanics 24(1983): 773-786.

16. Behrmann LA and Elbel JL: “Effect of Perforations onFracture Initiation,” paper SPE 20661, presented atthe 65th SPE Annual Technical Conference andExhibition, New Orleans, Louisiana, USA, Septem-ber 23-26, 1990.

17. Nolte KG: “Application of Fracture Design Based onPressure Analysis,” SPE Production Engineering 3(February 1988): 31-42.

18. Screenout occurs when the fluid carrying proppant islost to the rock matrix, interrupting fracture growth.It results in rapid increase in pumping pressure.

Tap water100-cp HEC solutionP

erfo

ratio

n di

amet

er/

aver

age

part

icle

dia

met

er

Maximum particle concentration, vol/vol

Bridging region

No bridging region

Maximum gravel content, lbm/gal

0 0.15 0.27 0.58

0

2

4

6

8

10

0 2 4 6 8 10 30

nImportance of selecting perforationentrance hole diameter to prevent bridg-ing of proppant in the perforation. Toavoid bridging, the ratio of the perfora-tion diameter to average diameter of theproppant must lie above the curve.These are data for tap water and car-boxymethyl hydroxyethyl cellulose (HEC),a water-based polymer. (After Gruesbeckand Collins, reference 12.)

with guns of 60° phasing or less (dashed lines). This means theperforation is never more than 30° from the fracture. (See Warpin-ski, reference 15.)

colleagues.13 Alignment of 180° phasedshots with the known fracture planereduced perforation friction and signifi-cantly improved fracture treatment. Gunswere aligned by mounting them on bearingsthat allowed rotation. Gun angle was con-trolled by a steering tool or, on TCP jobs,with a weighted half-cylinder that seeks thelow side of the hole. This practice, however,is not widespread. The most practical wayto approach this today is by perforating witha phase angle that increases the likelihood

October 1992

of having shots parallel to the induced frac-ture plane.

Laboratory experiments by Daneshy showthat fracture initiation pressures are higherwhen the fracture and perforation are notparallel and do not intersect.14 Later,Warpinski reported that hydraulic fracturesmay not lie in the same plane as the perfo-ration.15 This observation was based on in-situ mineback experiments in which a shal-low, perforated wellbore was excavated tosee how the fracture propagated. Warpinskialso found that if the perforation and mini-mum stress planes differ by more than 30°,the fracture may initiate in a plane differentfrom that of the perforation. This indicatesthe phase angle should be 60° or less so theperforation is always within 30° of a frac-ture. Minimum phasing of 60° is further sup-ported by recent work of Behrmann andElbel, who showed that minimum fractureinitiation pressure and maximum fluid com-munication between perforations and frac-tures are achieved by minimizing “annularflow”—slurry traveling an annular patharound the casing to communicate with thefracture.16 This occurs when the fractureplane and perforation lie within 30°, prefer-ably 10° (top). Nolte17 previously pointed outthat if the hydraulic fracture does not initiateat the perforations, annular flow may causepremature screenout18 and asymmetric pen-etration of the fracture wings.

65

Standoff as a percent of gun diameter

45°Gunpositioned

in casing

Casing

Nor

mal

ized

ent

ranc

e ho

ledi

amet

er, %

0

40

80

120

0 10 20 30

nCross section of a gun in casing (top) andthe effect of gun/casing standoff on entrancehole diameter for a bighole charge.

The Sand Control Completion—Home ofBig HolesSanding is a problem in weak or unconsoli-dated sandstones. The objective of a sandcontrol completion is to eliminate sandingwhile maintaining a production rate that iseconomic, minimizes reservoir damage andthus maximizes recovery. Near the well-bore, sand movement can reduce perme-ability locally. Produced sand can erodedownhole and surface equipment and itsremoval can be costly. In sufficient quanti-ties, sand can plug the completion or sur-face facilities.

