GB2158487A - Gelled acid composition for acidizing subterranean formations - Google Patents

Abstract

Gelled acid compositions which comprise: an aqueous acid, acrylamide or methacrylamide polymers cross-linked with an aldehyde(s), and a reducing agent or chelating agent. The reducing or chelating agent acts to prevent iron ions forming troublesome insoluble residues when the compositions are used for acidizing subterranean formations. The reducing agent may be ascorbic or erythorbic acid, hydrazine or iodide salts, for example, and as chelating agent EDTA, HEDTA, or citric or lactic acids are preferred.

Description

SPECIFICATION Gelled acid composition for acidizing subterranean formations This invention pertains to improved compositions and method for acid treating or acidizing of subterraneanformations.

Acid treating oracidizing of porous subterranean formations penetrated by a well bore has been widely employed for increasing the production of formation fluids, e.g., crude oil, natural gas, etc., from said formations. The usual technique of acidizing a formation comprises introducing a non-oxidizing acid into the well under sufficient pressure to force the acid out into the formation where it reacts with the acid-soluble components of the formation. The technique is not limited to formations of high acid solubility such as limestone, dolomite, etc. The technique is also applic abletoothertypesofformationssuch asasandstone containing streaks orstriations of acid soluble components such as the various carbonates.

During the acid treating operation, passagewaysfor fluid flow are created in the formation, or existing passageways therein are enlarged, thus stimulating the production offluidsfromtheformation.This action oftheacid on the formation is often called etching. Acid treating or acidizing operations wherein the acid is injected into the formation at a pressure or rate insufficient to create cracks or fractures in the formation is usually referred to as matrix acidizing.

Hydraulic fracturing is also commonly employed to increase the production of fluids from subterranean formations. Hydraulic fracturing comprises the injection of a suitable fracturing fluid down a well penetrating a formation and into said formation under sufficient pressure to overcome the pressure exerted by the over-burden. This results in creating a crack or fracture in the formation to provide a passageway which facilitates flow of fluidsthrough the formation and into the well. Combinationfracture-acidizing processes are well known in the art.

Thus, it is within the scope ofthe present invention to inject the gelled acidic compositions ofthe invention into the formation under insufficient pressure to cause fracturing oftheformation and carry out a matrix acidizing operation, or inject said gelled acidic composition at sufficient rates and pressure to cause fracturing and carry out a combination fractureacidizing operation.

One ofthe problems commonly encountered in acidizing operations is insufficient penetration ofthe formation bytheacid. It is desirablethat good penetration is obtained in orderto realise maximum benefits from the operation. Too often the acid is essentially completely spent in the area immediately adjacent and surrounding the well bore. The severity ofthe problem increases as the well temperature increases because acid reactivitywith the formation increases with increasing temperatures as in deeper wells.

Poor penetration can also be caused, and/or aggravated, by fluid loss to the more porous zones of the formation where low permeability is not a problem.

Poor penetration can also be caused and/oraggra- vated by leak-off atthe fracture faces in combination fracturing-acidizing operations. Fluid loss or leak-off can frequently worsen the situation by leaving the tight (low permeability) zones of the formation unchanged and merely opening up the already high permeability zones.

One solution which has been proposed forthe above discussed problem is to incorporate various polymericthickening orviscosifying agents into the acid solution. Such agents thicken the acid solution and increase the viscosity thereof. It has been rported that polymer-thickened acid solutions have reduced fluid loss properties. For example, see U.S. Patent No.

3,415,319 issued in the name of B. L. Gibson; and U.S.

Patent No. 3,434,971 issued in the name of B. L. Atkins.

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 No. 3,749,169 issued in the name of J. F. Tate; U.S. Patent No. 3,236,305 issued in the name of C. F. Parks; and U.S. Patent No.

3,252,904 issued in the name of N. F. Carpenter.

Higherviscositiesarealso advantageous in combination fracturing-acidizing operations in thatthe more viscous acidic solutions producewiderand longerfractures. More viscous acid solutions are also more effective in carrying dropping agents into the formation when propping agents are used.

