56. Remedial Cleanup, Sand Control, And Other Simulation Treatment

7/29/2019 56. Remedial Cleanup, Sand Control, And Other Simulation Treatment

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Chapter 56Rem edial C leanup, Sand Control,and Other Stimulation TreatmentsA.W. Coulter Jr., Dwell Schlumberger*S.J. Martinez, Information Services Div., U. of Tulsa*K.F. Fischer, Dwell Schlumberger*

IntroductionAlthough fracturing and acidizing are the most commo ntypes of well stimulation used today, other type s of stimu-lation treatments also are used. Some of these treatmentsuse acid-type materials but are not generally classed asacidizing jobs. These treatments are specifically designedfor the removal of a blocking agent such as gypsum(gyp), drilling mud. paraffin, formation silicate, par-ticles, or other materials on the wellbore face and in theformation immediately adjacent.

The first stimulation treatments used in oil and gas wellsinvolved explosives such as dynamite or nitroglycerin.This method was used for many decades before being dis-continued for safety reasons. More recent attempts tostimulate with explosives involved displacem ent of explo-sive material into the producing formation in a fracture-type treatment. The material was then detonated. Becausethis method was hazardous, research involving explosiveshas been discontinued.ReperforationIn some cases it is useful to reperforate a well in the samezone in which it was originally perfo rated. The detona-tion of the gun loosens blocking materials in the forma-tion adjacent to the well and in the previous perforation s,and simultaneously creates mo re drainage holes into thewellbore. Also, over a period of time, some of the origi-nal perforation tunnels mig ht become totally blocked bymigratory fines, scale, gyp, or paraffin. Reperfo ration insuch cases could greatly increase drainage area into thewellbore.Abrasive Jet CleaningAnother method used to clean up shot holes or to removegyp contaminating the formation near the wellbore make s

use of a jetting tool. One or mo re strea ms of sand-ladenfluid are forced through a hardened, specially designednozzle at pressures of 1,00 0 psi and up, to impinge againstthe wall of the borehole.These jets, striking against th e face of the open h ole,loosen and break up gyp deposits, and may penetrate theformation. If the tool is moved up and down while jet-ting, the entire bor ehole can be cleaned. This same toolmay be used for perforating pipe; the high-pressure jetsof sand-laden fluid are able to cut throug h %-in-thick steelpipe in 15 to 30 seconds. They can then penetrate the for-mation to a depth of 12 or 15 in. in another 5 minutesor so, forming large unobstructed channels for the pro-duction of reservoir fluids.Mud RemovalSeveral materials are used to remov e drilling mud fromthe borehole and the adjacent formation. The most com-monly used material is a mud-dissolving acid consistingof inhibited hydrochloric acid (HCI) with an added fluo-ride. This material dissolves part of the mud and loosensthe remainder so that it may be flushed out. Mud-removalagents are often used ahead of fracturing, acid jobs, orcement, to clean the face of the pay, to allow a lowerbreakdo wn pressure, and to minimize mud contamination.Acid cleanup solutions, containing special surfactantsthat increase penetration and provide special mud-dispersing prope rties, are also used when an infiltrationof mud into the formation is suspecte d.

Other solutions, containing phosphates or other chem-icals, may be used to loosen an d disperse mud particlesso they can be more easily flushed from their position inor adjacent to the wellbore.Special blends of surfactants, iron chelating ag ents, andmud-dispersing agents also have been effective in remov-ing mud from the formation.


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56-2 PETROLEUM ENGINEERING HANDBOOK

Water Blocks and EmulsionsOil- and/or water-base d solutions containing low-surface-tension, emulsion-breaking agents have been used suc-cessfully to remov e w ater blocks or emulsions from a for-mation. M ore recently, solutions o f special surfactants andalcohols have become popular. These materials arepumped into the formation to contact the water or emul-sion block. B y changing the blocking materials physicalcharacteristics , the solution enables the blocking fluid tobe produ ced. Treatments of this type usually con sist ofa specialized, commercially available p roduct with an oil-carrying agent. If a large zo ne is to be treated, divertingagents, ball sealers, or packers should be used to ensurethat the solution con tacts the blocking fluids. Oth erwise ,the chemicals will probably enter the more permeable andnonblocked portions of the formation and miss the block-age completely.Scale DepositsWhen a well produces some water, gyp deposits may ac-cumulate on the formation face and on downhole equip-ment and thereby reduce production. These deposits mayhave low solubility and be difficult to remove. Solutionsof HCI and ethylenediaminetetraacetic acid (EDTA) canoften be used to remove such scales. So luble portions ofthe scale are dissolved by the HCl while the chelating ac-tion of EDTA breaks up and dissolves much of the re-maining sc ale portions. When deposits containhydroca rbons mixed with acid-soluble scales, a solvent-in-acid blend of aromatic solvents d ispersed in HCl canbe used to clean the wellbore, down hole equipment, andthe first few inches of formation around the wellbore (crit-ical area) through which all fluids must pass to enter thewellbore. These blends are designed as a single stage thatprovides the benefits o f both an organic solvent and anacid solvent that contact the deposits continuously.Paraffin RemovalSeveral goo d comme rcial paraffin solvents are on the mar-ket. These materials can be circulated past the affectedparts of the wellbore or simply dumped into the boreholeand allowed to soak opposite the trouble area for a peri-od of time. Soaking, how ever, is much less effective be-cause the solvent bec omes saturated at the point of contactand stagnates.In the past, many paraffin so lvents havt contained chlo-rinated materials having an organic chloride ion. Presuma-bly such materials have been taken off the market becauseof problem s encountered in refineries with poisoning ofcertain catalysts by organic chlorides. The nonorganicchloride ion from HC l is wate r soluble, and it can be read-ily extracte d from th e oil during refining p rocesse s. There-fore, the problem of catalyst poisoning does not arise whenHCl is used in paraffin-removal formulations such as aciddispersions.

