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Society of Petroleum Engineers6200 North Centra/ Expwy. ;~;E;ER SPE6379Dallas, Texas 75206

CurrentCompletionPracticesin TightReservoirs

by

Kale Webster, Member SPE-AIME

THIS PAPER IS SUBJECT TO CORRECTION

@ Copyright 1977

American Institute of Mining, Metallurgical, and Petroleum Engineers, inc.

This paper was prepared forthe 1977 Permian Basin Oil and Gas Recovery Conference of the SocietyofPetroleumEngineers ofAIME, heldinMldland, Texas, March l(kl 1, 1977. Permission tocopyisrestricted toanabstract ofnotmore than 300words. liiustrations maynotbe copied, Theabstractshouid contain conspicuousacknowledgement of where and by whom the paper is presented. Publication elsewhere afterpub!ication in theJOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL isusually granted upon request to the Editor of the appropriate lournai, provided agreement to give proper credit ismade, Discussion of this paper is invited.

ABSTRACTtechnique or a gelled weak acid and 1imitedentry. COZ is used in many of the gelled

This paper describes the current comple- acid treatments which include non-emulsifiers~tion methods being used in tight or low iron stabilizers and KCL to prevent claypermeability reservoirs of the Austin Chalk, swel1ing.Canyon Sand and Morrow Sand.

Most of the Morrow Sand wells of eas-Formation damage has been a major con- tern New Mexico are drilled with very low

sideration in all of these reservoirs and has water loss muds, drill stem tested and thenstrongly influenced the drilling mud, testing! cemented with low water loss cement usingcementing, perforating and stimulation. All KCL in the water to prevent cement filtragqof these factors influence the completion damage to the formation. The most commonpractices and are described for each reser- method of perforating is through tubing withvoir. a differential into the wellbore. Some wells

are completed natural but most respond to aThe Austin Chalk in the Pearsall Field small acid clean-up treatment. Data from

is often described as producing from natural a study of field results are included in thefractures. The method of finding these frac- paper. Many of the completion problems at-.tures includes coring, logging, and in some tributed to formation damage were due tocases slant hole drilling through the pay poorly develwed porosity and permeabilitysection to intersect more of the fractures. often associated with edge wells. This meansThree basic fracturing fluids and methods well cost could be substantially reduced byhave been used to treat the Austin Chalk and re-evaluating the mud, casing, tubing andthe average treatment now costs $35,000. stimulation programs.

The Canyon Sands of Sutton and Ozona INTRODUCTIONCounties, Texas areair or gas drilled. Mostof the wells are fracture treated using the This paper describes the current cor-nballsealer multistage and gelled water pletion methods being used in tight or low

permeability reservoirs in the Austin ChalkReferences and illustrationsat end of paper. near Pearsall, Texas, the Canyon Sand near

94 CURRENT COMPLETION PRACTICES IN TIGHT RESERVOIRS SPE 6379 -

Sonora and Ozona, Texas, and the Morrow GasSand of New Mexico. Formation damage hasbeen a major consideration in all of thesereservoirs and has greatly influenced thedrilling fluids, perforating, acidizing andfracture treating. In some cases, preventionof formation damage has been overly empha-sized and greatly increased well costs.

AU!71N CHALK

The Pearsall Field was discovered in1933 and is located about 60miles southwestof San Antonio in Frio County, Texas. Pro-duction is from the Austin Chalk which isencountered at a depth of about 5500 feetnear the town of Pearsall. The structure isan anticline with a large nose plunging tothe southwest across the central portion ofFrio County, Top of the pay is 1500 to 2000feet deeper to the southwest near Divot,Texas. Gross pay section is 350 to 450 feetin thickness and is generally divided intoan upper and lower with 20 to 40 feet ofshale separation.

Limited development took place from 1933through 1941 and the wells were completed byshooting in the open hole with nitroglycerin.Further development took place between 1948and 1956 and acidizing was used to stimulatethe wells.

