CA1220930A - Methods for acidizing subterranean formations and gelled acid compositions - Google Patents
Methods for acidizing subterranean formations and gelled acid compositionsInfo
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- CA1220930A CA1220930A CA000461843A CA461843A CA1220930A CA 1220930 A CA1220930 A CA 1220930A CA 000461843 A CA000461843 A CA 000461843A CA 461843 A CA461843 A CA 461843A CA 1220930 A CA1220930 A CA 1220930A
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Abstract
ABSTRACT OF THE DISCLOSURE
Gelable and gelled acid compositions comprise an aqueous acid, a water soluble polymer comprising, for example, acrylamide, an N-sulfoalkyl ethylenically unsaturated amide and a polyvinyl crosslinking monomer, and a reducing agent such as erythorbic acid. The gelled acids are useful in acidizing subterranean formations and do not yield insoluble residues when used in the presence of ferric ions.
Gelable and gelled acid compositions comprise an aqueous acid, a water soluble polymer comprising, for example, acrylamide, an N-sulfoalkyl ethylenically unsaturated amide and a polyvinyl crosslinking monomer, and a reducing agent such as erythorbic acid. The gelled acids are useful in acidizing subterranean formations and do not yield insoluble residues when used in the presence of ferric ions.
Description
~L~Z~
METEIODS FOR ACIDIZING SUBTERRANEAN
FORMATIONS AND GELLED ACID COMYOSITIONS
This invention pertains to improved composi-tions and method for acid treating or acidizing of subterranean formations in order to stimulate the production of formation fluids.
Acid treating or acidizing of porous subter-ranean formations penetrat~d by a well bore has been widely employed for increasing the production of fluids such as, crude oil, natural gas, etc., from said forma-tions. The usual techni~ue of acidizing a formation comprises introducing a nonoxidizing acid into the well under sufficient pressura to force the acid out into the formation where it xeacts with the acid-soluble components of the formation. The technique is applied to formations of high acid solubility, such as lime-stone, dolomite, etc., and is also applicable to othertypes of formations containing streaks or striations of acid-soluble componen-ts such as sandstone containing carbonate striations.
During the acid treatments, passageways in the formation for fluid ~low are created or enlarged, thus stimulating the production of fluids from the 32,152~F -1- ~r, lL~2(~3Q
~2--formation. The ac-tion of the acid on the formation is o~-ten called etching. There are essentially two well--knowrl types of acid treatments used in the field, and these include matrix acidizing and fracturing acidizing.
S Both types of treatments utilize acid compositions as pumpable fluids. In matxix acidizing operations, the acid is injected into the formation at a pressure ox rake sufficient to force the fluid into the formation but insufficient to hydraulically fracture the formation.
In fracture acidizing operations, the acid composition, which is usually in the form of a viscous gel, is used as a fracturi.ng fluid which is pumped through the well bore into the formation at a sufficient rate and pressure to overcome the overburden pressure and thus fracture the formation.
One of the problems commonly encountered in acidizing operations ls insufficient penetration of the formation by the acid. It is desirable that good penetration is obtained in order to realize maximum benefits from the operation. Too often the acid is essentially completely spent in the area immediately adjacent and surrounding the well bore. Th~ severity of the problem increases with well depth and with increasing temperatures which enhance the reaction of the acid with the formation.
Poor penetration can also be caused, and/or aggravated, by fluid loss to the more porous zones of the formation or "thief zones." Poor penetration can also be caused and/or aggravated by leak-off at the fracture faces in combination fracturing-acidizing operations. Fluid loss or leak-off can frequently worsen the situation by leaving the tight ~low perme-ability) zones of the formation unchanged and merely 32,152~F -2-~Z20~
_3_ openiny up the already high permeability zones. One solution w~lich has been proposed ~or the above-discussed problem is to incorporate various thickening or gelling agents into -the acid solutions. Such agents thicken the acid solution and increase the viscosity thereof.
- It has been disclosed that polymer-thickened acid solutions have improved fluid loss properties. For example, see U.S. Patent Nos. 3,415,319 and 3,434,971.
It has also been reported that the reaction rate of said polymer~thickened acid solutions with the acid-soluble portion of the formation is lessened or retarded.
See, for example, U.S. Patent Nos. 3,749,169; 3,236,30S;
3,252,904; 4,055,502; 4,103,742 and 4,191,657. It has been disclosed that foams, including foamed acids, have improved fluid loss properties. For example, U.S.
Patent Nos. 3,937,283-and 4,235,734 disclose foamed acid systems which can be used for foam fracturing.
However, such foams have stability problems.
Higher viscosities are also advantageous in combination fracturing-acidizing operations in that the more viscous acidic solutions produce wider and longer fractures. More viscous acid solutions are also more effective in carrying propping agents into the formation when propping agents are used.
Another problem encountered in acidizing operations, particularly when empl~yins acidizing compositions having thickening or viscosifying agents incorporated therein, is stability to heat. By "stability to heat" is meant the retention of the increased or greater viscosity properties under the conditions of use. To be satisfactory, such compo-sitions should be sufficiently stable to resist thermal 32,lS2-F -3-~Z~Og3~
clegenerat.ion in the presence of acid ~or a period of time su~:Eicient to accomplish the intended purpose, e.y., good penetration and significant etching of the formation. The degree of stability required in any particular operation will vary depending upon the type of formation being -treated, the temperature of the formation, the well depth, acid concentration, pump rates, shear rates, etc. For exclmple, acidizing o~ a kight low permeabi.lity formation will proceed more slowly (i.e., at lower pump ra-tes and lower pressures) than with a more permeable formation, other factors being the same, because a longer time will be required to obtain a significant amount of etching.
The temperature of the formation usually has lS a pronounced effect on the stability of the acidizing compositions and, generally speaking, is one of the most important operating variables when considering stability. Increased formation temperatures usually have at least two undesirable efects. One such effect is deganeration of the composition, which results in a decrease in viscosity. Another such effect is increased rate of reaction of the acid wi-th the formatlon. Thus, some compositions which would be satisfactory in a low temperature formation such as in the ~ugoton field in the Anadarko basin might not be satisfactory in forma-tions encountered in deeper wells as in some West Texas fields. Another problem which is sometimes encountered when using thickened compositions in treating formations involves flow-back or removal of the treating composi-tion after the operation is completed.
In ordinary acidizing operations using unthick-ened acids there is usually no problem in removing the 32,152-F -4-~Z;~:)93~
spent acid because it is essentially wat~r. However, some residues from the sperlt thickened or viscous acid solutions are difficult to remove from the pores of the formation of the fracture after the operation is complete.
Sometimes the polymeric gelling agent precipitates from the spent acid and forms a clogging residue in the pores of the formation or in the fracture. This can inhibit the production of fluids from the formation and can require costly cleanup operations.