Hot-oil treatments also are commonly used to removeparaffin. In such a treatment, heated oil is pump ed downthe tubing an d into the formation. The hot oil dissolvesthe paraffin deposits and carries them out of the wellborewhen the well is produ ced. When this technique is used,hot-oil treatments are usually perfo rmed on a regularlyscheduled basis.

Paraffin inhibitors are a recent development. These aredesigned to create a hydrophilic surface on the metal well

equipment. This in turn minimizes the adheren ce ofparaffin accum ulations to the treated surfaces.Large-Volume Injection TreatmentsA simple technique often used to free or to open block-ages within the formation consists simply o f pumping largevolumes of crude oil, kerose ne, or distillate into the for-mation. These treatments are especially eff ective whenthe formation is blocked by fine silicates o r other solids.Pumping the oil into the formation may rearrange thesefine particles so that flow channels to the wellbore arereopene d. Sometim es it is helpful to add surface-activeagents or emulsion-breaking agents to the oil.Steam InjectionIn some areas where low-gravity crude is produced, steamis used to heat and reduce the viscosity of the oil and there-by allow the oil to move more easily to the wellbore. Twotypes of steam injection are used. In some areas, steamis injected into a central injection well and the oil pro-duced from adjacent or surrounding wells.The other ty pe of injection is often referred to as huffn puff. This consists of alternate steam-injection andoil-production cycles from the same well.General Comments

In any case of production decline, it is important to haveall available facts and to make the best possible analysisfrom these facts as to the factors contributing to thedecline. If the problem is not analyzed as completely aspossible before treatment, a great deal of money may bewaste d in the use of an incorrect treatment. Also,whenev er a fluid is to be pump ed into a specific part ofa zone, some chemical o r mechanical method should beused to ensure that the fluid enters the proper zone.Sand ControlSand Formation Properties and GeologyMost oil and gas wells produce through sandstone for-mations th at were deposited in a marine or detrital envi-ronment. Marine-deposited sands, where most of thehydrocarbons are found, are often cemented with calcare-ous or siliceous minerals and may be strongly consoli-dated. In contrast, Miocen e and younger sands are oftenunconsolidated or only partially con solidated with soft clayor silt. These structurally wea k formations may not re-strain grain movement. When produced at high flow rates,they may produce sand along with the fluids.Why Sand is ProducedFluid movement through sandstone reservoirs createsstresses on the sand grains because of fluid pressure differ-ences, fluid friction, and overburden pressure s. If thesestresses excee d the formation-restraining forces, then sandgrains and fines can move and may be produced with thefluid. Rapid changes in fluid production rates and fluidphase s cause unstable conditions that can result in in-creased sand production. When a well starts to pro ducewater, it will often start to produce sand. M uecke dem-onstrated that particle movem ent takes place in a mul-tiphase system when the wetting phase starts to move.Even consolidated sandstone can be mechanically andchemically damaged with time as the reservoir is pro-duced. O verburden stress on sand grains increases as the


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REMEDIAL CLEANUP, SAND CONTROL, & OTHER STIMULATION TREATMENTS 56-3

reservoir pressure decreases. Water movement can dis-solve minerals that cement sand grains as well as changethe carrying capacity o f the formation fluids. Fines migra-tion can reduce the permeability in the perforation tun-nels. This can result in a higher pressure drop into thewellbore and a change in formation stresses. A calcite-cemented formation can be damaged by an improperlydesigned acid treatment, and increased sand productioncan result.Consequences of Sand ProductionSand movemen t in unconsolidated formations and its ul-timate production with oil and/or gas creates a numberof costly and potentially dangero us problems. Mos t com-mon among these problems are the following.2,31, Production interruptions can be caused by sand plug-ging the casing, tubing, flow lines, or separa tor.2. Casing collapse can be caused by changes in over-burden pressure and stresses within the formation.

3. Downhole and surface equipment can be destroyed,resulting in downtim e for equipment replacemen ts, spills,cleanup or even an uncontrolled blowou t.