An objective of perforating in these highlyproductive and often unconsolidated sandsis to reduce the near-wellbore pressure gra-dient during production (see “Sand ControlCompletion,” next page). There are twoschools of thought on the best way do this.The established method is to perforate in away that takes advantage of protectionafforded by subsequent gravel packing. The-oretical studies show that perforation geom-etry can sometimes be optimized to obviategravel packing.19

For gravel packing, many large-diameterperforations are preferred to few smallholes. This is because larger holes provide alarger area open to flow and therefore lesspressure drop on production. To achievethis, perforators producing large diameterholes and high shot density are used. A uni-form shot distribution further reduces forma-tion stress in addition to preserving casingstrength (below). Because of the high pro-ductivity of the reservoir, deep penetration is

66

Rem

aini

ng c

asin

g st

reng

th

1

0.9

0.8

0.7

0.6

0.5

0.4

0.30 0.25 0.5 0.75 1.0 1.25 1.5

Entrance hole diameter, in.

• 5 shots per foot, 60° phasing• 6 spf, 60° phasing• 12 spf, 135° phasing• 12 spf, 120° phasing• 12 spf, 120° phasing

nRelationship between perforationentrance hole diameter and phasing oncasing strength. The 135°/45° phased HSDguns achieve the greatest area open to flowwhile maintaining maximum casingstrength. Here, casing strength is normalizedto 1, the strength of unperforated casing.

a lower priority. Depth of penetration is suf-ficient if it assures good communicationwith the reservoir.

To create large, clean perforation tunnels,these wells are typically shot underbalancewith TCP using high shot density guns. Theideal underbalance will sufficiently cleanperforation tunnels without breaking downthe formation. Sand control could perhapsbe provided by maintaining productionrates low enough to prevent collapse of theperforation tunnel’s stable arch—interlock-ing grains, like a keystone arch over a door-way. But such a low production rate is gen-erally uneconomical and arches areunstable when flow conditions change.Instead, the arch is usually stabilized by fill-ing the perforation with gravel (see “SandControl: Why and How?” page 41).

In gravel packing, a wire-wrapped screenor slotted liner is positioned along the perfo-rated interval. A slurry of thickened brinecarrying gravel of closely controlled size ispumped downhole. The gravel fills the per-foration tunnels, creating a “pack.” One keyto tightly packing the perforations is use of afluid that rapidly leaks off into the perfora-tions so the gravel slurry continues flowinguntil the perforation is completely full. Thisslurry is followed by an additional slurry tofill the screen-casing annulus with gravel.The pressure drop during production cannow be distributed across both the near-wellbore area and the gravel pack, whichhelps to reduce stress at the formation.

Clearance between gun and casing has asignificant effect on entrance hole size ofbig hole charges (see “A Perforation Glos-sary,” page 57). Adverse effects of standoffare much less for deep penetrating charges.Running the largest gun size practical for thewell casing provides entrance holes thatminimize the differential pressure across for-mation and pack (left).

Optimizing Perforation Operations—Envi-ronment and SafetyEach perforating system has ratings intendedto ensure safety and minimize operatingrisk. Foremost are ratings for maximum andminimum pressure and “time-at-tempera-ture”—the duration of exposure at a giventemperature. The time-temperature rating isdetermined by the explosive material,which degrades at elevated temperaturesand extended exposure times. This degrada-tion results in a loss of sensitivity—leadingto a potential misfire—and loss of strength—leading to reduced charge performance.Today’s ratings have been established byboth laboratory tests and extensive fieldexperience.

The most common explosives in currentuse are cyclotrimethylene trinitramine, RDXfor short, and cyclotetramethylene tetran-itramine, HMX for short. When conveyedby wireline, RDX is limited to exposure of 1hour at 330°F [166°C], or when tubing con-veyed, to 100 hours at 200°F [93°C]. Simi-larly, HMX survives 1 hour at 400°F[204°C], and 100 hours at 300°F [149°C].At higher temperatures or longer exposures,explosives and special elastomers and lubri-cants are available to perforate reliably at upto 500°F [260°C] for wireline-conveyedapplications and up to 460°F [238°C] forTCP. Explosives for high temperatures,called HNS and PYX, are much more expen-sive and generally stocked only in areaswhere high-temperature wells are common.