During such acid treatments, the treatment acid often dissolves iron scale in pipes and iron-containing minerals in the formation. The dissolved iron normally remains in solution until the acid is spent. Upon spending, ferric hydroxide begins to precipitate and plug the formation. Complete precipi-tation of ferric hydroxide is reached at a pH of about 3.2. Ferrous hydroxide, being more soluble, does not begin to precipitate un-til a pH of approximately 7.7 and is not generally a problem.
The deleterious effects of ferric hydroxide in wells was recognized by Grebe in U.S. Patent No. 2,175,081 as early as 1937 where a strong acid containing sulfurous acid is disclosed as a means of countering the precipitation pro~lem. Numerous other procedures have been proposed for avoiding the ferric hydroxide problem. For example, u.S. Patent No.
METEIODS FOR ACIDIZING SUBTERRANEAN
FORMATIONS AND GELLED ACID COMYOSITIONS
This invention pertains to improved composi-tions and method for acid treating or acidizing of subterranean formations in order to stimulate the production of formation fluids.
Acid treating or acidizing of porous subter-ranean formations penetrat~d by a well bore has been widely employed for increasing the production of fluids such as, crude oil, natural gas, etc., from said forma-tions. The usual techni~ue of acidizing a formation comprises introducing a nonoxidizing acid into the well under sufficient pressura to force the acid out into the formation where it xeacts with the acid-soluble components of the formation. The technique is applied to formations of high acid solubility, such as lime-stone, dolomite, etc., and is also applicable to othertypes of formations containing streaks or striations of acid-soluble componen-ts such as sandstone containing carbonate striations.
During the acid treatments, passageways in the formation for fluid ~low are created or enlarged, thus stimulating the production of fluids from the 32,152~F -1- ~r, lL~2(~3Q
~2--formation. The ac-tion of the acid on the formation is o~-ten called etching. There are essentially two well--knowrl types of acid treatments used in the field, and these include matrix acidizing and fracturing acidizing.
S Both types of treatments utilize acid compositions as pumpable fluids. In matxix acidizing operations, the acid is injected into the formation at a pressure ox rake sufficient to force the fluid into the formation but insufficient to hydraulically fracture the formation.
In fracture acidizing operations, the acid composition, which is usually in the form of a viscous gel, is used as a fracturi.ng fluid which is pumped through the well bore into the formation at a sufficient rate and pressure to overcome the overburden pressure and thus fracture the formation.
One of the problems commonly encountered in acidizing operations ls insufficient penetration of the formation by the acid. It is desirable that good penetration is obtained in order to realize maximum benefits from the operation. Too often the acid is essentially completely spent in the area immediately adjacent and surrounding the well bore. Th~ severity of the problem increases with well depth and with increasing temperatures which enhance the reaction of the acid with the formation.
Poor penetration can also be caused, and/or aggravated, by fluid loss to the more porous zones of the formation or "thief zones." Poor penetration can also be caused and/or aggravated by leak-off at the fracture faces in combination fracturing-acidizing operations. Fluid loss or leak-off can frequently worsen the situation by leaving the tight ~low perme-ability) zones of the formation unchanged and merely 32,152~F -2-~Z20~
_3_ openiny up the already high permeability zones. One solution w~lich has been proposed ~or the above-discussed problem is to incorporate various thickening or gelling agents into -the acid solutions. Such agents thicken the acid solution and increase the viscosity thereof.
- It has been disclosed that polymer-thickened acid solutions have improved fluid loss properties. For example, see U.S. Patent Nos. 3,415,319 and 3,434,971.
It has also been reported that the reaction rate of said polymer~thickened acid solutions with the acid-soluble portion of the formation is lessened or retarded.
See, for example, U.S. Patent Nos. 3,749,169; 3,236,30S;
3,252,904; 4,055,502; 4,103,742 and 4,191,657. It has been disclosed that foams, including foamed acids, have improved fluid loss properties. For example, U.S.
Patent Nos. 3,937,283-and 4,235,734 disclose foamed acid systems which can be used for foam fracturing.
However, such foams have stability problems.
Higher viscosities are also advantageous in combination fracturing-acidizing operations in that the more viscous acidic solutions produce wider and longer fractures. More viscous acid solutions are also more effective in carrying propping agents into the formation when propping agents are used.
Another problem encountered in acidizing operations, particularly when empl~yins acidizing compositions having thickening or viscosifying agents incorporated therein, is stability to heat. By "stability to heat" is meant the retention of the increased or greater viscosity properties under the conditions of use. To be satisfactory, such compo-sitions should be sufficiently stable to resist thermal 32,lS2-F -3-~Z~Og3~
clegenerat.ion in the presence of acid ~or a period of time su~:Eicient to accomplish the intended purpose, e.y., good penetration and significant etching of the formation. The degree of stability required in any particular operation will vary depending upon the type of formation being -treated, the temperature of the formation, the well depth, acid concentration, pump rates, shear rates, etc. For exclmple, acidizing o~ a kight low permeabi.lity formation will proceed more slowly (i.e., at lower pump ra-tes and lower pressures) than with a more permeable formation, other factors being the same, because a longer time will be required to obtain a significant amount of etching.
The temperature of the formation usually has lS a pronounced effect on the stability of the acidizing compositions and, generally speaking, is one of the most important operating variables when considering stability. Increased formation temperatures usually have at least two undesirable efects. One such effect is deganeration of the composition, which results in a decrease in viscosity. Another such effect is increased rate of reaction of the acid wi-th the formatlon. Thus, some compositions which would be satisfactory in a low temperature formation such as in the ~ugoton field in the Anadarko basin might not be satisfactory in forma-tions encountered in deeper wells as in some West Texas fields. Another problem which is sometimes encountered when using thickened compositions in treating formations involves flow-back or removal of the treating composi-tion after the operation is completed.
In ordinary acidizing operations using unthick-ened acids there is usually no problem in removing the 32,152-F -4-~Z;~:)93~
spent acid because it is essentially wat~r. However, some residues from the sperlt thickened or viscous acid solutions are difficult to remove from the pores of the formation of the fracture after the operation is complete.
Sometimes the polymeric gelling agent precipitates from the spent acid and forms a clogging residue in the pores of the formation or in the fracture. This can inhibit the production of fluids from the formation and can require costly cleanup operations.
During such acid treatments, the treatment acid often dissolves iron scale in pipes and iron-containing minerals in the formation. The dissolved iron normally remains in solution until the acid is spent. Upon spending, ferric hydroxide begins to precipitate and plug the formation. Complete precipi-tation of ferric hydroxide is reached at a pH of about 3.2. Ferrous hydroxide, being more soluble, does not begin to precipitate un-til a pH of approximately 7.7 and is not generally a problem.