4. Disposal of produced sands is costly because regu-lations require the disposed sands to be essentially oil-free.Methods of Sand ControlHigher allowable production rates have increased the needfor more effective and durable sand-control systems,which exhibit minimal permeability impairment. Experi-ence indicates that sand control should be implementedbefore the formation is seriously d isturbed by sandremoval.4 Four general types of sand-control methodshave been developed to reduce or to prevent the move-ment of formation sands with produced fluids.1. In some cas es, sand production can be preventedmerely by restricting the production rate and thus reduc-ing the drag forces on the sand grains. 4 This simple ap-proac h is usually uneconomical. Increasing perforationsize and density along with the use of clean, nondamag-ing completion fluids will help to decre ase fluid velocityand drawdown pressure at higher production rates.2. Gravel packing is the oldest, simplest, and most con-sistently reliable metho d of sand control. It has wide ap-plication bo th on land and offsho re. Advances ingravel-pack technology, which use a viscous fluid to car-ry high gravel concentrations around a screen, have re-sulted in faster and more prod uctive gravel pac ks.Improv ed completion tools and through-tubing tools thateliminate the need for costly workover rigs have expand-ed the application of gravel packing.

3. Sand consolidation plastic treatments inject resinsinto the producing interval, binding the formation sandgrains together while leaving the pore spaces open. Withuse of special preflush systems an d diverting agents, in-tervals up to 30 ft thick ha ve been successfully consoli-dated and provided with the strength necessary to allowhigh production rates.4. Resin-coated gravel packing places gravel coatedwith a resin both inside and outside the perforations andin the casing. As the resin cures, the sand grains are boundtogeth er. A strong, highly permea ble, synthetic sandstonefilter results. After curing, the excess resin-coated grav-el is drilled fr om the casing, resulting in a full-open well-bore. This gravel pack can be used with or without a

screen, in primary or remedial work, and through coiledor concentric tubing.The type of sand-control method selected will dependon the specific well conditions. Important variables suchas grain-size distribution, clay content, interval length,bottomh ole temper ature, wellbore deviation, mechanicalconfiguration, bottomh ole pressure, anticipated produc-tion rates, and cost should be considered before decidingon the method of sand control best suited to the well.Formation SamplingThe most important design param eter in sand control isthe formation sand grain size. The success of gravel-packmethods relies upon formation particles being restrainedby the larger pack gravel. Chances for a successful sandcontrol job are highest when representative samples ofthe formation are available for sieve analysis. This ena-bles selection of the prope r size of gravel.

The most representative formation samples are obtainedfrom rubber-sleeve core barrels. Sidewall cores, althoughthey contain crushed grains and mud contamination, arethe second choice. Bailed or separator samples are notrepresentative since sand grains may have beensegrega ted-the larger grains remaining in the hole andthe smaller grains being produ ced with the well fluids.Formation AnalysisFormation sand sieve analyses shou ld be conducted to de-termine formation properties under bottomhole conditions.Treatment steps in acidizing, clay stabilization, and sandcontrol can be determined . Typical sand stone analyses in-clude permeability, porosity, response to acid, mineral-ogy, petrog raphic analysis, scanning electron microsc opeanalysis, X -ray diffraction analysis, and optical emissionspectrographic analysis.Well PreparationA successful sand-control installation is dependent on fol-lowing the recommended procedures for all phases of thedrilling and completion operations. Selection of the prop ergravel size or resin, use of nondamaging drilling and com-pletion fluids, perforation density and cleanliness, andgravel placement are among the important factors affect-ing well productivity. The following are some of the fac-tors that should be considered before any sand controlproced ures are initiated.Cleanliness. Clean Tubing. Steps must be taken to en-sure that new or used tubing is as free as possible of rust,scale, mill varnish, and other contaminants or obstruc-tions. Many o f the contaminants can be remov ed chemi-cally through the use of solvents o r mechanically byrabbiting the joints before running.Acidizing the tubing string af ter it has been run willremove rust and some of the pipe dope that accumulates(see next section) inside th e tubing. The string can be acid-ized by pumping HCl down the string to within 100 to200 ft of the bottom and then immediately reverse-circulating the acid out. Complexing or reducing agentsshould then be used. The acid should not be allowed toexit the tubing string nor to reach the perforations. Anyiron present in the ferric state could precipitate asgelatinous ferric hydroxide (FeOH), which can be ex-tremely dam aging to the formation.


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TABLE 56.1-PRESSURE DROP ACROSS A SAND-FILLED PERFORATION

GravelU.S. 10120 mesh,

500 darcies

U.S. 20/40 mesh,119 darcies

U.S. 40/60 mesh,40 darcies

Formation sand,1 darcy

Flow Rate, q(BPDlperforaiion)

102550

100102550

100102550

10010

Pipe Dope. Pipe dope, when improperly applied, maybe squeezed inside the tubing at the joints and can be trans-ported into the formation by treatment fluids or becom elodged in the screen or liner slots, resulting in decre asedproduction. It is virtually impossible to remov e the solidsfound in pipe dope from an oil or gas well chemically.Pipe dope should therefore be applied sparingly to the pinend of the tubing. It should not be used inside the collarwhere it can be squeezed into the tubing strmg w hen thejoint is made up.

