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Acidizing

Chapter 54 Acidizing A.W. Coulter Jr., A.R. Hendrickson, S.J. Martine2.u. of Dwell-Schlumberger Dowell-Schlumberger Tulsa * Introduction The use of acids to stimulate or to improve oil production from carbonate reservoirs was first attempted in 1895. Patents covering the use of both hydrochloric and sulfuric acids for this purpose were issued at that time. Although several “well treatments” were conducted, the process failed to arouse general interest because of severe corrosion of well casing

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  Chapter 54 Acidizing A.W. Coulter Jr., Dwell-Schlumberger A.R. Hendrickson, Dowell-Schlumberger S.J. Martine2.u. of Tulsa * Introduction The use of acids to stimulate or to improve oil produc-tion from carbonate reservoirs was first attempted in 1895.Patents covering the use of both hydrochloric and sulfur-ic acids for this purpose were issued at that time. Althoughseveral “well treatments” were conducted, the processfailed to arouse general interest because of severe corro-sion of well casing and other metal equipment, The nextattempts to use acid occurred between 1925 and 1930.These consisted of using hydrochloric acid (HCl) to dis-solve scale in wells in the Glenpool field of Oklahomaand to increase production from the Jefferson Limestone(Devonian) in Kentucky. None of these efforts were suc-cessful and “acidizing” once again was abandoned.The discovery of arsenic inhibitors, which allowed HClto react with the formation rock without seriously damag-ing the metal well equipment, revived interest in oilwellacidizing in 1932. At that time, Pure Oil Co. and DowChemical Co. used these inhibitors with HCl to treat awell producing from a limestone formation in IsabellaCounty, MI. Results of this treatment were outstanding.When similar treatments in neighboring wells producedeven more spectacular results, the acidizing industry wasborn.Throughout the years following those early treatments,the acidizing industry has grown to one using hundredsof millions of gallons of acid applied in tens of thousandsof wells each year. Technology has developed with in-creasing rapidity, and many changes and innovations havebeen made to improve the effectiveness of acidizing treat-ments. Because of new techniques of application and de-velopment of additives to alter the characteristics of theacid itself, acidizing has become a highly skilled science.A knowledge of available materials, chemical reactions ‘Authors of the OrigInal chapter on this topic I” the 1962 editlon Included th!s aulhor(deceased). P E. Rlzgerald. and Harold E Staadt at treating and well conditions, reservoir properties, androck characteristics are required to design an effective andefficient acidizing treatment. Since it is beyond the scopeof this text to cover all aspects of acidizing in detail, thisdiscussion will be limited to a general description of ma-terials, techniques, and design considerations. A bibli-ography is provided for those requiring a more detaileddiscussion of a particular subject. Also, the major wellstimulation companies providing acidizing services offerliterature and technical assistance for problem analysis andtreatment design. General Principles The primary purpose of any acidizing treatment is to dis-solve either the formation rock or materials, natural orinduced, within the pore spaces of the rock. Originally,acidizing was applied to carbonate formations to dissolvethe rock itself. Over a period of time, special acid for-mulations were developed for use in sandstone formationsto remove damaging materials induced by drilling or com-pletion fluids or by production practices.There are two primary requirements that an acid mustmeet to be acceptable as a treating fluid: (1) it must reactwith carbonates or other minerals to form soluble prod-ucts, and (2) it must be capable of being inhibited to pre-vent excessive reaction with metal goods in the well. Otherimportant considerations are availability, cost, and safe-ty in handling. While there are many formulations avail-able, only four major types of acid have found extensiveapplication