3653

4t3653

IntroductionIn 1932 the oil industry started using hydrochloricacid to stimulate oil wells. Immediately the problemarose of diverting the acid treatments into a desiredzone, and over the years the diverting methods usedhave been dictated by the reservoirs being developed.In the period from 1932 to about 1945, the PermianGrayburg-San Andres carbonate reservoirs were pri-mary drilling objectives in the Permian Basin. Thesereservoirs were from 3,000 to 5,000 ft deep and weregenerally prolific. Stimulation treatments were usuallysmall in volume (less than 2,000 gal) and the majorconcern was to avoid acidizing into bottom water.During the period from about 1946 to 1960, Permianreservoirs such as the Clearfork, Tubb, and Spraberrywe~~.exninitd Tht=c~ ~,~~()- tfi R fUMLftA__n WS-----.. ~..?----- - . ...”” .“ “,””” –.. —w”wy .“o”m —

voirs were stimulated with large volumes of acid orfracturing fluids, or both. During the 1960’s, deep gasfields (15,000 to 22,000 ft), principally Ellenburger,required new diverting materials to achieve effectivestimulation at high reservoir temperatures and pres-sures.

1936 to 1946The earliest documentation of a diverting agent wasin 1936 when a patent was issued to Halliburton OilWell Cementing Co. for the use of a soap solution thatreacted with calcium chloride to form a precipitate.This was a water-insoluble, oil-soluble calcium soapthat acted as a diverting material for the acid.’ Onevear later Halliburton was issued a patent for a divert-ing material that utilized locust bean gum to gel cal-cium chloride and sodium chloride for blanking off a

f

zone and thus diverting acid into untreated intervals.2Sulfuric acid was used as a diverting agent in con-

nection with a conventional hydrochloric acid treat-ment. After the sulfuric acid was pumped into thewellbore, the pumps were shut down for a short timeand then pumping of the hydrochloric acid was re-sumed. When in contact with the calcium carbonate,the sulfuric acid formed insoluble calcium sulfate,which was the diverting agent. This system was notwidely accepted, owing, it is assumed, to the recogni-tion that it could cause potentially permanent produc-tivity damage.

A major concern in the development of the Gray-burg-San Andres carbonate reservoirs was that treat---”* ,.fi..l,-l-v+a”A :“+- ●L- a.,.+4,......,.-.+-- ..,.A:,...ll.-l.,UIQIIL t,UUIU GALGUU UILU UIG UULLU1ll WaLG1 , ~dl UVUICX1lY

since the wells were open-hole completions. Begin-ning in the late 1930’s, various materials were de-signed to avoid this problem. Dowell developed“Blanket,” which was ~ h~avv calcium chlrmide dII-, --------- --------- “___tion that depended on its viscosity to divert the acidup the hole. Dowell later introduced “Jelly Seal” asthe second-generation blocking material. This was anatural gum (locust bean gum) that when mixed withwater formed a viscous, gelatinous fluid. When difll-culty was encountered in obtaining a plug because ofhigh porosity and permeability, “Jelflake,” or cello-phane, was mixed with the Jelly Seal to accomplishthe shut-off. An internal breaker had to be used tolower the viscosity of Jelly Seal after treatment. Forthis purpose, a bacteria mixture was prepared by ex-“posinga Jelly Seal mixture to air circulating in a warmroom. It was stirred from time to time and liquefaction

When porous zones or perforations in porous zones are opened jor production, thestimulation fluids must be diverted if proper treating is to be assured. It seems that jrom thetime the need for diverting agents was first recognized in the early 1930’s, everything hasbeen tried — from soup to nuts.

.777MAJ, 1972 _ . 593Page 1 of 6

signaled that the bacteria were ready for use. The ori-ginal bacteria mixture was then used to inoculatelarger quantities. Each station kept a crock of bac-teria mixture for use with Jelly Seal. At temperaturesbelow 100”F, 1 gal of bacterial solution was requiredfor each 20 gal of Jelly Seal.’