Filtering of Fluids. Significant reduction of oil and gasproduction can be caused by formation damage causedby solids in the fluids used in well completion or work-over operations. Clays, silt, or organic solids injected intoa perforated interval can become trapped in the forma-tion matrix or in the perforation tunnels whe re they canact as a low-permeability chok e, reducing the productivityof the well.Cement Bond. A good bond between the cement and theformation and between the cement and the casing is es-sential to isolate the producing zone. Primary cementingis one of the most critical ph ases in a successful well com-pletion. and goo d cementing practices sho uld be followe d.

Fig. 56.1-Hrgher perforatron density rncreases the success ra-tro for sand consolrdatron treatments.

Ap (psi) WithPerforation Diameter of

3/8 n. 12 in. Vi in. 1 in.0.6 0.2 0.1 0.05

24.0 8.0 2.3 1 oo132.0 44.0 10.0 4.00495.0 175.0 37.0 13.00

2,079.o 666.0 137.0 48.002.0 1 o 0.4 0.20

55.0 21.0 6.0 3.00272.0 99.0 25.0 11 .oo983.0 357.0 81 .O 31 .oo

4,037.o 1,298.O 282.0 104.006.0 3.0 1.3 0.70

177.0 67.0 20.0 9.00893.0 324.0 80.0 33.00

3,250.O 1,178.O 260.0 98.0013,400.o 4,360.O 927.0 323.00

450.0 190.0 64.0 32.0027,760.O 9,280.O 2,091 .O 808.00

An in-gauge borehole is important, and the string shouldbe equippe d with adequ ate centralizers, particularly indeviated holes. Spacer fluids and cement slurry proper-ties must be controlled. The string should be equipp edwith scratchers through critical zones and either rotatedor recipro cated during placement of the slurry. Turbu-lent flow is recommended.If there is any indication of poor bonding, a squeezecement job should be perform ed. Intervals to be treatedseparately should be effectively isolated by bonded ce-ment to reduce the possibility of communication betweenzones.Perforations. Th e success of sand-control treatments incased holes, m easured in terms of well productivity andtreatment life, is greatly affe cted by perforation size anddensity and by perforating dama ge. Perforation tunnelsmust be open so they can be filled w ith pack gravel toprevent filling with formation sand. If perforation s areplugged , gravel cann ot be depos ited in the tunnels (as car-rier fluids flow into the formation) and formation con-solidation c hemicals cannot be injected.If formation sand is lodged in the perforation tunnels,the pressure drop within the perforation can be excessive,even though th e permeability of the formation sand is rcla-tively high. Experimental results indicate that fluid flowcan be turbulent in gravel-filled perforation tunnels withpressure drops far greater than those predicted by Dar-cys linear flow equations. As shown in Table 56.1, pres-sure drop within a gravel-filled perforation tunnel can bequite significant. 4m6The greater the perforation density, the less the draw-down thro ugh each perforation tunnel and the less the ve-locity throug h each effective perfora tion. Intervalsperforated 4 shotsift show cumulative production beforesanding to be seven times greater than intervals perforat-ed with only 1 shot/ft. Two perforation s per foot showtwo-thirds the capacity of 4 shots/ft.

In wells gravel-p acked effectively. the lower fluid ve-locity resulting from high perforation density and large-diameter perforations reduces screen erosion and increasesthe ltfc of the sand-control treatment. Pressure drop


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through higher-density, large-diameter p erforations alsois reduced , resulting in higher we llhead pre ssure andgreater oil or gas production.

For sand consolidation, closely spaced perforations (8to I2 shots/ft) increase the likelihood of a uniform plas-tic pattern around the wellbore, even if some of the per-forations are plugged. Fig. 56.1 illustrates wha t canhappen when a perforation is plugged. Note that the sandis not consolidated behind the plugged per foration on theright side of Fig. 56 .1(a), and that this is the spot wherea failure is most a pt to occur.Perforation Cleaning. The high-pressure jet from a per-forating gun pierces the casing and forms a hole by pul-verizing cement and formation into comp acted particles.Cement and material from the jet charge are mixed withthe formation material in the comp acted zone while loosedebris fills the perforation tunnel. It is necessary to re-move this debris from the perforation tunnel to increasethe probability of success in sand consolidation or gravelpacking. Fig. 56.2 show s the dama ge that can occur inthe perforation and on the face of the formation duringdrilling and perforating. 8

Perforation-cleaning metho ds include backflow , under-balanced perforating, backsurging, washing, and acidstimulation, or combinations of these. These method s arediscussed next.

Backflow. Flowing the well may not clean up more thana few perforations and, if enough differential is availableto purge debris from the perforations, the well may sandup. Backflow must be done slowly and carefully.

Underbalance. Perforating with hydrostatic pr essureless than formation pressure allows tunnel debris to becarried into the wellbore with the first surge of fluid fromthe formation.