It is generally recognized that guns have amaximum pressure rating. Exceeding thisvalue can cause the gun to collapse or fluidto enter, possibly splitting the gun and stick-ing it in the casing if detonation occurs. Lesswell-recognized are gun limitations whenperforating in gas. Not all guns that can befired in a liquid-filled borehole can toleratethe higher shock associated with firing in agas-filled borehole, which lacks the damp-ing effect of wellbore fluid. Some guns must

Oilfield Review

Sand Control CompletionBig Hole Perforation Technique Selection

No

No

No

Wireline-conveyed guns.Select TCP firing systemand accessories.

Select largest high shotdensity gunsrecommended forcasing size.

To minimize risk ofsticking guns, selectguns of reduceddiameter.

Select largest high shot density guns recommended for casing size.

Yes

Yes

Yes

Select TCP firing system and accessoriesto integrate with DSTstring.

Select TCP firingsystem andaccessories to provideperforation surging.

Does improvement in well cleanup from underbalance perforating justify added complexity?

Surge Perforation(Brief, high-intensity flow toclean perfs; no rateinformation collected.)

Drillstem test

Are downhole pressure/flow rate measurementsrequired?

Is well deviation ≥ 60° or is the interval long enough to justify running on tubing?

Perforate underbalance with high shot density gunsand big hole charges

(tubing conveyed).

Perforate overbalance with high shot density guns and big hole charges

(tubing conveyed).

NoYes

Is a significant amount of sand production expectedregardless of underbalance choice?

Boxes in red outline denote final decision points.

be supported by liquid at atmospheric pres-sure or higher. Special carriers are availablefor some guns for use in gas and high-pres-sure settings.

Although perforating guns are sometimesexposed to hostile environments, ratings arerarely specified. The reason is both practicaland technical. A hostile environment com-

October 1992

prises many variables—wellbore tempera-ture, pressure, hydrogen sulfide [H2S], treat-ment acid, carbon dioxide [CO2], durationof exposure and stress during exposure. Notall can be quantified to determine if seriousrisk exists. Because of the demands of perfo-rating, hardware must be robust and of highquality steel, well suited to hostile environ-ments. For wireline-conveyed guns, expo-sure time is minimal. In TCP, where gunsand accessories may be exposed for an

extended period, H2S-resistant accessoriesare available.

Each perforating system has a number offeatures, often redundant, to ensure safe

19. Santarelli FJ, Ouadfel H and Zundel JP: “Optimizingthe Completion Procedure to Minimize Sand Pro-duction Risk,” paper SPE 22797, presented at the66th SPE Annual Technical Conference and Exhibi-tion, Dallas, Texas, USA, October 6-9, 1991.

67

wellsite operations. Guns themselves con-tain only secondary explosives (charges,detonating cord, boosters) and are armedwith the primary explosive (detonator) justprior to running in the well. This allows forsafe loading and handling. Guns are com-monly transported to the wellsite loaded, butarmed only just before being run in the hole.Firing assemblies are designed to protect thedetonator and position it to initiate the deto-nating cord. In the event guns are retrievedunfired, disarming is simple and may beperformed immediately.

In wireline-conveyed perforating, electri-cal detonators are used, fired by applyingpower from surface. The detonators are dis-abled if fluid floods the gun, preventingaccidental detonation.

Surface equipment is shut off andgrounded prior to running and pulling theguns, eliminating accidental application ofpower. In addition, radio transmission,welding and cathodic protection systemsare shut down to eliminate possible strayvoltages. This requirement can be a seriousoperational limitation, for example, elimi-nating radio communication to offshoreplatforms. To safely overcome this limita-tion, the S.A.F.E. Slapper Actuated FiringEquipment system has been developed.20 Inthe S.A.F.E. system, a special initiator is usedthat fires only from a very high voltage pulseof short duration—a pulse not produced byroutine rig operations. The S.A.F.E. initiatorcontains no primary explosive and initiatesonly from a specific signal from surface.

TCP has safety features common to manyother techniques. In Schlumberger systems,firing heads are connected to the top of thegun string with a blank interval of at least 10feet [3 m] above the top shot. This allowsarming of guns only after the charges arebelow the rig floor, away from personnel.Firing pins require a minimum of 150 to300 psi to drive into detonators, ensuring nopossibility of firing until below surface.