The deleterious effects of ferric hydroxide in wells was recognized by Grebe in U.S. Patent No. 2,175,081 as early as 1937 where a strong acid containing sulfurous acid is disclosed as a means of countering the precipitation pro~lem. Numerous other procedures have been proposed for avoiding the ferric hydroxide problem. For example, u.S. Patent No.
2,175,095 suggests including withi~ the acidizing fluid a material such as lactic acid, ammonium acetate, glycine, glycolic acid, citric acid, or the like, which is capable of preventing the precipitation of iron or aluminum hydrates at normal precipitation pH values.
U.S. Patent No. 2,335,689 suggests adding an iron 32,152-F - -5-
U.S. Patent No. 2,335,689 suggests adding an iron 32,152-F - -5-
3~
sequestering agent, such as a polyhydric phenol, wi-thin the injec-ted acids. U.S. Patent No. 3,142,335 suggests the use of a seques-teri~lg agent containing a mixture of ingredients that function as a pH buffer, such as citric acid or a citrate salt mixed with acetic or formic acids or thelx salts. U.S. Patent No. 3,150,081 suggests using mixtures of hydroxyacetic and citric acids, which mixtures are alleged to be cheapar and more efective than the use of either acid alone.
The most common se~uestering agents in commercial practice are citric acid, ethylenediamine-tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), acetic acid, lactic acid, and citric acid/acetic acid mixtures. Data on these sequestering agents is found in the paper by Smith et al., Paper No. SPE 2358, Society of Petroleum Engineers of AIME (presented November 7-8, 1968).
The problem with most iron sequestering agents is that they are not particularly effe~tive at temperatures beyond about 125 to 150F (51.7 to 65.6C). Only NTA, citric acid and EDTA hava shown any effectiveness at higher temperatures. Of these, EDTA
is the only practical sequesteriny agent because citric acid tends to form insoluble citrates during the course of the well treatmentO Such citrates can block the well pro~uction almost as effectively as the ferric hydroxide.
The presence of ferric ions in the treatment acid solution is known to cause other serious problems 30 as well. For example, U.S. Patent No. 4,096,914 teaches that ferric ion reacts with asphaltenic oil to form 32,152-~ l6-1220~30 insoluble iron asphaltene compounds. These compounds are insoluble pxecipitates which likewis~ plug the formation channels and inhibit production of the des.ired fluid. The pa-tent teaches that the problem can be solved by incorpoxating salicylic acid into the treatment acid.
Ferric ion corrosion can also be a problem.
Two moles o~ ferric ion reacts with base metal to form three moles of ferrous ion. Almost any oxidizing source readily converts the ferrous ions to fexric ions, and a vicious circle results. Additives used to control problems associated with ferric lons in treat-ment of wells have been called "iron stabilizexs" or "iron control agents" by practitioners in the field.
Another problem associated with the presence of ferric ions in a polymer thickened acid composition involves the precipitation of the polymer. That is, a precipitate forms which is believed to contain iron in a chemically combined form (i.e., such a metal ion crosslinker) with the polymer. The precipitate is usually in the form of a gummy, insoluble mass that is very difficult, if not impossible, to resolubilize under the conditions of use for acid treatment fluids.
In view of the deficiencies of the prior art, it would be highly desirable to provide a gelable acid composition for use in acidic well treatments and the method of using the sama, which composition is viscous, stable and generally free of insoluble precipitates during use 32,152-F ~7-ln one aspect, the present invéntion is a gellableacid composition which compri.ses (A) an aqueous acid thickened with a functlonally effective amount of a water-soluble polymer comprising (1) from greater than 0 to 50 weight percent of a nonionic water-soluble ethylenically unsatu-rated monomer, (2) from 50 to less than lO0 weight percent of a water-soluble ethylenically unsaturated monomer con-taining an anionic moiety and (3) from 1 to 1000 ppm of a polyvinyl crosslinking monomer, wherein said polymer is crosslinked in amounts sufficient to provide increased viscosity to the composition during acid treatment of porous subterranean formations, which amount of polymer is sufficient to cause gelation and to form a gelled acid composition and sufficiently stable to degradation by heat of porous subterranean formationsi and (B) at least one reducing agent in an amount sufficient to prevent or sub-stantially prevent the foramtion of an insoluble residue as the gelled acid reacts with the acid-soluble components of said formation in the presence of dissolved ferric ion.
In another aspect, the present invention is a method for acid treating a porous subterranean formation susceptible to attack by an acid which acid is penetrated by a wellbore, which method comprises injecting into said formation via said wellbore a gellable or gelled acidic composition comprising (A) an aqueous acid i.n an amount capable of, and sufficient for, reacting with a significant amo~mt of the acid-soluble components of said formation;
and a water-thickening amount of a water-soluble polymer comprising (1) from greater than 0 to 50 weight percent of a nonionic water-soluble ethylenically unsaturated monomer, (2) from 50 to less than 100 weight percent of a water-soluble ethylenically unsaturated monomer containing an anionic moiety and (3) from 1 to 1000 ppm of a polyvinyl crosslinking monomer; and (B) at least one reduciilg agent 32,152-F -8-~.
"`` ~L22~930 g in an amount sufEicien~ to prevent or substantlally prevent the ~ormation of an insoluble residue as the gelled acid reacts with the acid-soluble components of said formation in t`he presence of dissolved ferric ion, said polymer and said acid in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit said gelation and thus form said composition having sufficient stability to degradation by the heat oE said formation to permit good penetration o said composition into said formation; and maintaining said composition in said formation in contact therewith for a period of time sufficient usually for the acid in said composition to react significantly with the acid~soluble components of said formation and stimulate the production of fluids therefrom.
The process of this invention yields a means of providing increased production of oil or gas. The acid composition can be used in fracture acidizing. The polymeric thickener provides an improved means for reducing the rate at which fluid is lost into the subterranean formation. The acid is also used for matri~ acidizing where the polymeric thickener provides a slow rate of reaction of acid within the subterranean formation to increase penetration of said acid into the formation. In either case, the reducing agent prevents problems associated with insoluble residues caused by ferric ions.
Ethylenically unsaturated, water-soluble monomers suitable for use in this invention are those which are sufficiently water-soluble when dissolved in 32,152-F -9-1~0~3~
water and wh.ich :reaclily undergo addit.ional polymeriza-tion to .~orm polymers which are at. least inheren-t:ly water-dispexsible and preferably water-soluble. By "inherently water-dispersible" is meant thak -the polymer, when contacted with an a~ueous medium, will disperse therein without the aid of surfactan-t to ~orm a colloid dispersion o polymer in the aqueous medium.