The recent chalk play started in late1974as a resultof higher oil prices andthe use of hydraulic fracturing. Developmenthas been near the town of Pearsall and in thesouthwestern part of the county toward Divit,Texas. More recent dri?ling has moved intoZavala, Dimmitt and LaSalle Counties, and al-so to the northeast into Atascosa and WilsonCounties as shown on Map No. 1. Leasing hasbeen strong further to the northeast whichcould extend the play into Robertson and Mad-ison Counties north of Houston. The play hasbeen described in more detail in recent tradejournal arttcles (1) (2).

The Austin Chalk is an Upper CretaceousMarl and is often described as a tight for-mation with no primary porosity that producesfrom a natural fracture system. This is par-tially erroneous, as some of the few cbresavailable show a matrix porosity of 8% withpenneabilityof 0.1 to 0.2md. Vertical frac-tures and numerous thin shale laminations areobserved in the core. This probably explainsthe high initial production with very rapiddecline. The oil in the natural fractures isproduced rapidly, then oil from the low per-meability matrix starts feeding into thefractures at a slow rate.

Drilling

Most operators drill to *200 feet andcement 8-5/8 inch surface casing. Drillingcontinues to total depth with 7-7/8 inchbits. Most wells are drilled without unus-ual problems to the chalk. The mud proper-ties are usually adjusted to give a very lowwa::o~oss while drilling through the pay

The low water loss is to preventshale c;ve in problems and is consideredessential by some operators in preventingformation damage to the Austin Chalk. Lostcirculation into the natural fractures ofthe chalk can be expensive if the mud weightis allowed to exceed 9.30r 9.4 lb/gal.This is probably one of the better problemsto encounter as manyof the good wells havelost circulation.

Deviated Holes

A few operators have drilled deviatedholes attempting to cut more natural frac-tures. The hole angle may be as much as 30or 40 degrees from vertical in the chalk.The deviated holes are expensive to drilland will cause excessive sucker rod and tub-ing wear when the wells are placed on thepump.

Mud Logginq

Mud logging units are used by many oper-ators. They provide information on the oiland gas shows, the type of formation and thedrilling rate. This type of information isuseful in conjunction with the open hole logsand is especially useful in stepout or wild-c;t wells.

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The open hole logging programs varywidely, but the most common are the induc-tion, laterolog and a fracture finding log,Some programs include one or more of theporosity measurement type logs along witha gamma ray log and in some cases, computeranalysis of a combination of these logs. Thelogging programs are being influenced byamixture of operators and industry personnelfrom the Gulf Coast and from the hard rockcountry of West.Texas. This may result ina combination ~f the best of both areas andan improved evaluation logging program forthe Austin Chalk. However, most operatorsfeel the present logs do not accurately pro-vide the needed information to determine thequalityor productive capability of the well..For this reason, production casing is runand a completion is attempted on a very highpercentage of the wells drilled.

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Cementinq. and insure treating all the pay section bythe limited entry technique. An acid and

Obtaining a primary cement job around ball sealer treatment is pumped prior tothe production casing has not been reported fracture treating for the purpose of break-to be a serious problem other than in isola- ing open the perforations to insure thatted cases. Sufficient cement is used to cover each hole will accept some of the fracturethe pay section and an additional few hundred treatment. The gelled water frac jobs canfeet, A second stage of cement is usually be divided into two basic approaches. Someneeded to protect the fresh water of the operators use a very thin gelled water andWilcox Sands that may be as deep as 3200 feet. low sand concentrations,where others use aA DY tool is run in the casing string to a more viscous gelled water to carry a sanddepth just below the Wilcox and cement is concentration of 2 or 3 lb/gal. Variouscirculated from this point back to the surface. combinations of thesetwo methods with al-This eliminates the expense of an intermediate ternating fast and slow pump rates are alsocasing string but does involve the cost of used by some operators. The average costdrilling out theDV tool. of a gelled water frac job is about $35,000.

Some operators are now running a sliding The gel and other additives used in thesleeve in the casing string in place of the frac fluid are mixed continuously while pump-DV tool. The sleeve is run into the hole in ing the large volumes from plastic lined pitsthe closed position which permits convention- directly into the well. The disadvantage isal cementing across the pay section. An open- that any problems or mistakes of mixing areing tool is then run on tubing, the sleeve hidden as they are pumped into the reservoiropened and cement is pumped through the sleeve without being observed at the surface. Lumpsand circulated back to the surface. The or small balls of the dry gel material cansleeve is closed and drilling out is elimin- cause severe plugging in the fracture system.ated.