Backsurging. Backsurging techniques dislodge gundebris and loose material from perforation tunnels by sud-den exposure of a perforated zone to an open chamberat atmosph eric pressure. The differential p ressure createdcauses formation fluids to surge throu gh the perforationsinto the casing, flushing the perforation tunnels. Back-surging has proved to be very successful in improvingproductivity.

Perforation Washing. Washing IS achieved by strad-dling a small increment of the perfora ted interval w itha special tool and injecting nondamaging fluids into theperforations in the increment. The fluid circulates out-side the casing and back through the perforations nearestthe tool seals, removing debris and formation sand fromthe perforations and from behind the casing. The tool ismoved in increments equal to the seal spacing until theentire perforated interval is wash ed.After the perforations are washed or surged and debrisis circulated out, a positive-depth indicator is placed inthe well below the perforations to establish a referencepoint from which the string is accurately spaced out. Thisis especially important in multiple com pletions with close-ly spaced producing intervals.M&-LXAcid Stimulation. The purpose of matrix acidiz-ing is to penetrate the formation at less than fracturingpressure and to remove d amage from perforation tunnelsand the critical area surrounding the wellbore. Mud filtercake, silt, and clay are typical damag ing materials thatmay be removed by mud acid to restore a wells naturalproductivity.

Fig. 56.2-Drawing of a perforation tunnel showing fluid inva-sion from the wellbore into the formation, and debrisin the perforation and the compacted zone surround-ing the perforation tunnel.

The formation solubility should be determined in bothmud acid and HCI. If mud acid is used, a formation solu-bility at least 10% gre ater in mud acid than in HCl is pre-ferred. M oreove r, the formation solubility in HCl shouldbe less than 20% to avoid calcium fluoride precipitation.

A typical three-step acid stimulation consists of an HCIpreflush, matrix treatment with mud acid, and an HCIoverflush.

HCl Prejlush. A preflush of 50 to 100 gal/fi of perfo-rations is advisable. The HC I is used to prevent contactof mud acid with calcareous materials o r formation brine.This prevents or reduces the chances o f precipitation ofcalcium fluoride and various fluosilicates.

Matrix Treatment With Acid. The proper volume ofmud acid should be injected to remove damage near thewellbore. Usually 50 to 200 gal/ft of perforations is re-quired. Success in matrix acidizing depends on acid con-tacting the entire production interval. This is achievedthrough the use of diverting agents.A mutual solvent comprised of ethylene glycolmonobutyl ether ).I0 is sometimes needed to achieve goodresults. It is an effective water-wetting agent, demulsifi-er. and interfacial-tension reducer.

Overflush. Completion or work over brine should notbe used as an overflush for mud acid because of the pos-sible precipitation of sodium, potassium, or calciumtluosilicates. The use of dilute HCI, amm onium-chloridesolution, light oil, or nitrogen as the overflush agent isrecommended.Clay ControlIn many cases, formation permeability may be damag edby the various clay materials present. Many clays arewater sensitive, and contact w ith foreign fluids may causedamag e by two mechanisms. The first, and probably themore critical, m echanism is dispersion and migration ofthe clay particles. Dispersion may be caused by charg edifferences or by fluid movement. The dispersed claysare then free to move thro ugh the formation until theyenter an opening too small to pass, thus lodging and reduc-ing permeability. The second mechanism is expansion orswelling of the clay particles. Water abso rbed between


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DIAMETER RATO PA CK MEDIAN/FORM MEDIAN

Fig. 56.3-Gravel-pack permeability impairment caused by for-mation sand invasion is illustrated by curve of changein ratio of increases. A ratio of 5 to 6 is the largestgravel that will stop all sand entry. Theoretical curvebeyond 14 indicates sand flowing freely through grav-el (after Ref. 12).

TABLE 56.2A-AVAILABLE GRAVEL SIZES

GravelSize I(in.) -

0.006 to 0.017 40/l 00 0.0120.008 to 0.017 40/70 0.0130.010 to 0.017 40160 0.0140.012 lo 0.023 30150 0.0170.017 lo 0.033 20/40 0.0250.023 to 0.047 16130 0.0350.033 to 0.066 12120 0.0500.033 to 0.079 1o/20 0.0560.047 to 0.079 10116 0.0630.066 to 0.094 8112 0.0800.079 to 0.132 6/10 0.106

J.S. SieveNumber

ApproximateMedian

Diameter[in.)

