FEATURE ARTICLE

Gas Well Stimulation Increases Production and Profits

PAUL CLINKENBEARD DOWELL INCORPORATED MEMBER AIME SHREVEPORT, LA. J. F. BOZEMAN HOUSTON, TEX. ASSOCIATE MEMBER AIME Downloaded from http://onepetro.org/JPT/article-pdf/10/11/21/2236770/spe-1067-g.pdf by guest on 24 September 2021 R. D. DAVIDSON AMARILLO, TEX. JUNIOR MEMBER AIME

Abstract acid's reaction rate on portions of the formation already contacted. However, the acid reacts rapidly with new, un­ Various acidizing and fracturing techniques have been touched formation rock. This retarded acid is especially developed to meet gas well stimulation .7Jroolems. Case his· valuable in formations which are highly acid-soluble be­ tories from fowr major gas producing areas show that con­ cause it penetrates far into the formation before becoming sidering such problems on an individual well basis, in light spent. By contrast, normal hydrochloric acid reacts rapidly of previous experience in the area, allows planning each in such a formation, quickly becoming spent in the imme­ stimulation treatment for maximum efJectiveness. Despite diate vicinity of the wellbore. the common opinion that stimulating old gas wells is un­ Acid-oil emulsions are also used in acidizing gas wells. profitable, results show that stimulating such wells has of­ Such emulsions penetrate deeplv into formations. They are ten resulted in substantial and profitable production in­ often employed as fracturing fluids, combining the action creases. of acidizing and fracturing. They are sufficiently viscous to suspend sand during such treatments. Basic Stimulation Methods Chemical additives are used in acid treatments when Gas producers have used many variations of the basic desirable. Both surface-active agents and silicate-swelling stimulation methods, acidizing and fracturing, to obtain control agents are frequently employed. maximum production from gas wells. These various tech­ niques are necessary to meet individual well conditions Fracturing Techniques found in different formations and fields. A knowledge of Fracturing has become an im~ortant and valuable meth­ the various stimulation techniques available and of the ad­ od for stimulating gas wells. Fracturing increases produc­ vantages of each should enable an operator to plan gas tion both by the eroding effect of sand grains rushing well stimulation treatments that will bring maximum pro­ through formation passages and bv opening new flow chan­ duction from either old or new gas wells. nels propped ooen by sand grains. Fracturing treatments originally used thickened oil as a sand-supporting fractur­ Acidizing Techniques ing fluid. Later, thickened water and thickened acid were Acidizing treatments using inhibited hydrochloric acid used. to enlarge pores and flow channels have been used suc­ For several years injection rates remained low (about cessfully for many years. Ordinarily, an acid treatment is 1 to 5 bbllmin). Later high injection rates were used. conducted by spotting acid opposite the zone to be treated, Advantages of high rates are that they (1) reduce frac­ and then pumping it into the formation. turing fluid costs because unthickened fluids can be used, High-injection-rate acid treatments are a variation of (2) penetrate deeper into the formation, and (3) produce the normal acid treatment. In such treatments, large vol­ more fractures in the pay section. umes of acid are injected into the formation at from 15 Both acidizing and fracturing treatments make use of to 75 bbl/min. Acid is usually pumped down both tubing temporary plugging materials to divert treating fluid from and casing. Advantages of this technique are that the rapid sections of relatively high permeability to sections of lower injection rate penetrates