United States Patent (19) 11) 4,371,443 Keeney 45) Feb. 1, 1983

(54) METHOD OF AND COMPOSITION FOR 3,157,232 11/1964 Ramos et al...... 166/307 X ACIDZNG SUBTERRANEAN 3,707,192 12/1972 Goins et al...... 166/307 X FORMATIONS 3,889,753 6/1975 Richardson ...... 252/8.55 X 4,044,833 8/1977 Volz ...... 166/307 75 Inventor: Bill R. Keeney, Duncan, Okla. 4,172,041 10/1979 Clampitt et al...... 252/8.55 73) Assignee: Halliburton Company, Duncan, Okla. Primary Examiner-Herbert B. Guynn Attorney, Agent, or Firm-Robert A. Kent; Thomas R. (21) Appl. No.: 232,785 Weaver 22 Filed: Feb. 9, 1981 57 ABSTRACT 51) Int. Cl...... E2B 43/27 52 U.S. Cl...... 252/8.55 C; 166/300 A method and composition for acid treating a subterra 58) Field of Search ...... 252/8.55 C, 166/300, nean formation. The composition comprises an alde 166/307 hyde, an ammonium salt capable of reacting with the aldehyde to produce a free acid, and a chemical additive 56) References Cited which retards the formation of the free acid. The pro U.S. PATENT DOCUMENTS cess is carried out by contacting the subterranean for 2,652,360 9/1953 Bond et al...... 252/8.55 mation with the composition. 2,778,427 1/1957 Cardwell et al...... 252/8.55 X 2,824,833 2/1958 Cardwell et al...... 252/8.55 14 Claims, No Drawings 4,371,443 2 formations with deep wells, the acid is largely neutral METHOD OF AND COMPOSITION FOR ized by the reactive material immediately adjacent to ACOZING SUBTERRANEAN FORMATIONS the well bore before the acid can reach other desired portions of the formation. The high temperatures exist This invention relates to acid treating or acidizing of 5 ing in these formations also increase the corrosion by subterranean formations. the acid of the casing and tubing of the well. The flow of oil from a subterranean formation to a Various solutions have been proposed to increase the well bore depends, among other factors, upon the de reaction time of various acidizing solutions on acidiza gree of permeability of the formation. Often, the perme ble formations and to decrease the corrosion problem ability is not sufficiently great to prevent a desired rate O related to the acidizing solutions. For example, in U.S. of flow of fluids, e.g., crude oil, natural gas, and so Pat. No. 3,157,232, which is assigned to the assignee of forth, from the formation. In such a case, the formation the present invention and is herein incorporated by can be treated to increase its permeability. reference, a method of acidizing an oil-bearing earthen Acidizing fluids have been utilized heretofore for formation to increase productivity thereof is disclosed. carrying out treatments in subterranean oil, gas or water 15 The method comprises combining formaldehyde and an producing formations. Such acidizing fluids have pri ammonium salt capable of reacting with the formalde marily been utilized to increase the productivity of oil hyde to form a free acid and directing the free acid and gas from calcareous formations by bringing out the through the oil-bearing earthen formation. In U.S. Pat. removal of reactive materials from naturally occurring No. 3,707,192, a slurry of reactive particles is displaced fractures and pore spaces in the formations whereby the 20 down a well and into a fracture to coat the faces of the sizes thereof are increased. Hydrochloric acid is ordi fracture at the desired radial distance from the well. narily used in acidizing limestone or dolomite forma tions and mixtures of hydrochloric and hydrofluoric Next, a non-acidic fluid reactant that will react with the acid are ordinarily used in acidizing sandstone forma reactant particles to form an acid capable of dissolving tions. 25 constituents of the formation is then injected into the Hydraulic fracturing is also commonly employed to fracture. increase the production of fluids from subterranean Although the acidizing compositions of U.S. Pat. formations. Hydraulic fracturing comprises the injec Nos. 3,707,191 and 3,157,232 are useful in stimulating tion of a suitable fracturing fluid down a well penetrat wells, certain problems have remained. For instance, ing a formation and into said formation under sufficient 30 when the reactants are mixed on the surface, the acid is pressure to overcome the pressure exerted by the in-situ immediately produced causing corrosion and a short stresses. This results in creating a crack or fracture in "reaction time'. When the reactants are injected sepa the formation to provide a passageway which facilitates rately into the fracture, there is often difficulty in mix flow of fluids through the formation and into the well. ing the two fluids in the fracture, particularly when