in well treatments: hydrochloric, hydrofluoric,acetic, and formic acids. Hydrochloric Acid (HCI) An aqeuous solution of HCl is most commonly used foracidizing treatments, for reasons of economy and becauseit leaves no insoluble reaction product. When HCl is  54-2 IT3500InI/ 300016I412IOnb4200 4n I2 16 20 24 STRENGTH OF ACID, PERCENT BY WEIGHT Fig. 54.1-Solution of limestone in acid.TABLE 54.1~-HYDROCHLORIC ACID DENSITY AT 6O“F% HCI SpecificGravity' Baume** lbmlgal psilftepth1.001.00480.78.3770.43512.001.0097 1.4 8.4180.43723.001.0147 2.1 8.4600.43924.001.0197 2.88.501 0.44155.001.0248 3.58.5440.44376.001.02994.2 8.5860.44597.001.03504.9 8.6290.44828.001.0402 5.68.672 0.45049.001.0447 8.28.710 0.452410.001.05006.9 8.7540.454711.001.0550 7.6 8 796 0.456812.001.0600 8.28.837 0.459013.00 1.0646 8.88.8760.461014.00 1.0702 9.58.922 0.463415.001.0749 10.1 8.9620.465416.001.0801 10.8 9.006 0.467717.001.0849 11.4 9.045 0.469818.00 1.090212.0 9.0890.472119.00 1.0952 12.6 9.132 0.474320.00 1.100213.2 9.171 0.476421.001.105713.99.218 0.478822.001.1108 14.5 9.2610.481023.00 1.115915.1 9.303 0.483224.00 1.121415.7 9.349 0.485525.001.1261 16.3 9.3850.487626.001.1310 16.9 9.433 0.489927.001.136817.59.478 0.492228.001.142218.09.523 0.494629.001.147118.69.563 0.496730.001.1526 19.2 9.609 0.499131.001.1577 19.8 9.663 0.501232.001.1628 20.39.6940.503533.00 1.1680 20.99.7380.505734.00 1.172721.4 9.7770.507835.00 1.177921.9 9.820 0.510036.001.182722.4 9.8600.512137.001.1880 22.9 9.9240.514438.00 1.1924 23.4 9.941 0.516339.001.196323.89.974 0.518040.001.200824.310.0110.519941.001.2053 24.7 10.049 0.5219145PETROLEUM ENGINEERING HANDBOOK pumped into a limestone formation, a chemical reactiontakes place, producing calcium chloride, CO*, and water.This reaction is represented by the following equation:One thousand gallons of 15% HCI will dissolve approxi-mately 10.8 cu ft (1,840 lbm) of limestone. It will liber-ate approximately 7,000 cu ft of CO1 , measured atatmospheric conditions, and produce 2,042.4 lbm of cal-cium chloride. This salt is dissolved in the original waterof the acid solution, plus 39.75 gal of water formed dur-ing the reaction. The specific gravity of this solution willbe 1.181 (20.4% calcium chloride). While 15 wt% HCIhas been the most commonly used, concentrations of 20and 28% have become extremely popular over the past2 decades. Regardless of the acid strength used, the reac-tion is the same and equivalent amounts of carbonate rockare dissolved. For example, 10,000 gal of 3% HCl solu-tion will dissolve the same amount of rock as 1,000 galof 28% HCl. Fig. 54.1 shows the effect of acid concen-tration on the amount of limestone dissolved. The maindifferences between the two solutions are their reactionrates (or spending times) and their physical volumes.Although lower concentrations of acid have greaterequivalent volumes, their reaction times and depth ofpenetration into the reservoir, from the wellbore, are con-siderably less than those of the higher-strength solutions.Reaction rates and penetration will be discussed later.Similar reactions occur when dolomite or impure lime-stone is treated with HCI. Dolomitic lime contains a largepercentage of magnesium combined as calcium magnesi-um carbonate. Although it reacts more slowly, this min-eral also dissolves in HCl, and the resulting magnesiumchloride is soluble in the spent acid. Other impurities oc-curring in limestone and dolomite are often insoluble inacid, and if appreciable percentages of such componentsare present, special additives must be included in the acidsolution to ensure their removal.HCl ordinarily is manufactured in concentrations of 32to 36 wt% HCl and is diluted at service company stationsto 15, 20, or 28% for field use. The concentrated acid,the various chemical additives, and water are mixed inthe tank truck used to haul the acid to the wellsite. Table54.1 lists the weights of various concentrations of HCI.These data are useful in calculating mixing proportionsfor acid dilution, using the following equation:vl-0 = vda cda 7 da Cca(HCI)Yca ' whereV/da= final volume of dilute acid, cdd = desired concentration of dilute acid, ?'du = specific gravity of dilute acid, V,, = volume of concentrated acid required, Cccr(HCI) percent of HCI in concentrated acid, andycO = specific gravity of concentrated acid.Approximate proportions of concentrated acid and water required for dilution are shown in Fig. 54.2.  