Amultistage method of fracing (3) hasPerforating been used to treat recent wells in the chalk.

This method consists of perforating each zoneMost wells are perforated with jet guns with the same number of large diameter burr-

using 40 to 60 holes to facilitate the limi- free holes. Sealer balls are used betweented entry technique of fracture treating the each stage to divert the treatment to thepay section. Some operators will evenly next zone. The number of sealer balls tospace the perforations across the entire pay use between each stage is the same as thesection. Others use peaks or criteria from number of holes per zone. The water used inthe logs to pick the points to perforate. these treatments was gelled and complexed in

tanks prior to pumping into the wells. ASome wells have been perforated and good gel mixture was obtained after consid-

treated in the lower portion of the chalk erable laboratory testing of the water andsection, a bridging plug set above the lower gel materials being used.zone, and then the upper zone is perforatedand treated, This is an expensive procedure The multistage method is less expensiveand most wells are now perforated across the due to eliminating the need for high injec-entire pay section and frac treated with one tion rates. A well with two intervals orsetup of the equipment. zones that requires 60 bbl/min. by the limi-

ted entry technique would only receive 30A few wells have recently been perfora- bbl/min. into each zone. The multistage

t,~dwith big hole burr-free guns and fracture treatment would only require 30 bbl/min. intotreated using the multistage ball sealer tech- one zone, diverting with ball sealers andnique to divert the treatment to different treating the second zone at 30 bbl/min. Thisintervals. eliminates much of the excessive pressure

caused by pipe and perforation friction andFracture Treatments greatly reduces the hydraulic horsepower re-

quirements. Cost for this type treatmentMostof the early frac treatments in averages about $15,000.

the Pearsall Field were oil-water emulsionswith volumes of 20Q,000 to 300,000 gallons The main benefits of hydraulic fractur-and sand concentrationsof about 2 lb/gal. ing the Austin Chalk are as follows:The oil-water emulsions gradually gave wayto gelled water and larger volumes of 300,000 1; The hydraulic induced fracture systemto 400,000 gallons which are usually pumped is vertical and connects the naturalat 50 to60 bbl/min. The injection rate is fractures and thin zones of porosity.designed to fit the number of perforations

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OK CURRENT COMPLETION PRACTICES IN TIGHT RESERVOIRS SPE 6379*7-

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2. Skin damage caused by-the drilling fluidand cement is eliminated when the frac-ture passes through the damaged rock andextends out into the reservoir.

3. The fracture extending into the reservoirhas the effect of a larger well bore andresults in greater productivity and alarger drainage area.

Any of the previously discussed fractur-ing methods will provide these benefits. Forthis reason it becomes very difficult toob-serve meas~rable production differences. Thebest wells are going to be those drilled inportions of the field where porosity and nat-ural fractures are best developed.

Re~epves

Many poor wells have been completed dueto our inability to determine well qualitywithout setting pipe and production testing.This has greatly reduced the average wellsreserves to an estimated 30,000 bbls. How-ever, wells drilled in the better producingareas will probably have ultimate recovery of60,000 to 80,000 bbls, and exceptionally goodwells may be double this value.

Economics

The cost of a completed well varies be-tween $200,000 and $300,000. There appearsto be very little relationship between ex-pense and the quality ofa well. The goodwells are in areas with better developedreservoir properties, In order to find theseareas, the operator needs the financing todrill a reasonable number of evaluation wells.

Conclusions

Much experimentation has taken placethe past two years including drilling, mud,logging, cementing, acidizing and fracing.It is time to choose the more desirable pro-grams, reduce the cost and continue withdevelopment drilling where the economic returnis reasonable. This approach has resulted inthe drilling and completion of wells into thetanks for $200,000 each.