one Combination fracture-acidizing processes are well 35 of the fluids is much more viscous than the other. known in the art. The present invention provides a composition and The rate at which acidizing fluids react with reactive method for acid treating a subterranean formation materials in a formation is a function of various factors which overcomes or at least mitigates the above de including acid concentration, temperature, velocity, the scribed problems. type of reactive material encountered, etc.. Whatever 40 It has been discovered that the addition of various the rate of reaction, the acidizing fluid can be intro chemical additives such as formic acid, or acetic acid or duced into the formation only a certain distance before the alkali metal or ammonium salt of formic acid or it becomes spent. The time required for the acidizing acetic acid, or hexamethylenetetramine (HMTA) will fluid to become spent is referred to herein as "reaction slow the rate of formation of a free acid when an alde time'. In creating new fractures in a formation, if the 45 hyde reacts with an ammonium salt and therefore re acidizing fluid is pumped further into the formation sults in delayed production of the acid which allows the after it has become spent, it may extend fractures in the acid to penetrate deeper into the formation and releases formation, but it may not increase the flow capacities of more of the desired products. the extended fractures which may close completely In accordance with the concept of the invention, when pressure is relieved. Thus, it is important to ex 50 there is provided a composition comprising a chemical tend the "reaction time' of acidizing fluids so that reac additive, an aldehyde, and an ammonium salt capable of tive materials are removed and/or new fractures are reacting with the aldehyde to produce a free acid. Op etched for as great a distance into the formation from tionally, a thickening or gelling agent can be added to the well bore as possible. the above described composition. Suitable chemical Another problem associated with acidizing subterra 55 additives that may be employed to carry out the inven nean formations is the corrosion by the acid of the tion are low molecular weight organic acids such as equipment which are used to carry out the treatment. acetic acid or formic acid, the alkali metal and the am The acid also presents a corrosive problem to the tanks monium salts of the low molecular weight organic used to transport the acid to and from the well bore and, acids, hexamethylenetetramine (HMTA) and mixtures on location, the acid has a tendency to attack to pumps, 60 thereof. The preferred composition is acetic acid. metal tubing leading to and within the casing along with Suitable aldehydes that may be used are formalde oil well tools and connecting lines. The expense of re hyde, acetaldehyde, propionaldehyde, glyoxal and pairing or replacing these corrosion damaged parts is butoxyaldehyde. Polymeric forms of formaldehyde extremely high. such as trioxine or paraformaldehyde can also be used Still another problem encountered in acidizing opera 65 as well as the monomer. Because of the effectiveness of tions is the high temperatures encountered in forma the reaction products in dissolving the subterranean tions with deep wells. Because of the quick "reaction formation and the cost and availability of the reactants, time' of the acids at high temperatures existing in the the preferred aldehyde is formaldehyde. 4,371,443 3 4. The ammonium salts that can be used to carry out the agent used will be in the range of from about 0.01 to present invention are generally any annonium salt that about 5 percent by weight. Small amounts of the gelling reacts with aldehyde to form a free acid. For example, agent will produce liquid mobile gels which can be ammonium chloride will yield hydrochloric acid, am readily pumped whereas large amounts of the gelling monium fluoride or ammonium bifluoride will yield agent will produce more viscous gels. There is, how hydrofluoric acid, ammonium citrate will yield citric ever, really no limit on the amount of gelling or thicken acid, ammonium formate will yield formic acid, ammo ing agent which can be used as long as the gelled acidic nium nitrate will yield nitric acid, ammonium sulfate composition can be pumped in accordance with the will yield sulfuric acid, ammonium carbonate will yield methods of the invention. carbonic acid, etc.. The list could be extended to in 10 Other materials may be included in the composition clude any acid where ammonium salt reacts with the of the invention. For instance, various treating agents aldehyde to form