ACIDIZING54-3 Determination of acid strength can be estimated in thefield using either a hydrometer or a field titration kit. Theaccuracy of hydrometer readings depends on the care andtechnique used by the field engineer. Both the hydrome-ter and the glass cylinder in which the test is made shouldbe free from oil or dirt. The spindle should float freelyin the acid, and all readings should be made at the lowestlevel of the acid meniscus. The temperature of the acidsample should be taken and the hydrometer reading cor-rected to 60°F. FORMULA FOR MIXING ACID IN ANY DESIREDCONCENTRATION:VOLUME OF STRONG = (VOL OF WEAK) (%WEAK) (SF? GR.OF WEAK)(=&OF STRONG) (SF’. GR.OF STRONG) GALLONS OFCONCENTRATEDt 7 HYDROCHLORIC ACID TO MAKE1,000 GALLONS OF DILUTE ACID -8 Determination of acid strength by titration is simplifiedby the use of 0.59 N standard sodium hydroxide solution.If a 2-mL sample of the acid is titrated with this standardsolution to a methyl orange end point, the burette read-ing (milliliters of sodium hydroxide used) will be equalto the acid strength (percent HCI). -9 & Acetic and Formic Acids Acetic acid (CH3COOH) and formic acid (HCOOH) areweakly ionized, slowly reacting, organic acids. They areused much less frequently than HCI and are suitableprimarily for wells with high bottomhole temperatures (BHT’s above 250°F) or where prolonged reaction timesare desired. The reaction of these acids with limestoneis described by the following equation: - 32- 30-283s-23’ 2:Q-202-I;-17k-16Z-15=-I4- 13k-12 2HOrg+CaCOj +CaOrgz +HzO+CO,.HAc is available in concentrations up to 100% as glacialHAc. while HCOOH is available in 70 to 90% concen-trations. For field use, HAc solutions normally are dilut-ed to I5 % or less. - IO-II $?-12 1-13 0-14 2I5 a -16 z -17 0-189 E-2om-2&2,EKBG-282:3oE: 32 z -34w:36g1380-400 Above this concentration, one of the reaction products,calcium acetate, can precipitate from its “spent acid” so-lution because of its limited solubility. Similarly, the con-centration of HCOOH normally is limited to 9 to 10%because of the limited solubility of calcium formate. Ata 10% concentration, 1,000 gal HAc will dissolve 740Ibm of limestone, whereas 1,000 gal HCOOH dissolves970 lbm. Where more dissolving power per gallon of acidis desired, HCI is sometimes mixed with HCOOH orHAc. Such blends still provide extended reaction times.when compared with HCl. HCOOH and HAc also maybe blended together. Table 54.2 illustrates some of themore common acid strengths and blends. Fig. 54.2-Dilution of concentrated HCITABLE 54.2-DIFFERENT ACIDIZING SOLUTIONSAcidConcentration Type RelatweCaCO, Eouivalent Reaction(Ibm/l,i)OOal acid) time’ Hydrofluoric Acid (HF) HF is used in combination with HCI and has been referredto as “intensified acid” or “mud removal” acid. depend-ing on the formulation and use. HF is used primarily toremove clay-particle damage in sandstone formations, toimprove permeability of clay-containing formations, andto increase solubility of dolomitic formations. Its utilityis based on the fact that some clays. silica, and other ma-terials normally insoluble in HCI have some degree ofsolubility in HF. For example, I .OOO al of an acid solu-tion containing 3% HF and 12% HCI will dissolve 500lbm of clay and up to I .450 lbm of CaC03, 7.5HCI8900.715HCI 1,8401.028HCI3,6706.036HCI 4,860 12.0 10 Formic9105.0 10 Acetlc 71012.015Acetic 1,06518.07.5Formic/14HCI mixture 2,4206.0IO14Acetic/HCI mixture2,380 12.08 Formic/14Acetic mixture1,70018.0 4HF+SiO? -tSiFJ +2H10andZHF+SiF,+HzSiF,. ‘Approwlmale time for acid react!on 10 be COmpleled ( %qx?nl ‘) to an equ~vaienfstrength of 2 5% HCI solution Values are compared by using spending lime 0115% HCI as 1  54-4PETROLEUM ENGINEERING HANDBOOK 0.4III \-MUD ACID 0.3 7’0.2I0. I 0/-REGULAR ACIDIIIII0 6 12 18 24 TIME OF CONTACT IN HOURS Fia. 54.3-Solubilitvof bentonite n mud removal acid ; 0.41 I I I z 0.3u2 TIME