In the middle 1940’s, methods used to divert acidswere the “Permeter” selective acidizing device (Chem-ical Process) and the “Electric Pilot” (Dowell).4 Eachtool located the contact between a conductive fluid(acid) and a nonconductive fluid (oil). The Permeterand Electric Pilot both used the principle of the “in-terface locater” and differed mainly in that the powersource for the former was located below the surface,whereas the power source for the latter was at thesurface. One or more resistors were spaced in an elec-trode run on a wire line and seated in a tubing nipple

-- set at a preselected depth in the open hole. Acid could“be pumped through the tubing and oii through thecasing. The interface could be determined by estab-lishing which fluid contacted the electrodes. Wheremore than one resistor was run, the actual depth ofthe interface could be controlled quite accurately byvarying the pump rates. The reverse of this with theacid in the anmdus would permit treating the upperportion of the hole. Disadvantages of this system werethat two pump trucks were required and that largevolumes of oil were usually needed. In a refinement of

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fig. l—Diversion in open hole using Permeter nipple.

594

this treating method, a blocking material was placedabove the electrode, which reduced the loss of oil tothe formation and permitted more accurate placementof the acid (Fig. 1).

After the middle 1940’s, open-hole formation pack-ers gained widespread use in acidizing. In the PermianBasin, open-hole hook wall packers had usually beenused. Many of them were pooriy designed and uncie-pendable. One packer used by Shell in the middle1930’s was a “Robinson 80 Hi Pressure Canvas Pack-er.” During World War II, packers were unreliablebecause rubber, a critical material, was diverted toother uses. After the war, robber element packerswere again used as a diverting method. In the late1940’s the Sweet formation packer was developed. Itproved to be reliable in diverting stimulation treat-ments. This packer, which was available with 48- or90-in. elements, had a side door choke assembly thatpermitted seiedVe treatmiit Z~@Veafid bf3iOiW tik

packer (Fig. 2). Even when fracturing above and be-low a formation packer with a side door choke, Shellexperienced little trouble with stuck packers. Thiswas probably because, as was the practice at that time,they used a large overfiush, which displaced the sandfrom the borehole.

A Lynes straddle packer was first used by Shell in1945 in the Monahans field. The first attempt to sepa-rate a zone for treatment with this packer proved un-

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JOURNAL OF PETROLEUM TECHNOLOGY

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Page 2 of 6

successful and it was 1949 before further refinementsbrought it into widespread use. The Lynes packer is ahydraulically operated inflatable packer, excellent fortreatment in open holes, particularly in those that havebecome so enlarged that a regular formation packercamot expand enough to effect a seal. When theLynes packers are used as a straddle tool, it is possibleto treat three intervals — below, between, and above~~~n!ll-lrmm—y..- . . . . withrmt mnvin u th~ ntarlr-re ~~c~I~s~.. . ...”... ...” . ...5 m.” p.--”.”.

it is time consuming to use packers as a diverting tool,it is a more expensive method.

1946to 1960 -In 1947 the use of emulsions to divert acid treatmentswas begun. Again the locust bean gum was the gellingmaterial, and the resulting emulsion, because of itshigh viscosity, was used to divert treatment acids.

In the early 1950’s, coincident with the introduc-tion of the fracturing process, many diilerent types ofdiverting agents were introduced. Some new stimula-tion materials were used for both treating and divert-ing. In about 1951, when the acid-kerosene emulsion-type fracturing fluids were introduced, Dowell devel-oped “Fixafrac,” a blocking agent to be used withthem. Fixafrac was a mixture of lime, kerosene, a gell-ing agent (a fatiy acid and a soap), and a graded cal-cium chloride salt. Oil-external-phase emulsions us-ing Batu gum as an emulsifier were used about thistime as diverting agents in fracture and acid treat-ments. Also in 1951, gelled acids were introduced foracidizing and fracturing. Dowell’s “Strata Lift,” ifused in connection with an acid treatment, would tendto block permeable zones and divert the treatment toless permeable zones. When used as a earner for stmdin less permeable zones, it was a fracturing fluid.3

In 1954 naphthalenes (mothballs) were first used asa blocking material. Crushed mothballs were mixedin a thickened acid. The oil-soluble naphthalene wasthought to be the ideal blocking agent because it sub-limes above 175 oF.’ Naphthalene that sublimes at ahigher temperature is now available. Although usedearlier in connection with well work, oyster shellswere first used in connection with stimulation treat-ment =hfint thic time. DwxJifiIIelI, tha.r hnA hnan I..aA...”...- -w”w. LS...7 u.,,”. A .“ . s“”m. J “*J “-U --u UO-U

to temporarily block a zone and were removed byacidizing. In about 1954, oyster shells were used withan oil-external-phase emulsion (water-internal-phase)as a blocking agent during stimulation jobs.