the free acid. such as acid corrosion inhibitors, mutual solvents, anti The present invention involves a known reaction sludging agents and friction reducing agents may be between an ammonium salt and an aldehyde. It has long added to the acid composition of the invention. been known that the reaction of formaldehyde and 15 When it is more desirable to contact the subterranean ammonium salt produces a free acid. (see Formalde formation with a foam containing the acidic composi hyde, Second Edition, J. Frederick Walker, 1953). The tion of the present invention, any foaming agent com reaction can be represented as follows: patible with the composition of the invention and capa ble of producing foam in an acidic environment may be 20 used. Many of these foaming agents are disclosed in U.S. Pat. No. 4,044,833. The method of the invention is carried out first by The different acids, hydrochloric acid and hydroflu adding the aldehyde to the chemical additive used to oric acid are the condensation products of the foregoing slow the formation of the free acid and then mixing the reactions and they vary depending solely on the substi 25 ammonium salt with the aldehyde chemical additive tution of different ammonium salts. mixture. The amount of chemical additive utilized in the Generally, the amount of free acid generated to treat invention can vary widely. Generally speaking, the formation and produced in the above described reaction can vary depending upon the type of formation amounts in the range of from about 1 to about 20 per being treated, and the results desired in the particular 30 cent by weight of the acid composition can be used but treating operation. The amount generated can vary amounts outside said range can also be used depending from less than 1 to greater than 60 weight percent based upon the "reaction time' desired in the acidic composi on the total weight of the acidic composition of the tion. Preferably, for economic reasons, the aldehyde invention. When the free acid generated is hydrochloric and ammonium salt are mixed together in stoichiomet acid, it is most preferred to generate an amount of hy 35 ric equivalent amounts. If a composition is desired with drochloric acid within the range of 1 to about 15 weight a viscosity greater than the above described mixture, percent based on the total weight of the acid composi the gelling or thickening agent is added to the composi tion. tion in amounts sufficient to achieve the viscosity de In certain acid treating operations carried out with an sired. acid composition, it is preferred that the viscosity of the 40 The acid compositions of the invention can be pre solution be kept at a minimum in order to facilitate the pared on the surface in a suitable tank equipped with penetration of acid into the pores of the treated forma suitable mixing means, e.g., a rotating, motor-driven, tion. In certain other applications such as fracturing of truck-mounted, cylindrical or oval shaped mixer, which the formation, a composition of higher viscosity may be is adapted for convenient removal of contents thereof as desired. In those instances, a gelling or thickening agent 45 by tipping, pumping, or application of gas pressure. The may be incorporated into the acid composition. Gener composition may be either transferred at a controlled ally, any conventional gelling or thickening agent may rate directly down the well bore or it may be removed be employed in accordance with the present invention. to a convenient storage tank for injection down the well Examples of suitable gelling or thickening agents are bore. galactomannans such as guar gum, gum karaya, gum 50 The composition is next introduced into the subterra tragacanth, gum ghatti, gum acacia, gun konjak, shariz, nean formation. The acid is introduced into the subter locus, psyllium, tamarind, gum tara, carrageenin, gum ranean formation whereby the calcareous rock in the kauri, modified guars such as hydroxypropyl guar, hy formation is dissolved thereby increasing the permeabil droxyethyl guar, carboxymethyl hydroxyethyl guar, ity, and permitting better flow of fluids through the carboxymethyl hydroxypropyl guar, cellulose ethers 55 formation. The pumping rate and pressures utilized will such as carboxyethyl cellulose, carboxymethyl cellulose obviously depend upon the characteristics of the forma (CMC), carboxymethyl hydroxyethyl cellulose tion and whether fracturing of the formation is desired. (CMHEC), hydroxyethyl cellulose, hydroxypropyl After the acid composition has been injected in this cellulose, methylhydroxypropyl cellulose, methyl cellu manner, the well may be shut in and allowed to stand