Exemplary nonionic monomers suitably employed in the practice of this invention are those ethylenically unsaturated monomers that are sufficiently water-soluble to form at least a 5 weight percent solution when disæolved in water and readily undergo addition polymeri-zatioIl to form polymers that are water-soluble. Examples of such nonionic monomers include ethylenically unsatu-rated amides such as acrylamide, methacrylamide andfumaramide; their water soluble N-substituted nonionic derivatives such as the N-methylol derivatives of acrylamide and methacrylamide as well as the N-methyl and N,N-dimethyl derivatives of acrylamide and methacryl-amide; hydroxyalkyl esters of unsaturated carboxylicacids such as hydroxyethyl acrylate and hydro~ypropyl acrylate; and the like. Of the foregoing nonionic monomers, the ethylenically unsaturated amides are pre~erred, with acrylamide being especially prPferred.
Examples of suitable water-soluble, ethylen-ical~y ur.saturated monomexs containing anionic moieties include the sulfonate-containing monomers including, for example, N-sulfoalkyl, a,~--ethylenically unsaturated amides such as 2~acrylamido-2-methylpropane sulfonic acid, 2-acrylamido propane sulfonic acid, 2-acrylamido ethane sul'fonic acid and the alkali metal salts thereo such as the sodium and potassium salts thereof as well 32,152-F . -10-.
~Z;2~)93~) a~ other such monome.rs listed iII U.S. Paten-t No.
3,692,673; sulfoalkyl esters of unsaturated carboxylic acids such as 2-sulfoethyl methacrylate and other such sulfoalkyl esters as listed in U.S. Patent No.
sequestering agent, such as a polyhydric phenol, wi-thin the injec-ted acids. U.S. Patent No. 3,142,335 suggests the use of a seques-teri~lg agent containing a mixture of ingredients that function as a pH buffer, such as citric acid or a citrate salt mixed with acetic or formic acids or thelx salts. U.S. Patent No. 3,150,081 suggests using mixtures of hydroxyacetic and citric acids, which mixtures are alleged to be cheapar and more efective than the use of either acid alone.
The most common se~uestering agents in commercial practice are citric acid, ethylenediamine-tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), acetic acid, lactic acid, and citric acid/acetic acid mixtures. Data on these sequestering agents is found in the paper by Smith et al., Paper No. SPE 2358, Society of Petroleum Engineers of AIME (presented November 7-8, 1968).
The problem with most iron sequestering agents is that they are not particularly effe~tive at temperatures beyond about 125 to 150F (51.7 to 65.6C). Only NTA, citric acid and EDTA hava shown any effectiveness at higher temperatures. Of these, EDTA
is the only practical sequesteriny agent because citric acid tends to form insoluble citrates during the course of the well treatmentO Such citrates can block the well pro~uction almost as effectively as the ferric hydroxide.
The presence of ferric ions in the treatment acid solution is known to cause other serious problems 30 as well. For example, U.S. Patent No. 4,096,914 teaches that ferric ion reacts with asphaltenic oil to form 32,152-~ l6-1220~30 insoluble iron asphaltene compounds. These compounds are insoluble pxecipitates which likewis~ plug the formation channels and inhibit production of the des.ired fluid. The pa-tent teaches that the problem can be solved by incorpoxating salicylic acid into the treatment acid.
Ferric ion corrosion can also be a problem.
Two moles o~ ferric ion reacts with base metal to form three moles of ferrous ion. Almost any oxidizing source readily converts the ferrous ions to fexric ions, and a vicious circle results. Additives used to control problems associated with ferric lons in treat-ment of wells have been called "iron stabilizexs" or "iron control agents" by practitioners in the field.
Another problem associated with the presence of ferric ions in a polymer thickened acid composition involves the precipitation of the polymer. That is, a precipitate forms which is believed to contain iron in a chemically combined form (i.e., such a metal ion crosslinker) with the polymer. The precipitate is usually in the form of a gummy, insoluble mass that is very difficult, if not impossible, to resolubilize under the conditions of use for acid treatment fluids.
In view of the deficiencies of the prior art, it would be highly desirable to provide a gelable acid composition for use in acidic well treatments and the method of using the sama, which composition is viscous, stable and generally free of insoluble precipitates during use 32,152-F ~7-ln one aspect, the present invéntion is a gellableacid composition which compri.ses (A) an aqueous acid thickened with a functlonally effective amount of a water-soluble polymer comprising (1) from greater than 0 to 50 weight percent of a nonionic water-soluble ethylenically unsatu-rated monomer, (2) from 50 to less than lO0 weight percent of a water-soluble ethylenically unsaturated monomer con-taining an anionic moiety and (3) from 1 to 1000 ppm of a polyvinyl crosslinking monomer, wherein said polymer is crosslinked in amounts sufficient to provide increased viscosity to the composition during acid treatment of porous subterranean formations, which amount of polymer is sufficient to cause gelation and to form a gelled acid composition and sufficiently stable to degradation by heat of porous subterranean formationsi and (B) at least one reducing agent in an amount sufficient to prevent or sub-stantially prevent the foramtion of an insoluble residue as the gelled acid reacts with the acid-soluble components of said formation in the presence of dissolved ferric ion.
In another aspect, the present invention is a method for acid treating a porous subterranean formation susceptible to attack by an acid which acid is penetrated by a wellbore, which method comprises injecting into said formation via said wellbore a gellable or gelled acidic composition comprising (A) an aqueous acid i.n an amount capable of, and sufficient for, reacting with a significant amo~mt of the acid-soluble components of said formation;
and a water-thickening amount of a water-soluble polymer comprising (1) from greater than 0 to 50 weight percent of a nonionic water-soluble ethylenically unsaturated monomer, (2) from 50 to less than 100 weight percent of a water-soluble ethylenically unsaturated monomer containing an anionic moiety and (3) from 1 to 1000 ppm of a polyvinyl crosslinking monomer; and (B) at least one reduciilg agent 32,152-F -8-~.
"`` ~L22~930 g in an amount sufEicien~ to prevent or substantlally prevent the ~ormation of an insoluble residue as the gelled acid reacts with the acid-soluble components of said formation in t`he presence of dissolved ferric ion, said polymer and said acid in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit said gelation and thus form said composition having sufficient stability to degradation by the heat oE said formation to permit good penetration o said composition into said formation; and maintaining said composition in said formation in contact therewith for a period of time sufficient usually for the acid in said composition to react significantly with the acid~soluble components of said formation and stimulate the production of fluids therefrom.
The process of this invention yields a means of providing increased production of oil or gas. The acid composition can be used in fracture acidizing. The polymeric thickener provides an improved means for reducing the rate at which fluid is lost into the subterranean formation. The acid is also used for matri~ acidizing where the polymeric thickener provides a slow rate of reaction of acid within the subterranean formation to increase penetration of said acid into the formation. In either case, the reducing agent prevents problems associated with insoluble residues caused by ferric ions.