With the accelerated search for other blocking ordiverting agents, various ones were rapidly intro-duced. In 1955, crushed limestone and sodium tetra-borate were used for diverting. Because the sodiumtetraborate is a poison, its use was discontinued and itwas replaced by rock salt. Rock salt has had wide-spread use in the Permian Basin because it is readilyavailable, inexpensive, and easy to handle. In 1956Gilsonite was used for the first time as an oil-solublefluid-loss additive and diverting agent.’ Perlite ingelled oil had already been tried as a diverting mate-rial. In 1956 guar gum was first examined for use asa diverting agent. Its high viscosity makes it an idealcarrying agent for many of the blocking materials, andit has been widely used in this capacity and as a gellingagent for water-based fracturing fluids.

A major change in diverting agents occurred whenball sealers were developed for better diversion in per-forated completions, After World War II and the in-troduction of jet perforating, the industry began tur-ningto the cased hole as the most efficient means ofcompletion. Since perforation density ranged from 3holes/ft to as much as 8 holes/ft over long gross inter-vals (up to 200 ft of 4 holes/ft), the problem of divert-ino the dirnnlstinn flnidc wac z+hcmt the wtme nc it hd—e ---- “--... -.-..=.. ..-.-” .. .-” -s.,-. ---- “---- -“ . . ----

been with the open-hole jobs. Although other meth-ods had been effective in some cases, determiningg thecorrect volume of blocking materials was a majorproblem. In 1956, The Western Co. introduced arubber ball to be used for sealing off casing perfora-tions and thereby diverting fluids to other perfora-tions.’ Although the first ball sealers were made ofsolid Hycar rubber and were %-in. in diameter, thereare now many dtierent types. Ball sealers have beenavailable in sizes from S/s-to 11A-in. in diameter. Ballscontaining many different kinds of cores have beenused for perforation sealing. Solid nylon balls havebeen made, as well as aluminum balls, rubber-coveredaluminum balls, rubber-covered phenolic balls (stableat high temperatures), and even permeable plastic-consolidated- walnut-hull balls (Chamelban-Dowell).SWL- .-.–,—-..-L-,, L–*, ..-, -—–—.––-—.–>- –– -. A–—–—1ne wamut-sneu DaII semen were maae so as to per-mit some bleed-off during a treatment; this wouldalleviate excessive pressure increases such as thosethat resulted when perforations were completely shutoff by the solid ball sealers.

1960 to 1970Another technique for diverting stimulation fluidswas introduced in 1961 with the concept of limitingthe number of perforations. Today, “limited entry” isone of the most efficient methods of diverting. Ex-perience proved that to achieve maximum benefitsfrom a fracture treatment, it is desirable to breakdown each perforation. This can be achieved withhigh-rate, large-volume acid treatments, but that ap-proach is expensive.” S A better way is to use acidand ball sealers prior to fracturing.

The Frac Baffle, introduced by The Western Co.A 1 (x< %,,0. k J4f.w.+ . “-X,, ,.---1,+:-” +,a#.hd”..c. 9au . z “.J > “an ,Ll wubkL a A,ti w bwllywuuu LbwnuyuG.

Used in the Permian Spraberry sandstone (the Upperand Lower Spraberry are approximately 1,000 ftapart), this system was designed for more economicalcompletions in multiple zones. In this method ofdiverting, one or more bathe rings (bafEes of decreas-ing diameter with increasing depth) were run in theoriginal casing string. These rings (4 in. and 3 in.in size for 51%-in. casing) are designed to accept a“giant ball sealer” or bomb and effect a completeseal as does a cementing plug. On the completion,the lower zone is perforated and fracture treatedthrough casing. While the flush is being pumped, abomb is inserted in the casing and the lower zone issealed off (Fig. 3). With the shut-in pressure main-tained, the next zone up is perforated and the nextstage of fracturing fluid is pumped. As many as threestages have been applied by this method.