lose, ethyl cellulose, propyl cellulose, and hydroxypro for a period of several hours or more depending on the pylmethyl cellulose, polyacrylamides, polyacrylates, composition and characteristics of the formation and oils which contain emulsifying agents such as diesel oil, the type of treatment being performed. If there is pres kerosine, or crude oil and polysaccharides. sure on the well, pressure is then released and the spent The amount of thickening or gelling agent used in the acid composition containing salts formed by the reac acidic composition of the invention can vary widely 65 tion of the acid composition, is permitted to flow back depending upon the viscosity, grade, and purity of the into the well bore and is pumped or flowed to the sur gelling agent and the properties desired in said composi face. The well may therefore be placed on production tion. In general, the amount of thickening or gelling or used for other purposes. 4,371,443 5 6 It is within the scope of the invention to proceed the TABLE I-continued injection of the acidic composition into the formation Reaction Rate Comparison with a preflush liquid. For instance, the injection of a Reaction cooling fluid, such as water, serves to cool the well Time Acid Spent Acid Live tubing and formation and extend the useful operating 5 Sample Composition Minutes % by Weight % by Weight temperature range of the acidic compositions. The vol CH3COONH4 ume of said cooling fluid so injected can be any suitable 8 NH4F 60 54.4 45.6 volume sufficient to significantly decrease the tempera HCHO ture of the formation being treated, and can vary de CH3COONH4 O 9 NH4F 20 54.8 45.2 pending upon the characteristics of the formation. For HCHO example, amounts up to 20,000 gallons, or more, can be CH3COONH4 used to obtain a temperature decrease in the order of O NH4F 30 40 60 100 to 250 F. HCHO The following example will serve to more compre CH3COONH4 5 11 NH3F 60 57.6 42.4 hensively illustrate the principals of the invention but is HCHO not intended to limit the bounds of the invention. CH3COONH4 12 NH4F 120 6.6 38.4 EXAMPLE I HCHO Laboratory tests were performed on silicon dioxide CH3COONH4 20 13 NH4F 30 52 48 using the acidic composition of the invention. The HCHO chemical additives used to slow the rate of formation of CH3COOH the acid were acetic acid and the ammonium salt of acetic acid. Based on the data of Table I, it can be clearly seen Samples 1, 2 and 3 were 25 cc. aqueous compositions that samples 4 through 12 containing ammonium ace containing 2.35g. of ammonium fluoride and 2.86 g. of 25 formaldehyde. tate and sample 13 containing acetic acid reacted slower Samples 4, 5 and 6 were 25 cc. aqueous compositions on silicon dioxide than samples 1 through 3 which do containing 2.35 g. of ammonium fluoride, 2.86 g of not contain acetic acid or ammonium acetate. EXAMPLE II formaldehyde and 1.23g of ammonium acetate. 30 Samples 7, 8 and 9 were 25 cc. aqueous compositions Laboratory tests were performed on silicon dioxide containing 2.35 g. of ammonium fluoride, 2.86 g. of using the acidic composition of the invention. The formaldehyde and 3.68 g. of ammonium acetate. chemical additive used to delay the rate of formation of Samples 10, 11 and 12 were 25 cc. aqueous composi the acid was hexamethylenetetramine (HMTA). tions containing 2.35 g. of ammonium fluoride, 2.86 g. 35 Sample 1 was a 25 cc. aqueous composition contain of formaldehyde and 4.90 g. of ammonium acetate. ing 2.35g. of ammonium fluoride and 2.86 g. of formal Sample 13 was a 25 cc. aqueous composition contain dehyde. ing 2.35 g. of ammonium fluoride, 2.86 g. of formalde Sample 2 was a 25 cc. aqueous composition contain hyde and acetic acid, 15% by weight of the aqueous ing 2.35 g. of ammonium fluoride, 2.86 g. of formalde composition. hyde and 1.83 g of hexamethylenetetramine (HMTA). After the components of each sample were prepared, Sample 3 was a 25 cc. aqueous composition contain 4 grams of 8 to 12 mesh silicon dioxide beads were ing 2.35 g. of ammonium fluoride, 2.86 g. of formalde placed in each sample. The samples were reacted with hyde and 3.66 g. of hexamethylenetetramine (HMTA). the silicon dioxide beads at 70 F. with constant shak Sample 4 was a 25 cc. aqueous composition contain ing. The samples were analyzed for silicon dioxide con 45 ing 2.35 g. of ammonium fluoride, 2.86 g. of formalde tent by atomic adsorption. The amount of spent acid hyde and 5.49 g, of hexamethylenetetramine (HMTA). was determined from the amount of silicon dioxide After the components of each sample were prepared, dissolved by the sample. The results of these tests are 4 grams of 8 to 12 mesh