deeper into the formation and permeability. One method employs solids of graded par­ breaks down a larger vertical extent of the pay zone. ticle sizes. These bridge and plaster over the most per­ A special acid is often used to achieve deeper pene­ meable section, diverting later fluids into other, less per­ tration. This acid contains an additive which retards the meable zones. The chemicals in the plug later liquefy, unblocking the permeable zone for continued production. Original manuscript received in Society of Petroleum Engineers of­ The other type of plug is the perforation ball sealer. These fice Mareh 3. 1958. Revised manuscript reoeived Aug. 19. 1958. Paper ball sealers are dropped into the well with the fluid and are presented at Gas Technology Symposium in Shreveport La. April 17-18. 1958. • • drawn into the perforations which are accepting fluid, di- SPE I067-G NOVEMBER, 1958 31 verting the fluid to other perforations. When the treatment Deep Acid-Insoluble Formations ends, the balls drop from the perforations to the bottom These formations include the Morrow and Tonkawa of the wellbore. sands from 4,000 to 8,000 ft. Typical stimulation treat­ ments in these formations consist of 15,000 gal of gelled Treating Techniques or refined oil, carrying 1 It> of 40-60 mesh sand per gal, injected at 30 bbl/min. These treatments usually begin San Juan Basin Gas Wells with low sand concentrations which are increased as the All these variations of acidizing and fracturing are used treatment progresses. Fracturing is often preceded by a in one or more areas with great success. In the Four Cor­ 500-gal mud-acid wash. This treatment cleans out perfora­ ners area, where New Mexico; Colorado, Utah and Ari­ tions and increases permeability in the critical area. A zona meet, gas production is confined largely to the San further advantage of the mud-acid wash is that it reduces Juan Basin in Northwest New Mexico. These wells are treating pressures, thus placing less stress upon the ce­ usually fractured, using water as the sand-carrying me­ ment. Typical results of fracturing treatments in the Mor·· dium. San Juan Basin operators were the first to employ row and Tonkawa sands are shown in Table 2. water as a fracturing fluid. Its use was first prompted by the excessive cost of transporting and storing oil. After Deep Acid-Soluble Formation~ experience proved that it was a satisfactory fracturing me­ The Chester and Hoover limestones are found at 4,000 dium, water quickly became popular in many other areas. to 8,000 ft. Because they are from 70 to 97 per cent solu­ ble in hydrochloric acid, various acid fluids are used in Gas wells in the San Juan Basin's three producing for­ stimulating them. Retarded acid, ordinary 15 per cent mations, the Cliff House, Menefee and Upper and Lower Downloaded from http://onepetro.org/JPT/article-pdf/10/11/21/2236770/spe-1067-g.pdf by guest on 24 September 2021 acid, 5 per cent acid-carrying sand, or gelled and emulsi­ Point Lookout, are usually fractured with 30,000 to 60,- fied acids carrying sand may be used. In the Chester for­ 000 gal of water, using 1.5 Ib of sand per gal of water. In mation, retarded acid, injected at 12 to 15 bbljmin, has some cases as much as 2 Ib of sand per gal of water has been highly effective. The Hoover formation has responded been used. Injection rates average about 60 bbljmin, al­ well to recent weak acid fracturing treatments. Acid used though rates as high as 100 bbljmin have been used. Table has varied from 2.5 to 5 per cent. A mud-acid wash is 1 gives results of fracturing treatments in several San Juan often used ahead of fracturing treatments in these forma­ Basin gas wells. tions. Suitable additives are incorporated in the fracturing The following case history is typical of treatments in this fluid when desirable. Typical results of treatments in these area. A San Juan gas well was perforated from 3,206 to formations are given in Table 3. 