Ethylenically unsaturated, water-soluble monomers suitable for use in this invention are those which are sufficiently water-soluble when dissolved in 32,152-F -9-1~0~3~
water and wh.ich :reaclily undergo addit.ional polymeriza-tion to .~orm polymers which are at. least inheren-t:ly water-dispexsible and preferably water-soluble. By "inherently water-dispersible" is meant thak -the polymer, when contacted with an a~ueous medium, will disperse therein without the aid of surfactan-t to ~orm a colloid dispersion o polymer in the aqueous medium.
Exemplary nonionic monomers suitably employed in the practice of this invention are those ethylenically unsaturated monomers that are sufficiently water-soluble to form at least a 5 weight percent solution when disæolved in water and readily undergo addition polymeri-zatioIl to form polymers that are water-soluble. Examples of such nonionic monomers include ethylenically unsatu-rated amides such as acrylamide, methacrylamide andfumaramide; their water soluble N-substituted nonionic derivatives such as the N-methylol derivatives of acrylamide and methacrylamide as well as the N-methyl and N,N-dimethyl derivatives of acrylamide and methacryl-amide; hydroxyalkyl esters of unsaturated carboxylicacids such as hydroxyethyl acrylate and hydro~ypropyl acrylate; and the like. Of the foregoing nonionic monomers, the ethylenically unsaturated amides are pre~erred, with acrylamide being especially prPferred.
Examples of suitable water-soluble, ethylen-ical~y ur.saturated monomexs containing anionic moieties include the sulfonate-containing monomers including, for example, N-sulfoalkyl, a,~--ethylenically unsaturated amides such as 2~acrylamido-2-methylpropane sulfonic acid, 2-acrylamido propane sulfonic acid, 2-acrylamido ethane sul'fonic acid and the alkali metal salts thereo such as the sodium and potassium salts thereof as well 32,152-F . -10-.
~Z;2~)93~) a~ other such monome.rs listed iII U.S. Paten-t No.
3,692,673; sulfoalkyl esters of unsaturated carboxylic acids such as 2-sulfoethyl methacrylate and other such sulfoalkyl esters as listed in U.S. Patent No.
4,075,134, as well as the alkali.metal salts thereof;
sulfoarylalkenes such as vinylbenzyl sulfonic acid and the various salts o vinylbenzyl sulfonate, p-styrene sulfonic ac.id and the salts thereof; sulfoalkenes such as vinyl sulfonic acid and salts thereo~; and the like.
Of the foregoing sulfonate monomers, the sulfoalkyl derivatives of acrylamide and methacrylamide are preferred with those of acrylamide being especially preferred, particula.rly 2~acrylamido-2-methylpropane sulfonic acid (AMPS), 2-acrylamido-2-propane sulfonic acid and the salts thereof. In the most preferred embodiments, the sulfo group is in the form of an alkali metal sulfonate salt such as sodium sulfonate.
The polyvinyl crosslinking monomers are copolymerized with the aforementloned ethylenically unsaturated water-soluble monomers including, for example, divinyl benzene, divinyl esters of polycar-boxylic acid, diallyl esters of polycarboxylic acids, diallyl maleate, diallyl fumarate, divinyl adipate, glyceryl trimethylacrylate, diallyl succinate, divinyl ether, the divinyl esters of ethylene glycol or diethy-le~e glycol diacrylate, polyethylene glycol diacrylates or methacrylates, and the like. Of the aforementioned polyvinyl crosslinking monomers, the most preferred is methylene bisacrylamide.
Polymers useful herein are anionic polymers.
Polymers are prepared by using techniques known in the art for preparing water-soluble polymers. That is, 32,152-F -11-3~
polymer~ are pr~pared by copolymerizing the afore~len-tioned mono~ers using techniques known in the art for prepariny water-soluble polymer~. For example, polym-exiæatio~ can be carried out in an aqueous medium in the presence of a small but effective amount of a water-soluble oxygen~containing catalyst at a tempera-ture of from 80F to 190F (26.7 to 87.8C). Tha resulting polymer is recovered from the aqueous tnedium, as by drum drying, and can be subsequently ground to the desired particle size. The particle size should be fine enough to facilitate the dispersion of the polymer in water. Polymers are also prepared using pol~meriza-kion techni~ues described in U.S. Patent No. 3,284,393 and U.S. Patent RE 28,474. Alternatively, the polymers can be prepared by reacting various known alkylating reagents with suitable copolymers to form anionic sites on the pol~mer.
Mclecular weights of the polymers of this i~vention can vary. Molecular weights typically range 20 from 100,000 to 25 million. Most preerably, molecular weights range from 1 million to 5 million.
The polymers of this invention comprise sufficient monomer containing anionic moieties in order that the composition of this invention does not yield an insoluble residue under acid treatment of subterranean formations. The polymers also comprise sufficient nonionic monomer in order that the polymer will provide sufficient thickening ability to the composition. The amount of polyvinyl crosslinking monomer which is employed will depend upon the type of crossllnker which is employed and the molecular weight of the linear polymer.
32,152~F -12-93(1 The pol.ymers of this i.nventlon are ~ypicall~
from greater than 0 to 50, preferably from greater than 0 to 35, most preferably from 20 to 30, weight. percent nonionic ethylenically unsaturated monomer, which is pre-ferably acrylamide; from 50 to less than 100, preferablyfrom 65 to less than 100, most preferably from 70 to 80, weight percent ethylenically unsaturated monomer comprising anionic moieties, which is prefereably 2-acrylamido-2-methyl propane sulfonic acid or a salt thereof; and from 1 to 1000 ppm, preferably from 50 to 100 ppm, most preferably from 75 to 100 ppm polyvinyl crosslinking monomer which is preferably methylene bisacrylamide. It is undersood that the amount of ethylenically unsaturated monomer containing anionic moieties within the polymer can vary depending on the anionic behavior of the monomer. It is also understood that the nonionic monomer within the polymer can undergo a small amount of hydrolysis.
The composition of this invention comprises from 0.4 to 35, preferably from 3 to 28, weight percent acid, from 0.1 to 5, preferably from 0.5 to 2, weight percent polymer, and from 60 to 99.5, preferably from 70 to 96.5, weight percent water based on the total weight of the composition. To this is added the reducing agent. In addition, it is desirable to add corrosion inhibitors, sequestering agents, demulsifying agents, surfactants, friction reduction reducers, etc., which are commonly added to such types of compositions.
In addition, -the acid compositions can be foamed by including a foaming agent and a gas, such a nitrogen.
Small amounts of polymer will usually produce liquid mobile gells which can be readily pumped. Large amounts of polymer will usually produce thicker, more 1;226)~3(~
viscous, somewhat elastic gels~ Gels having a viscosity "too thick to measure" by conventional methods can still be used in the pxactice of the invention. Thus, - there is really no fixed upper limit on the amount o polymer which can be used so long as the gelled acidic composition can be pumped in accordance with the methods of the invention.