silicon dioxide beads were shown in Table I. placed in each sample. The samples were reacted with TABLE I 50 the silicon dioxide beads at 70 F. with constant shak Reaction Rate Comparison ing. The samples were analyzed for silicon dioxide con Reaction tent by atomic adsorption. The amount of spent acid Time Acid Spent Acid Live was determined from the amount of silicon dioxide Sample Composition Minutes 7% by Weight % by Weight dissolved by the sample. The results of these tests are l NH4F 30 66.4 33.6 55 shown in Table II. HCHO 2 NHF 60 70 30.0 TABLE I HCHO Reaction Rate Composition 3 NH4F 20 72 28.0 Compo- Reaction Time Acid Spent Acid Live HCHO Sample sition Minutes % by Weight % by Weight 4. NH4F 30 56 440 60 HCHO NH4F 30 66.4 33.6 CH3COONH4 HCHO 5 NH4F 60 62 38 2 NHF 30 63 37 HCHO HCHO CH3COONH4 HMTA 6 NH4F 120 63.2 36.8 65 3 NH4F 30 47.2 52.8 HCHO HCHO CH3COONH4 HMTA 7 NHF 30 46.8 53.2 4. NH4F 30 33.2 66.8 HCHO HCHO 4,371,443 7 8 the relative proportions of HMTA and ammonium ace TABLE II-continued tate present in the acidic composition. Reaction Rate Composition This invention is not limited to the above described Compo- Reaction Time Acid Spent Acid Live specific embodiments thereof; it must be understood, Sample sition Minutes % by Weight % by Weight 5 therefore, that the detail involved in the descriptions of HMTA the specific embodiments is presented for the purpose of illustration only, and that reasonable variations in modi Based on the data of Table II, it is clear that samples fications, which will be apparent to those skilled in the 2, 3 and 4, which contained various amounts of hexa art, can be made in this invention without departing methylenetetramine (HMTA), reacted slower than sam 10 from the spirit and the scope thereof. ple 1 which contained no HMTA and the degree of I claim: retardation increased with increasing amounts of 1. An acidic composition comprising: HMTA. (a) an aldehyde selected from the group consisting of formaldehyde, acetaldehyde, propionaldehyde, EXAMPLE III 15 glyoxal, butoxyaldehyde, trioxine, and paraformal Laboratory tests were performed on silicon dioxide dehyde; using the acidic composition of the invention. The (b) an ammonium salt selected from the group con chemical additive used to delay the rate of formation of sisting of ammonium fluoride, ammonium bifluo the acid was various mixtures of hexamethylenetetra ride, ammonium chloride, ammonium sulfate and mine (HMTA) and ammonium acetate. 20 ammonium nitrate present in an amount sufficient Sample 1 was a 25 cc. aqueous composition contain to react with said aldehyde to form an acid; and ing 2.35 g. of ammonium fluoride and 2.86 g. of formal (c) a chemical additive selected from the group con dehyde. sisting of acetic acid, an alkali metal salt of said Sample 2 was a 25 cc. aqueous composition contain acetic acid, an ammonium salt of said acetic acid, ing 2.35 g. of ammonium fluoride, 2.86 g. of formalde 25 formic acid, an alkali metal salt of formic acid, an hyde, 1.83 g. of hexamethylenetetramine (HMTA) and ammonium salt of said formic acid, mixtures 2.46 g. of ammonium acetate. thereof and mixtures with hexamethylenetetra Sample 3 was a 25 cc. aqueous composition contain mine, said additive being and present in an amount ing 2.35 g. of ammonium fluoride, 2.86 g. of formalde sufficient to retard said reaction of said aldehyde to hyde, 3.66 g. of hexamethylenetetramine (HMTA) and 30 form said acid. 1.23g. of ammonium acetate. 2. The acidic composition recited in claim 1 further Sample 4 was a 25 cc. aqueous composition contain comprising a gelling agent. ing 2.35 g. of ammonium fluoride, 2.86 g. of formalde 3. The acidic composition recited in claim 2 wherein hyde, 3.66 g. of hexamethylenetetramine (HMTA) and said gelling agent is selected from the group consisting 35 of polyacrylamides, oils, polysaccharides, polyacryl 2.46 g. of ammonium acetate. ates, gum karaya, carboxyethyl cellulose, carboxy After the components of each sample were prepared, methyl hydroxyethyl cellulose, hydroxyethyl cellulose, 4 grams of 8 to 12 mesh silicon dioxide beads were hydroxypropyl cellulose, methylhydroxypropyl cellu placed in each sample. The samples were reacted with lose, methyl cellulose, ethyl cellulose, propyl cellulose the silicon dioxide beads at 70 F. with constant shak and hydroxypropylmethyl cellulose. ing. The samples were analyzed for silicon dioxide con 4. The acidic composition recited in claim 2 wherein tent by atomic adsorption. The amount of spent acid said gelling agent is selected from the group consisting was determined from the amount of silicon dioxide of gum tragacanth, gum ghatti, gum acacia, gum kon dissolved by the sample. The results of these tests are jak, shariz, locus, psyllium, tamarind, gum tara, carra shown in Table III. 45 geenin, gum kauri, hydroxypropyl guar, hydroxyethyl TABLE III guar, carboxymethyl hydroxypropyl guar and carboxy Reaction Rate Comparison methyl hydroxyethyl guar. Reaction 5. The acid composition recited in claims 1 or 2 Sam- Time Acid Spent Acid Live wherein said ammonium salt capable of reacting with ple Composition Minutes % by Weight % by Weight 50 said aldehyde to form said acid is selected from the 1 NH4F 30 66.4 33.6 group consisting of ammonium chloride and ammonium HCHO fluoride. 