CANYON SAND

The present development programs in theCanyon Sands of Sutton, Edwards and OzonaCounties, Texas started in 1970 just priorto the increasing gas prices. Most of theexperimental phase has passed, and the pre-sent programs consist of an orderly drillingand completion plan. There are about 80wellsdrilled per month and approximately 20% are

dry holes. The sand is found ata depth of5000 near the town of Sonora, Texas. Thegross sand thickness rapidly thickens from100 to 1000 feet as the structure dips tothe southwest to a depth of 9000 feet.

Drillin~

The wells are drilled with air whichprovides a means of observing gas productiofiwhen a productive sand zone is drilled. Sev-eral gas increases are often observed whiledrilling through the lenticular sands and thedepths recorded. Water and soap are injectedwith the air to remove the cuttings and main-tain circulation when water productive sandsare encountered. Some operators drill to thetop of the Canyon Sands, then use gas todrill through the pay section. This is toprevent the possibility ofa downhole fireif liquid hydrocarbons are encountered in anyof the sands.

A temperature log is run in the openhole while the well is flowing and is usedto determine which of the many lenticularsands are gas productive. The basic openhole logs consist of one or more of theporosity logs, gamma ray logs, caliper logsand some operators will run electric logs.

Cementin~

It is unusual to enc~unter channels orcement bonding failures across the pay sec-tion. The air drilled holes are loaded withbrine water, the casing run and then cemented.A mud filter cake is not formed across thepay section as only brine water is placed inthe hole after air drilling. This providesnear ideal conditions to obtain a good pri-mary cement job.

Some operators run the casing in the dryair drilled hole, then pump water in front ofthe cement to wet and wash the formation.This also provides good cement jobs as longas sufficient water and cement are used tostop all gas from flowing into the well andmixing with the cement.

Fracture Treatinq

Many different types of fracture treat-ments have been tried in the Canyon Sands,including gas frac, foam frac, gelled alcohol,gelled condensate, oil-water emulsion, gelledwater, and gelled weak acid. Gelled waterand weak acid is the predominately used fracfluid at this time.

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One independentoil company has comple-ted several hundred wells in the Canyon Sands(4). Their completion method consists ofmultistage ball sealer treating with gelledfresh w~ter. The sands are perforated in acidwith burr-free holes. Ball sealers are mixedin small volumes of 15% HCL acid between eachstage of the treatment to divert into eachzone. Three to five stages are used in mostwells. The frac fluid consists of guar gumgelled and complexed in frac tanks to give aviscosity of 30 cps. The propping agent con-sists of 1 pound per gallon of 20-40 mesh sand.The treatment volume is 36,000 to 60,000 gal-lons, depending on the number of stages. Tub-ing is run and the well is swabbed to a flow-ing condition. Cost for this type of treat-ment will be about $10,000 to $12,000, depen-ding upon the volume.

Another method used to stimulate theCanyon Sand consists of limited entry perfor-ating and a separate acid ball out treatmentto open the perforations. Fracture treatingfollows using a combin~tion of limited entryand ball sealers scattered throughout thetreatment to divert and treat the varioussand sections. The treating fluid is a weak(3 to 5%) gelled acid. Other additives suchas KCL, surfactants and iron stabilizers areadded to the fluid. The propping agent is 1and 2 lbs/gal of 20-40 mesh sand, and fluidvolumes average about 60,000 gallons. Carbondioxide is blended into the frac fluid at1000 scf/bbl and is used to energize the fluidto give a rapid recoveryof the fluid afterthe treatment. However, the well must bekilled after clean-up or recovery of thefrac fluid to install tubing. An alternateto killing for tubing installation is tofrac down tubing and/or the annulus at reducedinjection rates and with high friction pres-sure. Some operators leave the C02 outof thetreatment to eliminate this problem. Cost ofa gelled acid treatment is about $25,000 PIuS$5,000 for the C02.

There is obviously considerable concernby some operators to prevent formation damage.A weak gelled acid is used as the frac fluidto provide a low ph and to insure thegellmaterial breaks and lets the fluid return atabout the viscosity of water. Potasium chlo-ride is added to prevent clay swelling in thereservoir. Surfactents are added to preventemulsions. Iron stabilizers are to preventdamage from the iron (siderite) released fromthe formation when acid is used. CO is add-

$ed to provide rapid recovery of the racfluids. Additional safety factors are re-quired as there is added risk of injur,ytopersonnel should a mechanical failure occurwhile pumping C02 or any expandable gas withthe treatment.