The "ixon control agents" useful herein are reducing agents which are known classes of compounds having many members. Any member(s) of these known classes of compounds can be used herein so lony as the chosen member(s) is compatible with the gelled acidic composition, i.e., the ¢hosen member(s) is solu~le or dispersible in the acidic composition and does not preven-t formation of the gelled acidic composition or cause premature breaking of the gel. Examples of suitable reducing agents include reducing organic acids (e.g., ascorbic acid, erythorbic acid, and the like) and soluble salts thereof (e.g., sodium erythorbate, potassium ~rythorbate, ammonium ascorbate, and the like), hydrazine, iodide salts (e.g;, sodium iodide and the like) and other s~ch compounds. Of these, the organic reducing acids are preferred, and ascorbic acid, erythorbic acid and/or the soluble salts thereof are most preferred. The reducing agent is included in the gelled acidic composition in an effective amount, i.e., the amount sufficient to prevent or substantially prevent the formation of an insoluble residue when the gelled acidic composition is contacted, for example, with calcium carbonate in the presence of dissolved iron.
32,152-F ~14-Iron control ayents are typically used in concentrations oE ~rom 0.01 to 0.36 g/100 ml o~
thickened acid composition, preferably from 0.06 to 0.24 g/100 ml of thickened acid composition.
Typically, iron control agents are employed in a concentration of 0.06 g/100 ml of thickened acid composition for every 1000 ppm ferrlc ion present and such concentrations are very effective in combating residue formation.
Acids useful in the practice of this invention include, for example, inorganic acids such as hydrochloric acid, phosphoric acid, nitric acid, hydrofluoric acid; and a mixture of hydrochloric acid and hydrofluoric acid; C1-C4 organic acids such as formic acid, acidic a~id, propionic acid, bu-tyric acid and mixtures thereof and combinations of inorga~ic and organic acids. The nonoxidizing acids are preferred.
The concentration or strength of the acid can vary depending upon the type of acid, the type of formation being treated and the results desired in the particular treating operation. Most preferably, the acid used in the practice of this invention is an inorganic acid such as hydrochloric acid.
The gelled acidic composition of the present invention can be prepared on the surface in a suitable tank equipped with a suitable means for mixing and then pumped down into the well and into the formation employing conventional equipment for pumping acidic compositions.
Most preferably, the pol~mer is mixed with the acid at temperatures of from 60~F to 90F (15.6 to 32.2C).
Viscosity development is quite rapid and complete viscosity development occurs in 60 to 90 minutes.
32,152-F ~15-~2ZOg3~
It is within the scope of this invention to precede the injection of the gelled acid composition wi-th a pad fluid or preflush such as gelled water~ or a~ueous potassium chloride, aqueous ammonium chloride, etc.
Such fluids serve to cool the well tubing and formation and extend the useful operating temperature range of said compositions. The volume of the pad fluid so injected can be any suitable volume sufficient to significantly decrease the temperature of the formation being treated and can vary depending upon -the char-acteristics of the formation. Typically, the compo-sitions of this invention are employed at temperatures up to 300F (148.9C), depending upon conditions of employment and the amount of polymer which is used.
The gelled acid compositions of the invention can be prepared on the surface in a suitable tank equipped with suitable mixing means, and then pumped into the formation employing conventional equipment for pumping acidic compositions. However, it is within the scope of the invention to prepare said compositions while they are being pumped down the well. This tech-nique is sometimes referred to as "on the fly." For example, a solution of the polymer in water can be prepared in a tank adjacent the well head. Pumping of this solution through a conduit to the weli head can be staxted. Then, downstream from the tank, a suitable connection can be provided for introducing-the acid into said conduit~ A foamed acid can be generated by subse~uently introducing a gas such as nitrogen into the flowing stream of acid composition. As will be understood by those skilled in th~ art, the rate of introduction of said components into said conduit will depend upon the pumping rate of the polymer solution 32,15a-F -16-~ZZ(~93C~ .
through said conduit. Any ot the above-mentioned orders of addition can be employed in said "on the fly"
techni~le. Mixing devices can be provided in said conduit, if desired.
S For fracture acidizing treatments, the composi-tions of this invention are injected into a bore hole at a sufficient rate and pressure to initiate and pxopagate a crack or fracture in the formation. Sand, bauxite, or other proppant material can be included in the treating fluid to prevent the fracture from closing.
In a matrix acidizing operation, the treatment fluid is injected into the borehole at a rate and pressure sufficient to force the acid out into the formation but insufficient to cause fracturing of the formation.
The following examples will serve to further illustrate the invention but should not be considered as limiting the scope thereof. Unless otherwise noted, all parts and percentages are by weight.
Example The present composition is prepared as follows.
A volume of concentrated hydrochloric acid is mixed with suffici~nt water to yield an acid concentration of 28 percent. To this solution is added enough ferric ~ chloride to yield a ferric ion concentration of 3000 ppm.
To this solution is added at a r~tio of 1.2 g/100 ml of acid solution, a random copolymer containing 70 percent by weight of a sodium salt of 2-acrylamido-2-methyl-propane sul~onic acid (AMPS) and 30 percent acrylamide monomer crosslinked with 100 ppm of methylene bisacryl-amide and having a molecular weight greater than 3million. To this solution is also added 0.6 g/1000 ml 32,152-F -17-0~3~
~18-o acid solution of a corrosion inhibitor. ~o this yelled solution is added sodium erythorhate at a ratio of 2 g/100 ml of said solution. Marble chips are added to the gelled solution and the mixture is heated to 150F (65.6C). After bubbling of the composi-tion is complete, the solution is examined for precipitate.
The polymer composition yields no residue formation.
Conversely, a similar formulation not containing sodium erythorbate but treated in a similar fashion yields substantial residue formation.
32,152-F -18-
sulfoarylalkenes such as vinylbenzyl sulfonic acid and the various salts o vinylbenzyl sulfonate, p-styrene sulfonic ac.id and the salts thereof; sulfoalkenes such as vinyl sulfonic acid and salts thereo~; and the like.
Of the foregoing sulfonate monomers, the sulfoalkyl derivatives of acrylamide and methacrylamide are preferred with those of acrylamide being especially preferred, particula.rly 2~acrylamido-2-methylpropane sulfonic acid (AMPS), 2-acrylamido-2-propane sulfonic acid and the salts thereof. In the most preferred embodiments, the sulfo group is in the form of an alkali metal sulfonate salt such as sodium sulfonate.
The polyvinyl crosslinking monomers are copolymerized with the aforementloned ethylenically unsaturated water-soluble monomers including, for example, divinyl benzene, divinyl esters of polycar-boxylic acid, diallyl esters of polycarboxylic acids, diallyl maleate, diallyl fumarate, divinyl adipate, glyceryl trimethylacrylate, diallyl succinate, divinyl ether, the divinyl esters of ethylene glycol or diethy-le~e glycol diacrylate, polyethylene glycol diacrylates or methacrylates, and the like. Of the aforementioned polyvinyl crosslinking monomers, the most preferred is methylene bisacrylamide.