TABLE 5&2B-FORMATION SANDSIEVE ANALYSIS

U.S. SieveNumber

30405070

100140200270325400PAN

Cumulative Wt% RetainedSample A Sample B

0.2 0.11.2 0.65.1 2.5

16.0 7.535.0 19.062.0 39.082.0 58.093.0 77.097.0 86.098.3 90.0

100.0 100.0

the clay particles causes the particles to expand , with acorresponding decrea se in pore volume and the pluggingof pore channels.To avoid a production decrea se, it is important to sta-bilize clays before or along with a sand-control treatment.Gravel SelectionGravel should be sized to prevent invasion of the packby the finest formation sand. I-13 For example, if 20%of a gravel pack is fine sands, the permeability will be35% less than if no fines were present. 13,14A gravel size should be selected that will restrict themovement of fine formation sand but will not reduce theflow of fluids to uneconomical rates. Saucier I2 suggeststhat the gravel size for controlling uniform sands shouldbe five to six times the diameter of the mean (median)formation sand grain size. Deg ree of pack impairment isillustrated in Fig. 56.3 , which show s the ratio of effec-tive to initial p ack permeab ility vs. the ratio of the packmedian diameter to the formation median diam eter.Prope r gravel size is determined by the following steps.1. Obtain a representative formation sample; closelyspaced samples from rubber-sleeve cores provide the bestdesign bases.

2. Perform a sieve analysis.3. Plot the sieve analysis data on either a cumulativelogarithmic diagram (S-plot) or a logarithmic probabilitydiagram.4. Calculate th e gravel median grain diameter using a

five-to-six multiple of the 50-percentile formation graindiameter. When multiple cores from a single zone areprovided , they should be analyzed and plotted separa te-ly. The samples should not be mixed. The sample withthe smallest 50-percen tile grain diameter is used to selectthe gravel. Tables 56.2A and 56.2B list some of the com-mercial grav el sizes available. Gravel sho uld be screenedand checked to verify size and distribution.

Sieve analysis data for two rubber-sleeve, core-barrelsamples, taken from the same zone, are tabulated and areplotted in Figs. 56.4 and 56.5. Note that Sample A is froma portion of the zone that contains coarser sand than theportion from which Sample B was taken. If Sample Aalone had been taken, a coarser gravel would have beenselected, and a gravel-pack failure could have resulted.In Fig. 56.6, produc ed, bailed, and core-barrel sam-ples from the same well zone have been plotted. The core-barrel sample plo ts as a straight line (minor variations aretypical, how ever). The dashe d variation shown could in-dicate a sieving or weight problem. A bailed sample wo uldtypically rise on the left because finer formation grainswould have been produced , leaving the larger grains inthe wellbore. Conversely, a produ ced sample would riseon the right, indicating an excess o f fines.The same data are shown in an S-plot (Fig. 5 6.7). Eachof the curves is typically S-shap ed, and any curve plottedby itself would not readily be interpreted as varying fromthe norm. An error in gravel selection could result if thesample type were not known.Gravel Quality. Studies have indicated that gravels con-taining fine particles outside the specified range will havelower permeability. I4 Some supply sources furnish grav-el with excessive amounts of particles smaller than spec-ified. Angular gravels may bc broken du ring shipping and


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CUMULATI VE LOG DIAGRAM (S.PLOT)U. S. SI EVE NUMBER

GRAI N DIAMETER, in

Fig. 56.4-Data from sieve analyses of Formation Sand Sam-ples A and B, taken from the same zone, are plottedhere. Note that using Plot A would result in the selec-tion of 20/40-mesh gravel rather than the better choiceof 40/60-mesh gravel for packing this zone, as indi-cated by Plot B.

handling, thereby creating fine particles th at reduce thequality of the gravel. Round ed gravels prov ide tigh ter,more uniform compaction and somewhat higher permea-bility than angular gravels.Screen SelectionMany types of wire-w rapped screens are available, in-cluding ribbed, all-welded, groov ed, and wrapped -on-pipe. The all-welded screen has the wrapping wireresistance-welded to wire ribs at each point of contact.Spacer lugs, solder strips, and weld beads are not requiredand, therefor e, the all-welded screen is stronger and more

LOGARI THM C PROBABILI TYDI AGRAM

; Em8;

IH g:::-* - P s*llPeP torn1rl;l iCUMULATI VE PERCENTBY WEI GHT

Fig. 56.5-Data for Samples A and B are again plotted here.Sieve-analysis data for sands with a normal grain-sizedistribution will plot as a straight line on a log proba-bility grid. The logarithmic probability diagram has anadvantage over the S-plot in that sampling errors aremore readily detected. Variations from the straight-line plot could be caused by sieving and weighingerrors, incorrect sample preparation, or by the sam-pling method itself.

Fig. 56.6-Logarithmic probability diagram of produced, bailed,and core-barrel samples from the same well.

corrosion-resistant; it also has a lower pressure drop, andit will not unravel if the wire is erode d or broken.The wrapping wire on these screens is usually madefrom 3 04 stainless stee l while the pipe core is Pipe GradeS or K. Othe r wire and pipe materials are available.

The configuration of the openings in all screens is veryimportant. If the sides of the slots are parallel, pluggingmay occur a s the small sand grains brid ge the slot. Toreduce the chance of this occurring, the wire used to wrapthe screen is wedge-shaped.

Fig. 56.8 show s the construction features of an all-welded screen.For gravel packing, the gauge of the screen should besmall enough to prevent the passage of the gravel-packsand. S lot width is usually taken as one-half to two-thirdsof the diameter of the smallest gravel-pack grains.