2 NH4F 30 45.6 544 HCHO 6. The acidizing composition recited in claims 3 or 4 HMTA further comprising a friction reducing agent. CH3COONH4 55 7. The acidic composition recited in claims 1 or 2 3 NH4F 30 49.6 50.4 HCHO wherein said acidic composition is foamed. HMTA 8. A method of acid treating a subterranean formation CH3COONH4 comprising: 4 NH4F 30 4.6 58.6 (a) contacting said formation with an acidic composi HCHO 60 tion comprising: HMTA an aldehyde selected from the group consisting of CH3COONH4 formaldehyde, acetaldehyde, propionaldehyde, glyoxal, butoxyaldehyde, trioxine, and para Based on the data of Table III, it is clear that samples formaldehyde; 2, 3 and 4, which contained hexamethylenetetramine an ammonium salt selected from the group consist (HMTA) and ammonium acetate reacted slower than ing of ammonium fluoride, ammonium bifluo sample 1 which contained neither chemical additive and ride, ammonium chloride, ammonium sulfate and the degree of retardation can be varied, depending upon ammonium nitrate present in an amount suffi 4,371,443 10 cient to react with said aldehyde to form a free pyl cellulose, methylhydroxypropyl cellulose, methyl acid; and cellulose, ethyl cellulose, propyl cellulose and hydroxy a chemical additive selected from the group con propylmethyl cellulose. sisting of acetic acid, an alkali metal salt of acetic 11. The method recited in claim 9 wherein said gel acid, an ammonium salt of acetic acid, formic ling agent is selected from the group consisting of gum acid, an alkali metal salt of formic acid, an am tragacanth, gum ghatti, gum acacia, gum konjak, shariz, monium salt of formic acid, mixtures thereof, locus, psyllium, tamarind, gum tara, carrageenin, gum and mixtures with hexamethylenetetramine, said kauri, hydroxypropyl guar, hydroxyethyl guar, car additive being present in an amount sufficient to boxymethyl hydroxypropyl guar and carboxymethyl retard reaction of said aldehyde to form said free O hydroxyethyl guar. acid; and 12. The method recited in claims 8 or 9 wherein said (b) withdrawing said composition from said forma ammonium salt capable of reacting with said aldehyde tion. to form said free acid is selected from the group consist 9. The method recited in claim 8 wherein said acid ing of ammonium fluoride and ammonium chloride. composition further comprises a gelling agent. 15 13. The method recited in claims 8 or 9 further com 10. The method recited in claim 9 wherein said gel prising incorporating a friction reducing agent into said ling agent is selected from the group consisting of poly acidic composition. acrylamides, polysaccharides, oils, polyacrylates, gum 14. The method recited in claims 8 and 9 wherein said karaya, carboxyethyl cellulose, carboxymethyl hydrox acidic composition is foamed. yethyl cellulose, hydroxyethyl cellulose, hydroxypro 20 k k k . .

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65 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. : 4,37l, 443 DATED ; February l, 1983 INVENTOR(S) : Bill R. Keeney It is Certified that error appears in the above-identified patent and that said letters Patent are hereby COrrected as shown below: In Column 6, Table I, Sample ll should read: Sample COmpOSition Reaction Time Acid Spent Acid Live (Minutes) % by Weight 8 by Weight ll NHFCAo 60 57.6 42.4 CHCOONH4. signed and Sealed this Twelfth Day of April 1983 SEAL Attest:

GERALD MOSSING OFF Attesting Officer Commissioner of Patents and Trademarks

------| UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. : : 4,37l, 443 DATED February l, l983 NVENTOR(S) : Bill R. Keeney It is Certified that error appears in the above-identified patent and that said Letters Patent are hereby Corrected as shown below: In Column 6, Table I, Sample ll should read: Sample CompOSition Reaction Time Acid Spent Acid Live (Minutes) % by Weight 3 by Weight ll NH.FHo?o 60 57.6 42. 4 CHCOONH4 eigned and Sealed this Twelfth Day of April 1983 SEAL Attest

GERALD J MOSSINGHOFF Attesting Officer Commissioner of Patents and Trademarks