Polymers useful herein are anionic polymers.
Polymers are prepared by using techniques known in the art for preparing water-soluble polymers. That is, 32,152-F -11-3~
polymer~ are pr~pared by copolymerizing the afore~len-tioned mono~ers using techniques known in the art for prepariny water-soluble polymer~. For example, polym-exiæatio~ can be carried out in an aqueous medium in the presence of a small but effective amount of a water-soluble oxygen~containing catalyst at a tempera-ture of from 80F to 190F (26.7 to 87.8C). Tha resulting polymer is recovered from the aqueous tnedium, as by drum drying, and can be subsequently ground to the desired particle size. The particle size should be fine enough to facilitate the dispersion of the polymer in water. Polymers are also prepared using pol~meriza-kion techni~ues described in U.S. Patent No. 3,284,393 and U.S. Patent RE 28,474. Alternatively, the polymers can be prepared by reacting various known alkylating reagents with suitable copolymers to form anionic sites on the pol~mer.
Mclecular weights of the polymers of this i~vention can vary. Molecular weights typically range 20 from 100,000 to 25 million. Most preerably, molecular weights range from 1 million to 5 million.
The polymers of this invention comprise sufficient monomer containing anionic moieties in order that the composition of this invention does not yield an insoluble residue under acid treatment of subterranean formations. The polymers also comprise sufficient nonionic monomer in order that the polymer will provide sufficient thickening ability to the composition. The amount of polyvinyl crosslinking monomer which is employed will depend upon the type of crossllnker which is employed and the molecular weight of the linear polymer.
32,152~F -12-93(1 The pol.ymers of this i.nventlon are ~ypicall~
from greater than 0 to 50, preferably from greater than 0 to 35, most preferably from 20 to 30, weight. percent nonionic ethylenically unsaturated monomer, which is pre-ferably acrylamide; from 50 to less than 100, preferablyfrom 65 to less than 100, most preferably from 70 to 80, weight percent ethylenically unsaturated monomer comprising anionic moieties, which is prefereably 2-acrylamido-2-methyl propane sulfonic acid or a salt thereof; and from 1 to 1000 ppm, preferably from 50 to 100 ppm, most preferably from 75 to 100 ppm polyvinyl crosslinking monomer which is preferably methylene bisacrylamide. It is undersood that the amount of ethylenically unsaturated monomer containing anionic moieties within the polymer can vary depending on the anionic behavior of the monomer. It is also understood that the nonionic monomer within the polymer can undergo a small amount of hydrolysis.
The composition of this invention comprises from 0.4 to 35, preferably from 3 to 28, weight percent acid, from 0.1 to 5, preferably from 0.5 to 2, weight percent polymer, and from 60 to 99.5, preferably from 70 to 96.5, weight percent water based on the total weight of the composition. To this is added the reducing agent. In addition, it is desirable to add corrosion inhibitors, sequestering agents, demulsifying agents, surfactants, friction reduction reducers, etc., which are commonly added to such types of compositions.
In addition, -the acid compositions can be foamed by including a foaming agent and a gas, such a nitrogen.
Small amounts of polymer will usually produce liquid mobile gells which can be readily pumped. Large amounts of polymer will usually produce thicker, more 1;226)~3(~
viscous, somewhat elastic gels~ Gels having a viscosity "too thick to measure" by conventional methods can still be used in the pxactice of the invention. Thus, - there is really no fixed upper limit on the amount o polymer which can be used so long as the gelled acidic composition can be pumped in accordance with the methods of the invention.
The "ixon control agents" useful herein are reducing agents which are known classes of compounds having many members. Any member(s) of these known classes of compounds can be used herein so lony as the chosen member(s) is compatible with the gelled acidic composition, i.e., the ¢hosen member(s) is solu~le or dispersible in the acidic composition and does not preven-t formation of the gelled acidic composition or cause premature breaking of the gel. Examples of suitable reducing agents include reducing organic acids (e.g., ascorbic acid, erythorbic acid, and the like) and soluble salts thereof (e.g., sodium erythorbate, potassium ~rythorbate, ammonium ascorbate, and the like), hydrazine, iodide salts (e.g;, sodium iodide and the like) and other s~ch compounds. Of these, the organic reducing acids are preferred, and ascorbic acid, erythorbic acid and/or the soluble salts thereof are most preferred. The reducing agent is included in the gelled acidic composition in an effective amount, i.e., the amount sufficient to prevent or substantially prevent the formation of an insoluble residue when the gelled acidic composition is contacted, for example, with calcium carbonate in the presence of dissolved iron.
32,152-F ~14-Iron control ayents are typically used in concentrations oE ~rom 0.01 to 0.36 g/100 ml o~
thickened acid composition, preferably from 0.06 to 0.24 g/100 ml of thickened acid composition.
Typically, iron control agents are employed in a concentration of 0.06 g/100 ml of thickened acid composition for every 1000 ppm ferrlc ion present and such concentrations are very effective in combating residue formation.
Acids useful in the practice of this invention include, for example, inorganic acids such as hydrochloric acid, phosphoric acid, nitric acid, hydrofluoric acid; and a mixture of hydrochloric acid and hydrofluoric acid; C1-C4 organic acids such as formic acid, acidic a~id, propionic acid, bu-tyric acid and mixtures thereof and combinations of inorga~ic and organic acids. The nonoxidizing acids are preferred.
The concentration or strength of the acid can vary depending upon the type of acid, the type of formation being treated and the results desired in the particular treating operation. Most preferably, the acid used in the practice of this invention is an inorganic acid such as hydrochloric acid.
The gelled acidic composition of the present invention can be prepared on the surface in a suitable tank equipped with a suitable means for mixing and then pumped down into the well and into the formation employing conventional equipment for pumping acidic compositions.
Most preferably, the pol~mer is mixed with the acid at temperatures of from 60~F to 90F (15.6 to 32.2C).
Viscosity development is quite rapid and complete viscosity development occurs in 60 to 90 minutes.
32,152-F ~15-~2ZOg3~
It is within the scope of this invention to precede the injection of the gelled acid composition wi-th a pad fluid or preflush such as gelled water~ or a~ueous potassium chloride, aqueous ammonium chloride, etc.
Such fluids serve to cool the well tubing and formation and extend the useful operating temperature range of said compositions. The volume of the pad fluid so injected can be any suitable volume sufficient to significantly decrease the temperature of the formation being treated and can vary depending upon -the char-acteristics of the formation. Typically, the compo-sitions of this invention are employed at temperatures up to 300F (148.9C), depending upon conditions of employment and the amount of polymer which is used.