Another problem encountered in acidizing operations, particularly when employing acidizing compositions having thickening orviscosifying agents in corporated therein, is stability to heat. By stability to heat, is meantthe retention ofthe increased or grater viscosity properties underthe conditions of use. Such compositions to be satisfactory should be sufficiently stableto resist degeneration bythe heat of the formation for a period of time sufficientto accomplish the intended purpose, e.g., useful penetration and significant etching oftheformation.The degree of stability required in any particular operation will vary with such operating variables as the type of formation being treated, the temperature ofthe formation, the well depth (time to pump the acidic composition down the well and into the formation), the acid concentration in the composition, etc. For example, acidizing of a tight low permeability formation will proceed more slowly than a more permeable formation, other factors being the same, because a longertime will be required to obtain a significant amount of etching and the composition must remain in place and effective for a longer period of time. Also, more time will be required to pump the acidic composition into place in the formation.

The temperature of the formation usually has 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 effects. One such effect is degeneration ofthe composition, e.g., decrease in viscosity. Another such effect is increased rate of reaction ofthe acid with the formation. Thus, some compositions which would be satisfactory in a low temperatureformationsuch as in the Hugotonfield in the Anadarko basin might not be satisfactory in formations encountered in deeper wells as in some West Texas fields.

In ordinaryacidizing operations using unthickened acids there is ususally no problem in removing the spent acid because it is essentiallywater. However, a problem which is sometimes encountered when using thickened compositions in treating formations is the case of removal of the treating composition afterthe operation is completed. Some thickened or highly viscous solutions are difficult to remove from the pores ofthe formation of the fracture after the operation is complete. Sometimes a clogging residue can be left in the pores ofthe formation, or in the fracture. This can inhibit the production of fluids from the formation and can require costly cleanup operations. It would be desirable to have gelled acidic compositions which break down to a lesser viscosity within a shorttime after the operation is completed.

Swanson addressed the problems set forth above and allegedly solved such problems by using certain of acrylamide or methacrylamide crosslinked with certain aldehyde crosslinkers in USP 4,103,742 and 4,191,657. From an academic standpoint, the Swan sontechnologywould appearto be viable. From a practical standpoint, however, the technology is rendered commercially untenable by the present discoverythatan insoluble residue is formed in copious quantities when the gel led acid reacts with the typical acid-soluble component(s) ofthe formation being treated ifthere is dissolved iron present. The adveFse effect of rdissol--ed iron on the polymeric gelling agents n Swanson was previously unknown.

The problem caused bathe ehe dissolved iron is particu- larly vexatious because it is almost impossible to eliminate it from acid solutions in contact with conventional steel (e.g., blending tanks, pumps, piping, etc.) and the insoluble residue that formes as the acid "spends" during the acidizing treatment could (and probablywould) cause severe blockage and damage to the formation. Dissolved iron in the ferric (+3) oxidation state seems to be more troublesome than ferrous (+2) iron in these gelled acidtreatment fluids.

it has now been discovered that the inclusion of a reducing agent and/or a chelant will prevent or significantly prevent the formation of the insoluble residue noted when the gelled acidic compositions of Swanson are reacted with acid-soluble components of a subterranean formation in the pressence of dissolved iron. Thus, the present ivertion is a substantial improvement over Swanson.

The present invention, in particular, relates to a method for acid treating a porous subterranean formation susceptible of attack by an acid and penetrated by a well bore, which method comprises: injecting into the formation via the well bore a gelled acidic composition comprising water, as a waterthickener, a water-dispersible polymer selected from polyacrylamides and polymethacrylamides, partially hydrolyzed polyacrylamides and partially hydrolyzed polymethacrylamides wherein a portion ofthe carbo xamide groups are initially hydrolyzed to carboxyl groups, cross-linked polyacrylamides and cross-link ed polymethacrylamides, partially hydrolyzed cross linked polyacrylamides and partially hydrolyzed cross-linked polymethacrylamides wherein a portion ofthe carboxamide groups are initially hydrolyzed to carboxyl groups, copolymers of acrylamides or methacrylamides with another ethylenically unsaturated monomer copolymerizable therewith, and mixtures thereof, an acid to react with the acid-soluble components of the formation, a small amount of at least one water-dispersible aldehyde to cause gelation of an aqueous dispersion ofthe polymer, the acid, and the aldehyde, at least one compatible reductant and/or chelant as an iron control agent to prevent or significantly preventtheformation of an insoluble residue as the gel led acid reacts with the acid-soluble components oftheformation in the presence of dissolved ferric iron, the polymer, the acid, and the aldehyde(s), in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit the gelation and thus form a said composition having sufficient stability to degeneration by the heat of the formation to permit penetration ofthe composition into the formation; and maintaining the composition in the formation forthe acid therein to reactwith the acid-soluble components of the formation and stimulate the production of fluids therefrom.