CUYULATYE LO6 O~GRAM (I-PLO11U 5 SIYE UMGE

Fig. 56.7-Cumulative log diagrams (S-plot) of produced, bailed,and core-barrel samples from the same well. Note thata bailed sample would result in the selection of acoarser gravel.


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56-8 PETROLEUM ENGINEERING HANDBC,Y

Fig. 56.B-All-welded screen

The screen diam eter should be as large as possible andyet leave adequate room for packing gravel. Table 56.3show s the dimensions and inlet areas fo r several sizes ofwire-w rapped screens. Sc reens are available with slotopenings from 0.00 6 to 0.250 in. in 0.001 -in. increments.

The screen length should overlap th e perfor ated inter-val both above and below by 3 to 5 ft. Blank pipe shouldbe run above the screen to provide a reservoir for extragravel. Blank pipe length sho uld be three to four timesthe screen length w ith a minimum length o f 60 ft.Gravel PackingMethods. The term gravel. as used here. refers to auniform, graded , com mercial silica sand that is placed inthe wellbore and perforation tunnels for the purpo se ofmechanically retaining fo rmation sands. These are de-scribed next.Circulating gravel pack s are done in two steps: an out-side pack and an inside pack. The outside pack or pre-pack places gravel outside the perforations, where voidsmay exist in the formation surrounding the casing and inthe perforations. An outside pack is, of course, not usedin openho le com pletions. The outstde pack is usually at-tained by pumping a gravel slurry throug h an open-endedworkstring with the application of fluid pressure . Toachieve goo d gravel placement, fluid must be lost to theformation.

The inside pack is achieved by pumping a slurry con-taining from 1/4 o 15 Ibm of gravel per gallon of fluiddown the workstring and through a crossover tool intothe annular space between the screen and the casing. Thegravel is held in place by the screen while the carrier fluid

(brine, diesel oil, etc.) flows through the screen and cross-over tool into the tubing/casing annulus and back to thesurface.A wash pipe extends from the crossover tool. insidethe blank pipe and screen, to the bottom of the screen.Returns are taken through the wash pipe. It is recom-mended that the wash pipe outside diameter be 0.6 timesthe screen liner inside diam eter and made up with flushjoints. This ratio of wash pip e outside diamete r to linerinside diameter op timizes gravel distribution along thescreen in deviated holes.

The screen and blank pipe should be centralized every15 ft, and the length of the screen should be such thatit extends above and below the perfo rated interval by 3to 5 ft. A calculated quantity of a high-density slurry a t15 Ibm of gravel per gallon of fluid (density of l3.8YIbm/gal if fluid is wate r) is circulated into place. A s thegrave1 settles out and packs in the hole outside the screen.injection pr essure will increase. When the injection pres-sure has increased to between 750 and 1.500 psi abovethe originally established injection pressu re, pumping isstopped. When the sandpack causes such a pressure in-crease, the condition is called a screenout.The slurry remaining in the reservoir above the screen(established by the blank p ipe) w ill settle out so that am-ple gravel exists abov e the top of the screen. Since about60% o f the slurry volume consists of gravel. 100 ft ofslurry will result in 60 ft of settled grave l.

In grave1 packing intervals with the circulating metho dand a high-density slurry, a lower tell-tale is recom-mended. The lower tell-tale is a short section of screen,not less than 5 ft long, located below the productionscreen. A seal sub, installed between the lower tell-taleand the production screen, seals the wash pipe. Return sare thus taken only throug h the lower tell-tale. By thismetho d, gravel is placed at the bottom o f the screen. anda denser pack can be achieved . After screenout of the tell-tale screen, the wash pipe is pulled up into the produc-tion screen to complete the gravel pack.Squeez e gravel pack ing uses a gravel slurry consistingof 1 5 lbm of gravel a dded to a gallon of viscous carrierfluid. This gives a high-density (13.89 lbmigal) slurry.The carrier fluid transports the gravel into place and issqueez ed awa y into the formation. A viscosity breakerin the carrier fluid reduces the viscosity according to apreplanned schedule; this allows fo r fast well cleanup.

Revers e circulating gravel pack ing is a low-densitymetho d using r/4 o 2 Ibm of gravel per gallon of carrierfluid. The slurry is circulated down the tubing/casing an-nulus. The gravel is retained on the outside of the screenwhile the carrier fluid flows through the screen and tothe surface through the tubing. After the pack is completedand all the gravel in the annulus has settled, the tubingis pulled out of the hole, leaving the screen assembly witha polished nipple on top in the hole. A productton packe rwith an oversh ot seal assembly is then run over thepolished nipple. If the pack is done in perfo rated casing,a prep ack is usually done first.

Several disadvantages associated with a reverse circulat-ing gravel pack include (1) long rig time, (2) pack voids.(3) slurry pum ped d own the annulus scouring dirt and millscale from the inside of the casing and the outside of thetubing, and (4) possible formation damage caused by largeamounts of dirty fluid circulated and lost to the formation.