Other operators feel that the simpleand far less expensive gelled water treat-ments provide good stimulation. rheironproblem does not.exist when only water isplaced on the formation to mix with the 40to 60% connate water that exists in the re-servoir. Proper mixing of the gel in tanksprior to treating will insure a clean breakwhen the fluid is exposed to the reservoirtemperature, Swabbing will be required inmost cases after installing the productiontubing and wellhead; however, it will notbe necessary to kill the well after it isflowing.

Conclusions

There are two bas?c methods used to frac-ture stimulate wells in the Canyon Sand. Thedifference is based on the concern for preven-tion of formation damage and economics.

The gelled acid treatments include sev-eral additives to prevent or reduce formationdamage, The cost is $25,000 to $30,000 andis greatly increased by the procedure andadditives used.

The gelled water treatments are far lesschemically complex as the additives are heldto a minimum. Operators using this methodare not overly concerned with formation damageand place more emphasis on lower stimulationcost. The average cost of a gelled watertreatment is $10,000 to $12,000.

MORROW SAND

The Pennsylvanian age Morrow Sands arefound at a depth of7,000 to 12,000 feet ineastern New Mexico. The rock is a coursegrained, well cemented sandstone, with few ornr~natural fractures. The productive inter-val usually consists of two zones, each about10 feet in thickness. The formation is consi-dered to be extremely sensitive and very easi-ly damaged by drilling fluid, water or acid.

Drilling Fluid

The mud system used to drill through thepay section usually has a very low water lossof about 5CC with 2 to 5% KCL to inhibit clayswelling.

Dr?ll Stem Test

About 80% of the wells are drill stemtested. The flow data, formation damage fac-tor and logs are used in making the decisionto run pipe or abandon. A damage factor be-tween 2 and 10 is comnon,

Some operators do not use !ISTSas they

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cm CURRENT COMPLETION PRACTICES IN TIGHT RESERVOIRS SPE 6379 - ~/

Ifeel the fluid dropped back on the formationafter testing causes excessive damage.

Cementin9

Sufficient cemenk for 1000 to 1500 feetof fill is pumped around the 4% or 5% inchproduction casing. A low water loss cementwith KCL is used to reduce filtrate damage tothe formation. Lost circulation is notaproblem while cementing the primary casingand chanelling or communication between zonesis seldom reported.

Perforating

Most of the wells are perforated throughtubing and below a packer. Two and seveneighths inch tubing is run in many wells toprovide sufficient inside diameter to permitthe passage of deep penetrating, larger dia-meter perforating guns. Many operators swabthe fluid out of the tubing and then perfor-ate through tubing with the pressure differen-tial into the wellbore. This keeps fluid offthe formation and is considered to be benefi-cial in preventing formation damage.

A few operators perforate overbalancedwith casing carrier guns. The hole is loadedwith a clean fluid and acid is placed acrossthe section to be perforated. Casing carrierguns are used to provide maximum penetration.This lets the perforations soak in acid toremove cement and mud damage while the tubingis installed.

Acidizing

A few Morrow Sand wells are completednatural or with no treatment after perfora-ting. However, most wells are acidized andrespond very favorably to small acid clean-up treatments. The acid volumes are 1000to 4000 gallons with ball sealers to divertacid and open all the holes. Additives usedin the acid are:

1. Clay stabilizers2. Iron stabilizers3. Non-emulsifiers4. Fine suspension agents

Nitrogen or CO is used in most treat-tments to energize t e acid for rapid fluid

removal.

Fracture Treatinq

A small percentage of the wells are frac-ture treated. Some use a very viscous KCLbase water frac of 20,000 to 40,000 gallonswith sand concentrations up to 4 pounds pergallon and C02 as an energizer for fast fluid

pecovery, Others use a gelled weak acid (3to 5%) with 1 lb/gal of sand and C02.