3,252 ft. Before the fracturing treatment this well showed no production. It was fractured with water carrying 30,- South Texas and Gulf Coast Gas Wells 000 Ib of sand. During the treatment perforation ball seal­ ers were dropped in three groups of 50 each. Average in­ The major gas-producing formations in South Texas and jection rate was 71.5 bbljmin. Actual gas flow on a three­ the Gulf Coast are the Frio and Wilcox. The Frio sands hour test was 2,427 Mcf/D. Calculated absolute open flow are soft, silty and shale laminated. Their solubility in hy­ was 5,978 Mcf. drochloric acid ranges from 10 to 30 per cent. The Wilcox sand averages less than 1 per cent solubility in hydro­ Oklahoma-Texas Panhandle and Southwest Kansas chloric acid. It is a hard, shale-laminated formation, characterized by streaks of lignite. The major Wilcox gas­ Gas well stimulation techniques in Southwest Kansas producing area is from Montgomery County, north of and the Oklahoma-Texas Panhandle are of three major Houston, southwest to Zapata County. types, depending upon the formations involved. These are In both formations fracturing is the major stimulation (1 shallow production from the Herrington, Krider, Win­ r method. A mud-acid wash is often used also to clean mud field and Fort Riley formations, (2) deep production from from the formation and from perforations. The principal the acid-insoluble Morrow and Tonkawa sands, and (3) fracturing fluid used in these formations is an acid-kero­ deep production from the acid-soluble Chester and Hoover limestones. sene gel. The amount of sand used has varied, but aver­ ages about 8,000 Ib for the Frio formation and about 15,- Shallow Gas Formations Practically all new completions and workovers in the shallow Hugoton series of formations are now stimulated TABLE 2-RESULTS OF FRACTURING MORROW AND TONKAWA SANDS by fracturing. In the past, acidizing was the major stimu­ Fluid Sand Injection Production (open flow, ~~U Formation Gallons Type ~ rote (bbl/min) Before After lation method. Because the Hugoton series is from 50 to Morrow 20,000 gelled 30,000 28 show 40,000 oil 85 per cent soluble in acid, fracturing treatments in these Morrow 20,000 water 20,000 34.8 850 26,000 formations frequently use weak acid as the treating fluid. Tonkawa 25,000 gelled 25,000 25 1,500 35.000 New completions in this area are being fractured with oil from 8,000 to 20,000 gal of 1 per cent hydrochloric acid, carrying 1 Ib of 20-40 mesh sand per gal, at injection TABLE 3-RESULTS OF STIMULATING CHESTER AND HOOVER LIIAESTONES rates of 30 to 40 bbljmin. Older wells, discussed later in Production Fluid Sand Iniection __ (open flow, Mcf) _ more detail, are fractured with larger quantities of ma­ Formation Gallons Type .J..I!>l rate (bbl/min) Before After terials and at higher injection rates. Chester 10,000 retarded acid 18 600 8,000 Hoover 30,000 3 per cent acid 30,000 55 show 40,000 Chester 10,000 retarded acid 28.5 349 30,000 TABLE 1-FRACTURING TREATMENTS IN SAN JUAN BASIN GAS WELLS Initial Water Sand Injection potential TABLE 4-COMPARISON OF G\jLF COAST GAS WELL POTENTIALS BEFORE Well Zone treated (gal) (I b) rate (bbl/min) (Mel) AND AFTER FRACTURING lower Point lookout 47,700 60,000 44 Before (Mcf/D) After Mcf/D) Upper Point Lookout 51,120 60,000 50 14,407 County Formation Depth (ft) Point Lookout 40,000 50,000 55 Dewitt Wilcox 7,800 Show 14,500 Menefee 21,200 31,000 35 Duval Wilcox 7,100 3,000 20,000 lower Cliff House 40,000 56,000 62 Matagorda Frio 10,000 2,600 10,600 Upper Cliff House 40,000 60,000 55 12,432 Frio 8,600 3,000 30,000