In 1962, a synthetic polymer (Dowell’s FLAX-2and Halliburton’s Matriseal) was brought out as afluid-loss additive and diverting agent. This material

‘.’. 1972 595Page 3 of 6

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was first designed for use in matrix acidizing jobs insandstones and has since been used in carbonate res-ervoirs. The material is inert, but in the presence ofacid it swells to 30 to 40 times its original size. Thepolymer comes in different sizes and shapes, regularand irregular. The particles are pliable, deformable,and degradable.g’5*e

One interesting method of diverting is the so-calledPine Island fracturing technique widely used in 1964in the Pine Island field of Louisiana, and in the Lulingfield of Texas. In this method of diverting a fracturetreatment, the lowermost zone is perforated and frac-tured. The next zone up the hole is then perforated,and before the next stage of fracturing fluid ispumped, the previous perforations are blocked off byplugging back with pea gravel or aand or both. Asmany as six stages have been performed on a well. ~One advantage could be that the pressure transmittedthrough the gravel-sand pack reduces the possibilityof communication between the zone being fracturedand the previously fractured zone.10

Union Oil Co. of California in 1965 introduced“Unibeads” as a blcdcing material.” This material,which is a special wax-polymer blend of a refineryby-product, is enjoying widespread acceptance. TheUnibeads are available with three dtierent 24-hoursolubllity temperatures (OS-90, soluble at 90°F;OS-1 30, soluble at 125 ‘F; and OS-160, soluble at155°F). The material is available in buttons (%- to%-in. in diameter) and in graded particles (from 8to 100 mesh). It can be used with either water or oilas the carrying medium, and it is oil soluble, waterinsoiuiie, deformatile, anti teniperritiure degradrtbk.Fig. 4 illustrates the pressure behavior for a 5,000-gal acid treatment in a well in Justis field, N. M. TheFusselman zone from 7,153 to 7,204 ft (20 holes)was treated with acid and Unibeads for diversion be-cause of a cement bond failure that had occurred afterthe initial acid treatment.

Shell’s experience with Unibeads in the Mid-f!nntinent D&, We~lern Rej@n is shown Orl Fig. 5.

The curves are based on results from both acid andfracture treatments. Curve A shows 12 months’ after-production on 23 jobs and indicates approximatelythe same decline rate bfore and after treatmen~which suggests that, by properly diverting the stimu-lation fiuids rather than by mereiy removing skin,additional pay was opened to production. Curve C,reflecting 51 jobs, indicates that production increased2.1-fold, from 1,400 to about 3,000 BOPD.

In about 1966, “chicken feed” diverting agent wasused by Dowell in deep dry-gas wells. This is agroundand graded grain (maize and other feed grain) usedas a blocking agent in stage acid treatments. It is ablend of protein, gum, and carbohydrates that willcompletely degrade in the presenee of water, brine,or acid.g

O.-..1:.’.1 Ph.-:..l P... Ati-d,.-zd :- ~ 967 +k-Galulllal vllGllll- w. WL1 UUU*GU -

treating technique “Temp-Trol,” which employe[~temperature survey in connection with a stimulationtreatment. A base temperature log is run and afterone stage of stimulation materird is pumped, a seeondtemperature survey is run to determine the intendtreated (cooled). Knowing the approximate length of

JOURNAL OF PETROLEUM TECHNOLOGYPage 4 of 6

—

the interval to be treated, one can estimate more ac-curately the volume of blocking material needed todivert the next stage. Since both the porosity valuesand the permeability values for any interval are rarelyavailable, any volumes are estimates at best. In thefirst treatments, rock salt was used as a divertingmaterial, but since then a variety of other blockingmaterials have been used.13

In the 1960’s, the fluids used for diverting changedvery little. The gelled waters, gelled acid, gelled oil,oil-water emulsions and oil-acid emulsions were mcd-fied to make them more efficient as diverting fluidsand earners. The variety of bridging materials avail-able was increased during the 1960’s and the newmaterials were basically the result of using morepolymer-base materials for blocking.

Paraformaldehyde was introduced as a blockingagent during the late 1960’s. This diverting materialis used in the deep (18,000 ft and deeper) Ellenburgerdry-gas wells now being developed. With the hightemperatures — 250° to 350”F — the conventionalblocking ageiits were not adequate for diverting stim-ulation treatments, Paraformaldehyde is temperaturedegradable and is soluble in both water and oil. Onecaution must be noted with paraformaldehyde: whenused with a guar gum fluid, the paraformaldehydewill destroy the breaker in the fluid and can preventit from breaking in the reservoir.