OF CONTACT IN HOURS1 Fia. 54.4-Solubilitv f silica and in mud removal acid Figs. 54.3 and 54.4 compare solubilities of bentonite andsilica in HCl and HF acids.In carbonates, application of HF/HCl mixtures must becontrolled carefully because of cost and possible precipi-tation of reaction products such as calcium fluorides orcomplex fluosilicates, which have a very limited solubil-ity. For reaction with silicates, such as natural clays orclays in drilling fluids, the blends usually contain 2 to 10%HF and 5 to 26% HCI. The concentration of HCl usedin the blend should be equal to or greater than that of theHF.The so-called “intensified acids” used in dolomitic for-mations are mainly HCl containing small concentrationsof HF, usually about 0.25 % Intercrystalline films of sil-ica, insoluble in HCl, often occur in the crystal structureof dolomite. When such are present, they prevent the acid from contacting the soluble portions of the rock. The pres-ence of fluoride intensifier in the acid will destroy such films, allowing the acid to react more completely withthe soluble portions of the rock. Fig. 54.5 illustrates thecomparative reaction rates of HCI and intensified acid ondolomite formations.More recent developments of HF involve the use ofdelayed-action agents in sandstone acidizing. The first ofthese was a self-generating mud acid system, reported byTempleton er al. ’ The system provides slow generationof acid from the hydrolysis of methyl formate. yieldingmethyl alcohol and HCOOH acid. The acid then reactswith ammonium fluoride to yield HF in situ. They attrib- ute the success of the system to getting the HF reactionaway from the wellbore into areas that conventional HFsolutions normally do not reach before spending. Equal-ly important factors are the techniques of application andof returning the well to production following treatment.The treatment technique mvolves use of an aromatic sol-vent and mud acid preflush, along with the self-generatingmud acid (SGMA). The wells are returned to productionby opening the choke gradually over a 90-day period and never allowing an excessive drawdown. The process isavailable from most service companies as SGMA.A significant development in this area of slow-reacting,HF-supplying, clay-dissolving acid has been the fluoboric acid system reported by Thomas and Crowe.’ This acidhydrolyzes to form hydroxyfluoboric acid and HF, whichwill dissolve clays.HBF4 +HZO-‘HBF30H+HF.This reaction provides a slow-release source of HF, whichcan penetrate deeply before spending. Perhaps more im-portant, the slowly generated hydroxyfluoboric acid reactswith clays to form a nonswelling, nondispersing productthat stabilizes fine clays and holds fine particles of silicain place. Acid Reaction Rates A knowledge of the factors affecting the reaction rate ofacids is important for several reasons. First, these fac-tors, correlated with reservoir and formation character-istics, form a guide for the selection of acid type andvolume for a given treatment. Next, a study of these fac-tors can furnish an understanding of what parametersgovern spending time, which will determine how far agiven formulation can penetrate into the formation beforespending. Many factors govern the reaction rate of anacid, such as pressure, temperature, flow velocity, acidconcentration, reaction products, viscosity, acid type,area/volume ratio, and formation composition (physicaland chemical). These factors have been the subject of ex-tensively reported research for many years. Details of suchstudies are available in published literature. Only a briefgeneral discussion will be presented here.PressureFig. 54.6 shows the effect of pressure on the reaction rateof 15% HCl with limestone and dolomite at 80°F. Above500 psi, pressure has little effect on reaction rate. At bot-tomhole treating pressures, there is only a small differ-ence (a factor of 1.5 to 2) in the comparative reaction ofacid with limestone and dolomite compared to the ratherlarge difference (a factor of about 10) at atmosphericpressure. Temperature Acid reaction rate increases directly with temperature. At140 to 150”F, the reaction rate of HCI and limestone is