22 JOURNAL OF PETROLEUM TECHl'"OLOGY 000 lb for the Wilcox formation. Table 4 shows results of to 6 bbljmin down tubing have often potentialed at much fracturing gas wells in the Wilcox and Frio formations. lower rates than offset wells acidized down both casing and tubing at rates ranging from 17 to 25 bbl/min. Ark-La-Tex Gas Wells The Ark-La-Tex area covers East Texas, North Louisi­ Cotton Valley D ana and Southern Arkansas. All major petroleum-bearing formations in this area produce gas in some localities. Gas production from the Cotton Valley D sand occurs The principal gas-producing formations are the Monroe principally in North Louisiana in Caddo, Bossier and Lin­ gas rock, Rodessa, Pettit, Cotton Valley D and Smackover. coln parishes. The formation is usually found at 7,400 to Results of stimulating Ark-La-Tex gas wells are given in 9,000 ft. This sandstone is characterized by low permeabil­ Table 5. ity and low porosity. However, many wells in the Cotton Valley D come in naturally. Those which are not natural Monroe GGiS Rock completions usually respond satisfactorily to mud-acid and/ The Monroe gas rock, in Louisiana's Union and Oua­ or fracturing treatments using emulsified acid or gelled oils. chita parishes, is a shallow, silty lime at about 2,400 ft. Emulsified acid is ordinarily used when bottom-hole Its solubility in 15 per cent hydrochloric acid ranges from temperatures are above 225°F. These treatments are con­ 50 to 77 per cent. It produces dry gas. The usual acid ducted at pressures from 4,500 to 9,000 psi. Injection treatment in this area is 3,000 to 5,000 gal of 15 per cent rates range from 1 to 4 bbl/min. The average treatment inhibited hydrochloric acid. Recently, this formation has consists of 6,000 gal of emulsified acid, carrying 4,800 lb been fractured with 18,000 gal of 5 per cent acid, using of sand, and spearheaded by 500 gal of mud acid. At lower Downloaded from http://onepetro.org/JPT/article-pdf/10/11/21/2236770/spe-1067-g.pdf by guest on 24 September 2021 20,000 Ib of sand. Although results in some cases were pressures and temperatures gelled oil is frequently used as excellent, they were not consistent. For this reason, 15 per the fracturing fluid. cent acid is again being used. Smackover Limestone Rodessa Sandstone The Smackover formation is an oolitic limestone found at depths from 7,500 to 13,000 ft. Smackover gas pro­ Many small fields produce gas from the Rodessa for­ duction occurs from Southwest Arkansas through East mation in East Texas. A major Rodessa gas field is the Texas. The Smackover is 85 to 100 per cent soluble in Willow Springs in Gregg County, Tex. This formation is 15 per cent hydrochloric acid. Fracturing treatments in an oolitic, sandy limestone with an acid solubility of this formation have been unsuccessful. However, acidizing about 80 to 95 per cent in 15 per cent hydrochloric acid. has been highly successful, particularly since large acid In the Willow Springs field, the Rodessa is found at about treatments have become commonplace. 6,700 ft. The average treatment in this field is a multiple­ Present practice in Smackover formation is to use 10,- stage acid job, using volumes of 8,000 to 10,000 gal. Tem­ 000 to 15,000 gal of acid in two stages. Temporary plug­ porary plugging agents are used between stages to insure ging material is used between stages. The first half of each treating each separate zone. stage consists of retarded acid, followed by 15 per cent in­ Results in this field point out clearly the value of acidiz­ hibited hydrochloric acid. ing, especially the importance of using temporary plugging In the New Hope field the standard treatment is now materials to divert stimulation fluids into low-permeability larger, consisting of three stages using a total of 21,000 zones. The production range of wells com]Jleted naturally, to 28,000 gal of acid. The first half of each stage is re­ without stimulation, has been from 2,950 to 12,000 Mcf. tarded acid and the second half is regular 15 per cent hy­ Wells in this field that were acidized without using tem­ drochloric acid. Additives are incoroorated in the acid to porary plugging materials produced from 4,600 to 60,- provide low surface tension and silicate control, and to 000 Mcf. When multiple-stage acid treatments employing prevent the occurrence of emulsions. temporary plugging materials were used, ]Jroduction rates The first large acid treatment in the New Hope field jumped to a range of 75,000 to 280,000 Mcf. was on a new well which had been washed with 2,000 gal of mud acid, then squeezed with 1,000 gal of acid con­ Pettit Limestone taining demulsifying and low-surface-tension additives. Fol­ lowing this treatment, the well produced 8,800 Mct/D. The Pettit formation is an oolitic limestone with acid The well was then given a selective acidizing treatment solubility ranging from 80 to 100 per cent. It produces consisting of 21,000 gal of acid injected in three separate gas primarily in West Louisiana and East Texas at depths stages. Temporary plugging material was used between ranging from 5,800 to 7,800 ft. stages. The first half of each stage consisted of retarded Treatments used in Pettit limestone vary considerably acid, while the second half was regular inhibited hydro­ from one locality to another. Conventional acid jobs using chloric acid containing suitable additives. On a 10-day as little as 1,000 gal of 15 per cent acid are sometimes em­ test the well's open-flow potential was 45,500 Mcf/D and ployed. In some areas, high-injection-rate acid treatments 152 bbl of condensate per 1,000 Mcf of gas. using 25,000 gal of ordinary 15 ]Jer cent acid or retarded acid may be used. In other areas the Pettit limestone has Stimulating Old Gas Wells responded well to a combination acidizing-fracturing treat­ ment consisting of from 3,000 to 5,000 gal of emusified One of the best opportunities for increasing profits in acid, carrying 0.8 lb of sand per gal, followed by 10,000 gas production lies in retreating old gas wells which no to 15,000 gal of retarded acid and/or regular acid. Rec­ longer produce their allowable. Many operators have long ommendations for a specific Pettit well depend upon pre- vious experience in the area. • TABLE 5-RESUl TS OF RETREATING HUGOTON SERIES GAS WELLS High injection rates have been particularly valuable in Treating fluid Sand Injection -~- lib) rate (bbl/min) Before After the Pettit limestone. For example, operators in Harrison 80,000 (water) none . 100 485 1.200 20,000 (15% acid) none 72 1,200 2,700 County, Tex., have found that wells treated at rates of 4 110,000 (I % acid) 60,000 75 650 1,4QQ