In another aspect, the invention provides the above-defined composition.

The "iron control agents" useful herein are reducing agents and/or chelating agents, both of which are known classes of compounds having many members.

Any member(s) of these known classes of compounds can be used herein so long as the chosen member(s) is compatible with the gelled acidic composition; i.e., the chosen member(s) is soluble or dispersible in the acidic comosition and doesn't prevent formation of the gelled acidic composition or cause premature breaking ofthe gel. Examples of suitable reducing agents include, reducing organic acids (e.g. ascorbic acid, and erythorbic acid) and soluble saltsthereof (e.g. sodium erythorbate, potassium erythorbate and ammonium ascorbate), hydrazine and iodide salts (e.g. sodium iodide). Of these, the organic reducing acids are preferred and ascorbic acid, erythorbic acid and/or the soluble salts thereof are most preferred.

Examples of the class of chelating agents for iron include, the polyalkylenepolyaminepolycarboxylic acids (e.g., N,N,N'N'-ethylenediaminetetraacetic acid, (EDTA), N-2-hydroxyethyl-N,N',N'-ethylene diaminetriacetic acid lHEDTA), and the like) and soluble salts thereof (e.g. tetrasodium EDTA, ammonium saits of EDTA or HEDTA), the hydroxy-containing organic acids (e.g. citric acid and lactic acid), for example.

We particularly prefer two use erythorbic acid, ascorbic acid, ethylenediaminetetraacetic acid, N - 2 hydroxyethyl - N,N',N" - ethylenediaminetriacetic acid or their soluble salts as the iron control agents.

The reducing agent and/or chelating agent is included in the gelled acidic composition in an effective amount. The acid treatment of subterranean formations is not an exact science and what constitutes an effective amount must be determined for each location, so no universal rule can be laid down as to the maximum and minimum effective concentrations of chelating andior reducing agent.

The amount of iron control agentto be used depends upon the amount of ferric iron present in the water, which will be determined by the operator in the field. An effective amount may be said to be an amount sufficient to prevent or significantly prevent theformationofan insoluble residue when the gelled acidic composition is contacted with calcium carbonate (e.g. small marble chips) in the presence of dissolved iron.

In the field, an amount of about 10 Ibsll 000 gal (1.2 kg/1 0001) is typically used and an amount of 20 Ibs/l 000 gal (2.4 kg/10001) may be recommended as being generally effective. However, we find that in practice 5 Ib/l 000 gal (0.6 kg/1 0001) (equivalent to about 0.04wt %) is a minimum effective amountfor most jobs with ferric iron present at about 20 ppm.

Thus, there would usually be used a composition containing at least 5 Ib/l 000 gallons (0.6 keg/10001) of control agent while we would recommend a range of from 10 to 20 Ibs/l 000 gal (1.2 to 2.4 kg/1 0001) as being the most useful range of concentration.

The ratio of acid to ferric iron should be about 1.2 for most effective operation.

The gelled acidic compositions, and the compo nentsthereof, as well as the process of using such gelled acidic compositions to treat subterranean formation is described by Swanson, supra.

The water-dispersible polymer, which is present in such an amountasto actas a water-thickener, is preferably a polyacrylamide or a polymethacrylamide wherein, optionally, up to 45% of the carboxamide groups are initially hydrolysed to carboxyl groups.

We further prefer the water-dispersible polymer to be a random copolymer oftheformula

wherein: R is hydrogen ora loweralkyl radical containing from 1 to 6 carbon atoms; R' is an alkylene radical containing from 1 to 24 carbon atoms or is an arylene radical containing from 6to 10 carbon atoms; R" is hydrogen ora methyl radical;M is hydrogen, ammonium, oran alkali metal;Ziseither-NH2or -OM in the above Type I monomer units, with the proviso thatthe copolymer contains at least 10 mol percent ofthe Type I monomer units in which Z is -NH2; and x and y are the mol percent values of the respective individual monomer units land II, with x being in the range offrom lotto 99 and with Y being in the range of from 1 to 90.