The gelled acid compositions of the invention can be prepared on the surface in a suitable tank equipped with suitable mixing means, and then pumped into the formation employing conventional equipment for pumping acidic compositions. However, it is within the scope of the invention to prepare said compositions while they are being pumped down the well. This tech-nique is sometimes referred to as "on the fly." For example, a solution of the polymer in water can be prepared in a tank adjacent the well head. Pumping of this solution through a conduit to the weli head can be staxted. Then, downstream from the tank, a suitable connection can be provided for introducing-the acid into said conduit~ A foamed acid can be generated by subse~uently introducing a gas such as nitrogen into the flowing stream of acid composition. As will be understood by those skilled in th~ art, the rate of introduction of said components into said conduit will depend upon the pumping rate of the polymer solution 32,15a-F -16-~ZZ(~93C~ .
through said conduit. Any ot the above-mentioned orders of addition can be employed in said "on the fly"
techni~le. Mixing devices can be provided in said conduit, if desired.
S For fracture acidizing treatments, the composi-tions of this invention are injected into a bore hole at a sufficient rate and pressure to initiate and pxopagate a crack or fracture in the formation. Sand, bauxite, or other proppant material can be included in the treating fluid to prevent the fracture from closing.
In a matrix acidizing operation, the treatment fluid is injected into the borehole at a rate and pressure sufficient to force the acid out into the formation but insufficient to cause fracturing of the formation.
The following examples will serve to further illustrate the invention but should not be considered as limiting the scope thereof. Unless otherwise noted, all parts and percentages are by weight.
Example The present composition is prepared as follows.
A volume of concentrated hydrochloric acid is mixed with suffici~nt water to yield an acid concentration of 28 percent. To this solution is added enough ferric ~ chloride to yield a ferric ion concentration of 3000 ppm.
To this solution is added at a r~tio of 1.2 g/100 ml of acid solution, a random copolymer containing 70 percent by weight of a sodium salt of 2-acrylamido-2-methyl-propane sul~onic acid (AMPS) and 30 percent acrylamide monomer crosslinked with 100 ppm of methylene bisacryl-amide and having a molecular weight greater than 3million. To this solution is also added 0.6 g/1000 ml 32,152-F -17-0~3~
~18-o acid solution of a corrosion inhibitor. ~o this yelled solution is added sodium erythorhate at a ratio of 2 g/100 ml of said solution. Marble chips are added to the gelled solution and the mixture is heated to 150F (65.6C). After bubbling of the composi-tion is complete, the solution is examined for precipitate.
The polymer composition yields no residue formation.
Conversely, a similar formulation not containing sodium erythorbate but treated in a similar fashion yields substantial residue formation.
32,152-F -18-
Claims (9)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A gellable acid composition which comprises (A) an aqueous acid thickened with a functionally effective amount of a water-soluble polymer comprising (1) from greater than 0 to 50 weight percent of a nonionic water-soluble ethylenically unsaturated monomer, (2) from 50 to less than 100 weight percent of a water-soluble ethyleni-cally unsaturated monomer containing an anionic moiety and (3) from 1 to 1000 ppm of a polyvinyl crosslinking monomer, wherein said polymer is crosslinked in amounts sufficient to provide increased viscosity to the composition during acid treatment of porous subterranean formations, which amount of polymer is sufficient to cause gelation and to form a gelled acid composition sufficiently stable to degradation by heat of porous subterranean formations; and (B) at least one reducing agent in an amount sufficient to prevent or substantially prevent the formation of an in-soluble residue as the gelled acid reacts with the acid-soluble components of said formation in the presence of dissolved ferric ion.
2. The composition of Claim 1 wherein said acid is hydrochloric acid.
3. The composition of Claim 1 wherein said polymer comprises from 20 to 30 weight percent acrylamide, from 70 to 80 weight percent 2-acrylamido-2-methyl propane sulfonic acid or a salt thereof; and from 50 to 100 ppm of methylene bisacrylamide.
4. The composition of Claim 1 which comprises from 3 to 28 weight percent acid, from 0.5 to 2 weight percent polymer and from 70 to 96.5 weight percent water.
5. The composition of Claim 1 wherein said reducing agent is ascorbic acid or a salt thereof, or erythorbic acid or a salt thereof.
6. The composition of Claim 1 wherein said reducing agent is employed in a concentration of about 0.06 g/1000 ml of acid composition for every 1000 ppm ferric ion present.
7. A method for acid treating a porous subter-ranean formation susceptible to attack by an acid which acid is penetrated by a wellbore, which method comprises injecting into said formation via said wellbore a gellable or gelled acidic composition comprising (A) an aqueous acid in an amount capable of, and sufficient for, reacting with a significant amount of the acid-soluble components of said formation; and a water-thickening amount of a water-soluble polymer comprising (1) from greater than 0 to 50 weight percent of a nonionic water-soluble ethylencally unsaturated monomer, (2) from 50 to less than 100 weight percent of a water-soluble ethylenically unsaturated monomer containing an anionic moiety and (3) from 1 to 1000 ppm of a polyvinyl crosslinking monomer; and (B) at least one reducing agent in an amount sufficient to prevent or sub-stantially prevent the formation of an insoluble residue as the gelled acid reacts with the acid-soluble components of said formation in the presence of dissolved ferric ion, said polymer and said acid in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit said gelation and thus form said composition having sufficient stability to degradation by the heat of said formation to permit good penetration of said composition into said formation; and maintaining said composition in said formation in contact therewith for a period of time sufficient usually for the acid in said composition to react significantly with the acid-soluble components of said formation and stimulate the production of fluids therefrom.
8. The method of Claim 7 wherein said acid is hydrochloric acid.
9. The method of Claim 7 wherein said polymer comprises from 20 to 30 weight percent acrylamide, from 70 to 80 weight percent 2-acrylamido-2-methyl propane sulfonic acid or a salt thereof; and from 50 to 100 ppm of methylene bisacrylamide.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US52868783A | 1983-09-01 | 1983-09-01 | |
| US528,687 | 1983-09-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1220930A true CA1220930A (en) | 1987-04-28 |
Family
ID=24106723
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000461843A Expired CA1220930A (en) | 1983-09-01 | 1984-08-27 | Methods for acidizing subterranean formations and gelled acid compositions |
Country Status (1)
| Country | Link |
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| CA (1) | CA1220930A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114805679A (en) * | 2022-06-30 | 2022-07-29 | 山东诺尔生物科技有限公司 | Gelling agent for acid fracturing operation and preparation method thereof |
-
1984
- 1984-08-27 CA CA000461843A patent/CA1220930A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114805679A (en) * | 2022-06-30 | 2022-07-29 | 山东诺尔生物科技有限公司 | Gelling agent for acid fracturing operation and preparation method thereof |
| CN114805679B (en) * | 2022-06-30 | 2022-09-02 | 山东诺尔生物科技有限公司 | Gelling agent for acid fracturing operation and preparation method thereof |
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