Many of the wells frac treated are edgewells with very low porosity and permeabil-ity. Deliverability after acidizing is lowand they are fractured to exhaust all possi-.bilities for obtaining commercial production.This results in a low success ratio. However,wells with moderate sand development respondfavorably to fracturing. This indicatescareful selection of frac candidates is thekey to success in fracture treating the Morrow

Study Area

An evaluation of field results from wellsin the South Carlsbad Area provides informa-tion that could be used to modify some of thedrilling and completion practices. The wellsin the study area are shown on Map No. 2.Following is a summation of the data:

1. The wells inside the circled.area (Map 2]were Morrow Sand producing wells.

a. Nine wells were originally completednatural.

b. Three natural completions were acid-ized after producing for a fewmonths and all responded with siza-ble production increases. An exam-ple is shown in Figure 1.

c. Four wells were acidized and tworeacidized during initial completion,Initial acid response was good, butreacidizing did not help.

d. Two wells near the edge of the pro-ducing area declined very rapidly.

e. DSTs were reported on 5wells andall flowed gas at rates of 1 to 5MMCFD.

f. Logs show the better wells havehigher porosity and thicker sections,

9. One well was frac treated duringinitial completion without priortesting.

2. Four small prodwcers were completedout-side the circled area (Map No. 2).

a. Two were DSTd and flowed gas atsmall rates. Logs show poor devel-opment, response to acid was smalland one was fraced but did not re-spond.

b. The other two wells were not DSTdand logs show poor development onone and moderation the other. Acidtreatments were not reported oneither well.

The wells in the study area were comple-ted prior to the strong interest in preventing

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lSPE 6379 KALE WEBSTER 99

formation damage. Low waterloss mud was notused to drill the Morrow Sands in most ofthese wells, and KCL was not added to thece-ment. However, a regular non-emulsifyingacid was effective in removing damage fromwells within the productive area. Wells inthe po-eveloped reservoir areas did notrespond to any type treatment, and the re-sults should not be confused with formationdamage. Re-evaluation of the mud, casing,tubing, and stimulation programs would sub-stantially reduce drilling and completion cost

Conclusions

The present practices used to drcomplete the Morrow Sand are stronglyenced by.the operators concern aboutprevention of formation damage; Veryter loss mud is used to drill the payLarger casing and tubing is used in~ito facilitate the useof bigger and betterpenetrating perforation guns. Several addi-tives are used in the acid stimulation treat-ments to prevent or remove damage. All ofthese increase the well cost.

Data from the study area does not sub-stantiate the need for all of these measuresto prevent formation damage, Most of thestudy area wells were drilled prior to thebig concernover formation damage, and con-ventional drilling fluid, cementing, perfor-

11 andinflu-the1ow wa-section.ny wells

sting and acid stimulation were used. Theproduction data shows the wells drilled inthe better developed porosity and permeabil-ityportion of the field responded normallyto stimulation and made good wells.

The data from the study area could beused to re-evaluate the muds casing, tubing,cementing and stimulation to substantiallyreduce drilling and completion cost.

REFERENCES

1.

2*

3.

4.

Stewart-Gordon,T. J.: High Oil Prices,Technology Support Austin Chalk Boom,World Oil (October 1976) 123-127.

Long, Mike: Austin Chalk Play SpreadsFar To Northeast in Texas, The Oil andGas Journal (September 6, 1976) 60-64.

Webster, K.R., Goins, W.C. Jr., andBerry, S.C,: A Continuous MultistageFracing Technique, J. Pet. Tech. (June1965) 619-625.

Weaver, Christopher R.: HNGOil CompanysDevelorxnentand Production Techniques inSuttonCounty Sand Pays, The 21st AnnualSouthwest Petroleum Short Course - 1974.

Another vision, Efficient O eratfons MakeE300 MCFD Wells Profitable, orldoil (August

1974) 45-470

1 t I t , , 1 I, -A._L

3 6 9 12 15MONTHS

Fig. 1 - Average daily rate vs. months.

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Fig.2 - Map I; SouthTexasAustinChalkArea.

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tFig.3 - Map II; South CarlsbadArea.