l'iOVEMBER, 1958 23 TABLE 6-RESUlTS OF GAS WELL TREATMENTS IN lHE ARK·lA·lEX AREA St.t... nd Treating fluid Sand Production (Mcf/D) cGunty/parish Field and form-ation (gal) (lb) Before Alter La-Union Monroe-Gas Chalk 18,000 (5% ~cid) 20,000 ---:w--- 200 la-DeSoto Bethany-longstreet-Pettit 7,000 (15% acid) 10,000 127,000 2,500 (retarded acid) la-DeSoto Keatchie-Pettit 10,000 (15% acid) old well, would 15,000 15,000 (retarded acid) nof feed line la-Bossier Sligo-Collon Valley 0 6,000 (acid·oil emulsion) 4,800 200 allowable (4,OOO) 500 (mud acid) la-Caddo Greenwood-Cotton Valley 500 (mud acid), 6,000 (acid· 4,800 show 8,800 oil emulsion), 1,.000 (15% acid) Tex-I'anola Carthage-Travis Peak 250 (mud acid), 2,000 (acid· 2,000 flowing on flowing on oil emulsion) 700 psi 2,700 psi Tex-Gregg Willow-Springs-Rodessa 7,000 (15% acid), 462 (temp. plug) new 290,000

felt that reworking such gas wells was unwise. However, Keatchie Field experience during the past year has shown that stimulat­ The development of high-injection-rate acidizing and ing gas wells in which bottom-hole pressure has not de­ effective new additives has also made retreatments of creased too greatly often brings excellent production in­ Pettit limestone wells profitable. For example, a Pettit well creases. Careful analysis of individual well situations prior in the Keatchie field had declined to the point that it would to stimulation is even more important in the case of old not feed into the line. The well was stimulated with 15,000 wells than new wells. The treatment for each well should gal of retarded acid, followed by 10,000 gal of regular be carefully planned to provide maximum results. inhibited hydrochloric acid. Average injection rate was about 19 bbl/min. Following the treatment, this well po­ Downloaded from http://onepetro.org/JPT/article-pdf/10/11/21/2236770/spe-1067-g.pdf by guest on 24 September 2021 Hugoton Gas Field tentialed 15,000 Mcf/D, and six months later was still The value of reworking old wells can be seen in current producing its allowable into the line. stimulation practices in the Hugoton gas field, located in Southwest Kansas and the Oklahoma-Texas Panhandle. Conclusion Many old wells in this area are being treated with 50,- 000 to 80,000 gal of 1 per cent inhibited hydrochloric Properly planned stimulation treatments are usually acid, carrying 1 lb of sand per gallon. Injection rates have beneficial to both new and old gas wells. Because proper approximated 65 bblfmin. By contrast, new wells being stimulation treatments for gas wells vary widely from one completed in this field are presently being treated with area to another, it is advisable to rely on past experience only 8,000 to 20,000 gal of 1 per cent hydrochloric acid, in the area under consideration. Acidizing or fracturing and at injection rates of only 30 to 40 bblfmin. Results treatments which are highly successful in a given forma­ of treating these old wells are shown in Table 5. tion in one area may be much less so in the same forma­ tion in another area. For this reason, each gas well stimu­ Carthage Gas Field lation treatment should be carefully planned in view of This gas field, in the central portion of Panola County, recent experience in the area and individual well condi­ Tex., is about 25 miles long and 20 miles wide. The three fum. H* important gas-producing horizons are the Hill and the Upper and Lower Pettit. Retreatments of old wells in this field have been highly profitable. For example, a Carth­ age gas well completed in 1947 had declined until it was making only 250 Mcf/D before treatment. Following a treatment of 1,000 gal of channel acid followed by 6,000 gal of regular acid, the well produced 1,250 Mcf/D. The entire cost of the acid retreatment was returned by addi­ tional production in only 10 days.

Sligo Gas Field

Another example of successful treatments of old gas PAUL CLINKENBEARD (center) is district sales manager wells is in the Cotton Valley D formation, Bossier Parish, for Dowell at Shreveport, La. He received a BS degree North Louisiana. These wells were originally completed in mechanical engineering from Oklahoma State V. in with a SOO-gal mud-acid wash in 1947 and 1948. About 1949 and joined Dowell as a junior service engineer. J. F. a year ago, stimulation treatments were begun which con­ BOZEMAN (right) is district sales manager for Dowell at sisted of 500 gal of mud acid followed by 6,000 gal of Houston. He joined Dowell in 1950 after graduation from emulsified acid carrying 4,800 lb of sand injected at the 'I. of Houston with a BS degree in mechanical engi­ neerzng. R. D. DAVIDSON (left) is Dowell's district engineer about 2 to 4 bbl/min and 4,500 psi. Production, which at Amarillo, Tex. He joined the company in 1951 after was as low as 200 Mcf,. was increased in all cases to the graduating from the V. of Oklahoma with a BS degree in maximum allowable of 4,000 Mcf. petroleum engineering.

24 JOURNAL OF PETROLEUM TECHNOLOGY