Benzoic acid flakes were successfully used in 1969and are rapidly becoming one of the most populardiverting agents. Benzoic acid flakes divert by forminga filter cake on the formation. The graded sizes resultin a block similar to other solid diverting agents —salt, for example. They have one distinct advantageover other diverting agents in that they are solublein oil or water. Volubility in aqueous fluids increasesas pH and temperature increase. The volubility of theflakes in oil and aqueous fluids is such that thesefluids can be safely used as earners.” Fig. 6 showsresults obtained by Shell in using benzoic acid flakesin fracture and acid treatments. The twofoid increasein production is similar to the results obtained withUnibeads. Curve C, reflecting 27 jobs, shows an in-crease in production from approximately 720 to1,450 BOPD.

ApplicationIn its completions in the Permian Basin, Shell hasused the limited-entry technique since its introduc-tion, or at least a modified limited entry (more holes).Since the number of perforations is limited, all stimul-ation treatments are diverted, if necessary, by usingball sealers. For economic reasons (excessive hydrau-lic horsepower is required) ball sealers are normallyused on the acid treatment after perforating to insurethat all perforations are open. Fracture treatmentsare usually performed without ball sealers unlessthere are too many perforations.

The original completion on the deep, dry-gas Ellen-burger wells usually requires some method of diver-sion. Because of the long gross interval-up to 1,000ft — from 50 to 70 holes are required to open allpotentially productive intervals. Shell has employedtwo approaches in these wells: (1) using ball sealers

and (2) pumping a viscous fluid (gelled water) aheadof an acid stage.

One major difficulty encountered with any block-ing material is in removing it from the permeablezone after treatment. Any material that depends uponvolubility in either water or acid will present a prob-lem in some wells. If a low-pressure reservoir (Fig. 7)is completely sealed off (which is the case in most ofthe Permian Basin wells) the ease with which the gelbreaks is of paramount importance. The perfectblocking material is one that lasts long enough todivert fluid during a treatment and then becomes in-effective without having to be dissolved by contactwith other agents or by internal breakage.

Currently, the diverting agents that most nearlymeet these requirements are Unibeads and benzoic .acid flakes. The specifications for the 0S-90 beadsindicate they have a 24-hour volubility at 90°F. How-ever, the melting point of the 0S-90 beads is about138”F. A wax bead with a melting point of 90°Fwould yield better results for treatments in manyshallow wells in the Permian Basin. Benzoic acidflakes have one distinct advantage as a divertingagent: they are soluble in both oil and water, and inlow-temperature reservoirs this represents a safetyfeature not available with most other diverting agents.

A s● uc n

Fig. 6-Results of treatments with benzoic acid,1970-1971.

Fig. 7—Forces acting upon a soluble diverting agent.

MAY, 1972 59?Page 5 of 6

A project for research to undertake is the develop-ment of a material that stays intact for only the dura-tion of the treatment. Historically, most of the avail-able agents have been designed for long-term breaks(24 hours on longer). Durability, however, is not arequirement for a blocking material.

The value of diverting material necessary for aspecific job is difficult to estimate. Since we are ableto determine, at best, only the porosity of the zonesto be treated and not the permeability, the volumeof block must be approximated. The Temp-Trolmethod of stimulation offers a technique for selectiveblocking, and when the system is carried to comple-tion it should result in the treatment of all the zones.Although this type of treatment is usually more effi-cient than normal diverting methods, the additionalexpense often precludes its use.

Guidelines for DeterminingDiverting Agent VolumesIn the absence of actual experience in any given field,rules of thumb are used to determine the volume ofblocking agent to be used. Following are guidelinesfor the application of various diverting agent3.3*st10J2J4

1. Thickened water or oil and emulsions — use 50to 100 gal per foot of zone to be protected (usually1,000 gal mtilmum). The higher range is used if thezone is fractured.

2. Graded Unibeads or graded naphthalene — use5 lb per foot of zone to be plugged. If in perforations,....- 11L 4- * IL /=. . ..L?--n----U3G 1 7.Z LU & lU/ ~Gl lU1 ilLIUU.

a. If limited-entry perforations, use 1 lb of ma-terial per gallon of earner.

b. If multiple perforations, use 2 lb of materialn-r mallnn of rnrnerr-. ~------ . . -- . . . . . .

c. If open hole, use 2 lb of material per gallonof earner.

d. In wells with low reservoir temperature, Uni-beads should be used in an oil-base earnerto promote dissolving.