The Trigger Charge Firing system allowsrunning and positioning the TCP guns in thewell with no detonator. The firing head issubsequently run on wireline. This providesadditional safety while running the guns andretrieving the firing head prior to pullingmisfired guns.

New Completion Methods—Access for BigGunsEfforts of well operators to be more cost-effective have led to variations in comple-tion techniques, and concomitant innova-tions in perforating. A completion that hasgained popularity in the North Sea andVenezuela is called the monobore. As thename implies, a monobore completion hasa production string of uniform diameter,from the reservoir to surface. Casing is setwell above the reservoir, up to half the welldepth. Then, a smaller diameter hole isdrilled to total depth and a long liner run(Liner is any casing that doesn’t reach sur-face). Once the liner is set, production tub-ing of the same diameter as the liner is runand engages a sealing assembly on top ofthe liner. The well now has a “monobore,”with the liner serving as both casing, provid-ing protection, and as tubing, conveyingproduction. This approach has the advan-tage of requiring a less expensive, smallerhole with lower tubular costs, yet provides alarge-diameter production string. The well isthen perforated with high shot density guns,either wireline conveyed or anchored in theliner after running on wireline or tubing.The guns are then dropped, either automati-cally upon firing or mechanically via a wire-line trip.

Variations of the monobore technique arealready in use. One is to set a permanentpacker on production tubing at the top ofthe liner with guns suspended below (left).This allows use of the largest possible highshot density guns, while retaining the eco-nomic advantage of the monobore tech-nique. Underbalance is established and awireline assembly is then run in and latchedto the guns, which are lowered to targetdepth. They are set using an anchor thathangs them in the casing. The wireline isthen pulled out and the guns fired by pres-sure actuation. The guns are then released

nA variation of the monobore completion,using a permanent packer. Monoborecompletions are most common in theNorth Sea and Venezuela.

68 Oilfield Review

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaa a aa aaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaa a a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaWireline latch/anchor setting

tool

Safety spacer

High shotdensity gun

Anchorgripping

casing

20. Huber K, Pousset M and White D: “New Technol-ogy for Saving Lives,” Oilfield Review 2, no. 4(October 1990): 40-52.

nHigh shot density guns run below awireline-set packer. This permanent com-pletion allows underbalance perforatingwith the largest possible diameter guns ina permanent completion. Guns are usu-ally dropped after firing.

as in the standard monobore method, eitherautomatically or by wireline trip. In perforat-ing long intervals, where gun string weightexceeds the wireline limit, the guns can berun and anchored in the monobore prior torunning the permanent packer and produc-tion tubing.

Another type of completion allows largerdiameter guns than could be stabbedthrough a permanent packer or run throughtubing (left). To achieve this, a permanentpacker is set on wireline, with high shotdensity guns suspended below. Then, tubingis run, underbalance established and theguns fired and dropped.

A third perforation system is used for dual-string completions—two tubing strings runadjacent to each other to isolate productionfrom two zones (right). This allows under-balance perforating of the upper intervalwithout killing the well prior to production.

Developing a perforation strategy involvesanalyzing the reservoir using all data avail-able to design the job for the anticipatedconditions. A common pitfall is to bypassthis process, repeating what is consideredtried and true or what worked last time. Thisresults in some wins and some losses. Thebest approach is to arrive at a perforationstrategy by combining both field and operat-ing experience. Only this approach allowsthe operator to duplicate what went right inprevious jobs, avoid repeating mistakes, andtest new techniques that hold promise.

—JMK

nA dual-string completion that allowsunderbalance perforating of both strings.In this instance, the lower zone is perfo-rated with high shot density gunsstabbed through a packer. The upperzone is perforated with high shot densityguns suspended below a dual stringpacker. These guns are loaded and ori-ented to perforate the half of the casingopposite the adjacent long string.

69October 1992

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaa a aa aaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaa a a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaWireline

Casing collarlocator

Permanentpacker

Productionports

High shotdensity gun

Wirelinepressuresetting tool

Firing headaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaa a aa aaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaa a a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaDual stringpacker

Oriented highshot density

guns

High shotdensity guns

Casing

Productionports