9/9

REMEDIAL C LEANUP, SAND CONTROL, & OTHER STIMULATION TREATMENTS 56-Q

TABLE 56.3-COMMON SCREEN SPECIFICATIONS AND SIZES

OD Weight ID Holes(in.) (Ibmlft) (in.) per ft

1.050 1.14 0.824 721.315 1.70 1.049 601.660 2.30 I ,380 721 soo 2.75 1.610 a42.063 3.25 1.751 a42.375 4.60 1.995 962.875 6.40 2.441 1083.500 9.20 2.992 1084.000 9.50 3.548 1204.500 9.50 4.090 1445.000 13.00 4.494 1565.500 14.00 5.012 1686.625 24.00 5.921 1807.000 23.00 6.366 1927.625 26.40 6.969 2049.625 36.00 a.921 264

Pipe Base Screen

HoleDiam.

(in.)/l

7 6?A6%6% 676%12121212121212/2/2

TotalHoleArea

(sq in.lft)3.534.605.526.446.44

10.6011.9321.2123.5628.2730.6332.9935.3437.7040.0651.84

1.551.822.162.402.562.883.384.004.505.005.516.017.147.52a.15

10.17

Cased Hole Comolet ion

Screen Su rface Open Area insq in./ft for slot opening

(in.)0.008

6.88.09.4

10.511.212.614.817.519.721.824.126.331.232.935.644.4

0.010 0.012 0.020a.2 9.6 14.49.7 11.3 16.9

11.5 13.4 20.112.7 14.9 22.313.6 15.9 23.915.3 17.8 26.817.9 20.9 31.521.2 24.8 37.323.9 27.9 41.926.5 31 .o 46.529.2 34.1 51.331.9 37.2 55.937.9 44.2 66.539.9 46.6 70.043.3 50.5 75.954.0 63.0 94.7

Casing Screen Pipe Size(in.) (in.)4% wl65 2%5% 2%6% 2%7 27/8 to 3%7% 27/8 to 3/28% 49% 4% t0 !h10 % 5 to 5%Openhole Complet ion

Openhole ScreenCasing Diameter Pipe Size(in.) (In.) (in.)

2%% 1265/s to 7% 14 to 1675/s to as/e 14 lo 1895/8 to 10% 16 to 20

45%7

Wash-down gravel packs are done by dumping the grav-el down the casing, allowing it to settle. and then run-ning a screen-and-w ash pipe assembly with a wash-do wnset shoe into the hole. Circulation is established throug hthe shoe, and the screen assembly is lowered as the grav-el is washed up the annulus. When the shoe tags bottom.circulation is immediately stoppe d, and the gravel allowedto settle around the screen. The screen is then released,and a production packe r with a seal sub is run into thehole to seal the production string.

References

5

6.

7

8.

9.IO.I I.

12 .13 .

I-1.

Brulst. E.H.: Better Perlormance nf Gulf Coast Well\. paperSPE 4777 pres ented at the 1974 SPE Symposium on FormationDama ge Control, New Orleans, Jan. 3l-Feb. I.Gurley, D.G., Cop eland. C .T.. and Hendrick. J.O. Jr.: Debign .Plan. and Execute Gravel Pack Operations for MaxImum Produc-tion. J. Prr. Tech. (Oct. 1977) 1X9-66.Stein. N., Odeh. A-S., and Jones, L.G.: Estimating MaximumSand-Free Production Rates from Friable Sands for Different WellCompletion Geometries. J. Pa. Tech. (Oct. 19 74) 1156-58.Klotz. J.A.. Krueger, R .F., and Pye. D.S.: Maximum WellProducttvity in Dam aged Formations Requires Dee p. Clean Per-forations, paper SPE 4792 presented at the 1974 SPE Symposi-um on Formation Dama ge Control. New Orleans, Jan. 3l-Feb I.Brooks, F.A.: Evaluation of Preflushes for Sand ConsolidaitonPlastcs, J. Par. TK~. (Oct. 1974) 1095-l 102.Gidley. J.L.: Stnnulation of Sand\tonc Formations W ith the Acid-Mutual Solvent Method.~ J PH. Tech. (May 19711 SSl-58.Gulati. M.S. and Maly. G.P.: Th in-Sectjon and PermeabilityStudtca Call for Smaller Gravels in Gravel Packtng, paper SPE4773 presented at the 1974 SPE Symposium on Formation Dama geControl. New Orleans. Jan. 3l-Feb. I.Saucier. R.J.: Gravel Pack Design Consideration\. J. Pet. Ted?(Feb. 1974) 205-12.Williams. B.B., Ellioct. L.S.. and Weave r. R.H.: Productwityof Inside Casing Gra\,el Psck Completions. J. Pc,r. Twh. (April1972) 419-25.Sparlin. D.D.: Sand and Gravel-A Srudy of Their Permeabili-tws, paper SPE 4772 presented at the 1974 SPE Symp wium onFormation Dama ge Conrrol. New Orleans, Jan. 3 I-Feb. I.

56. Remedial Cleanup, Sand Control, And Other Simulation Treatment

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