3. Benzoic acid flakes — use 10 lb per porous footaf 7nna tn he mlIIom4“. -“u” .“ ““ y...~~”-.

4. Ungraded rock salt — use up to 60 lb per footto be plugged. If in limited-entry perforations, use30 lb/perforation.

5. Graded rock salt — use a maximum of 20 lbper foot to be blocked.

6. On a fracturing job, without temperature sur-vey, estimate that 50 ft of zone will be fractured at20 to 25 bbl/min.

7. Paraformaldehyde — use 5 lb of material perfoot to be blocked.

8. Dowell’s J-175 ground and graded feed grains— use 5 lb of material per foot to be blocked.

9. Ball sealers (not for limited entry):a. At a rate of 25 bbl/min and greater, use 10

percent excess.b. At rates of 15 to 25 bbl/min, use 30 per-

cent excess.

Original manuscript received in Society of Petroleum Engineersoffice Sept. 1, 1971. Revised manuscript received Feb. 2, 1972.Paper (SPE 3653) was presented at SPE Illinois Basin RegionalMeeting, held in Evansville, Ind., Nov. 18-19, 1971. @ Copyright1971 American Institute of Mining, Metallurgical, and PetmlaumEngineers, Inc.

c. On acid jobs at 4 to 6 bbl/min, use 100percent excess.

ConclusionsThe limited-entry technique or the modified limited-entry technique (using ball sealers) offers the bestmethod of diverting stimulation treatments of newwell completions.

In open-hole completions, or in completions withmany perforations, Unibeads and benzoic acid flakeshave proved to have the most desirable divertingcharacteristics. Except where temperatures are toolow, the plastering characteristic of Unibeads makesthem a desirable diverting agent. When reservoirtemperatures are too low for Unibeads, the benzoicacid flakes make an efficient blocking material that issoluble in oil or water.

Rock salt, naphthalene or any other solids that de-pend upon volubility in only one phase (oil or water)to degrade should be used only if no other type ofdiverting material can be used, and it must be recog-nized that there is a possibility of damaging the pro-ducing zone.

Research is necessary to develop a diverting agentfor stimulation treatment that would retain its divert-ing property for only the duration of the treatment.The diverting agents that depend upon viscosity aloneto divert do not appear to offer a positive method ofblocking because of the variation and unpredictabilityof gels and emulsions. A solid that would degrade:—-..-.>, –..,-. .-.–-.,> L. .L. —–-. J.-:--L,- >:-.--&--Immealately woula oe me most aeswaole mvernngagent.

AcknowledgmentsT nmm.sr.; .t,a th.a m-;c.;Am m.nmtarl h., Chnll (%1 Pm. q.J~,uwscALeU,u p, 1,11..1”’, & -1.*U u, “.s”11 -.1 -“.

to publish this paper. I also appreciate the help inaccumulating historical data given by Byron-Jackson,Inc., Cardinal Chemical, Inc., Dowell Div. of TheDow Chemical Co., Halliburton Services, and TheWestern Co.

References1.Clason, Charles E.: Halliburton Oil Well Cementing Co.:

U. S. Patent No. 1.122,452 (1936).2. Menaul, Paul L.: Halliburton Oil Well Cementing Co.:

U.S. Patent No. 2,122,483 (1937).3. Personal communications with Dowell.4. Hefley, Dana G. and Fitzgeraldj P. E.: “Selective Acidiz-

ing and Permeability Determmation by an ElectricalMethod,” Trans., AIME (1944) 155,223-231.

S. Personal communication with Halliburton Co.6. ~~;~t Aid to Multi-Stage Fracturing,” Pet. Eng. (Dec.,

7. Lagrone, K. W. and Rasmussen, J. W.: “A New Development in Completion Methods — The Limited EntryTechniquefl J. Pet. Tech. (July, 1963) 695-702.

8. Murphy, W, B. and Juch, A. H.: “Pin-Point Sandfractur-ing — A Method of Simultaneous Injection into Selected%nda~ J. Pet. Tech. (Nov., 1960) 21-24.

9. Carpenter, N. F. and Ernst, E. A.: “Acidizing with Swell-able Polymers: 1. Pet. Tech. (Sept., 1962) 1041-1047.

10. Personal communication with The Western Co.11. ‘*New Beads Help Acidizing, Fracturing: Oil and Gas J.

(Aug. 30, 1965) 52-54.12. Personal communication with Byron-Jackson, Inc.13. ~larl$al Chemical, Inc.: U. S. Patent No. 3,480,079

14. Personal communication with Cardinal Chemical, Inc.~T

598 JOURNAL OF PETROLEUM TECHNOLOGY

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