More preferably, the water-dispersible polymer is a random copolymer represented by the formula

wherein: R' and R"' are each independently a hydrogen atom, ora methyl radical; M is hydrogen, sodium, or potassium; Z is either -NH2 or -OM in the above Type I monomer units, with the proviso that the copolymer contains at least 10 mol percent of said Type I monomer units in which Z is -NH2; and xand y are the mol percent values ofthe respective monomer units land II with x being in the range of from 20 to 95 and withY being in the range offrom 5 to 80.

The aldehyde may be a C1-C10 aliphatic monoaldehyde, glyoxal, glutaraldehyde or terephthaldehyde but we particularly preferformaldehyde or acetaldehyde or mixtures thereof, especially when in combination with hydrochloric acid as the acid and a reductant as the iron control agent.

The amount of the aldehyde is preferably in the range of from 0.001 to 5weight percent based upon the total weight of the composition, more preferably from 0.004to 2 weight percent. It is particularly desirableforthislatter rangeto be adopted when one or more C1-C10 aliphatic monoaldehydes are used as the aldehyde.

The preferred amount of polymer is offrom 0.2 to 3, more particularly 0.75 to 2, weight percent based upon the total weight ofthe composition.

ltisadvantageousfortheacidto be present in an amount offrom 0.4 to 60 weight percent, based on the total weightofthe composition. It is more preferable forthe acid to be present in an amount of not more than 35 weight percent, and in this case an especially preferred composition results if HCI is used as the acid.

It will be understood that it is beneficial for the polymer, the acid and the aldehyde all to be present in amounts falling within the preferred ranges set out above.

The invention includes within its scope both methods of recovering formation fluids (e.g. oil or gas) from subterranean formations which comprise the acid treatment method of the invention and also the thus-recovered formation fluids.

Afurther aspect is iron control agents intended for the method or composition ofthe invention. Such an agent is a reductant orchelant not only suitablyfor the method or composition but specifically intended therefor, as an example of which may be cited an appropriate compound "got-up" for use with the method or composition ofthe invention.

Another aspect of the invention is the use of a reductant and/or chelant as an iron control agent for the method or composition ofthe invention.

Experimental: The following experiments will further illustrate the invention: Aqueous 28 percent hydrochloric acid was gelled by dissolving 0.8 weight percent, total weight basis, of a copolymer having about 70 mole percent of AMPS monomer and 30 mole percent acrylamide monomer in interpolymerized form. The polymer in the gelled acid was then crosslinked by blending in 0.3weight percentformaldehyde, various amounts of ferric chloride were dissolved into aliquots of the crosslinked gelled acid and the acid subsequently spent by contacting it with marble chips. The results of this series of experiments are shown in Table I below.

TABLE I Example Fe(pom) Appearance of Spent Fluid 1 0 no residue 2 250 small amount of residue 3 500 small amount of residue 4 1000 large amount of residue 5 2000 large amount of residue 6 3000 large amount of residue The residue was a gelatinous mass that floated on the surface ofthe spent fluid. The residue contained the AMPS/acrylamide copolymer. In a similar series of experiments, a ferrous salt was added in place of ferricchloride-no residue was observed.

In anotherseries of experiments, various amounts of erythorbic acid was added to aliquots ofthe above crosslinked gelled acid containing 3000 ppm ferric ion (added as ferric chloride). These novel gelled acids were then spent by contact with marble chips in the same manner as above. The results ofthis series of experiments are shown in Table II below.

TABLE II ExamPle Erythorbic Acid Appearance of Spent Fluid (wt. percent) 7 0 large amount of residue 8 0.05 large amount of residue 9 0.10 large amount of residue 10 0.15 no residue 11 0.20 no residue This series of experiments show the effectiveness of erythorbic acid in preventing the formation ofthe insoluble polymer-containing residue. Erythorbic acid and ascorbic acid are unusually effective in the present invention. For example, HEDTA was also effective in preventing formation ofthe insoluble residue, but more was required (on a mole basis relative to the amount of initial ferric ion in solution).

The above experiments also show the relative ease by which a skilled artisan can determine the effective concentration level of a reducing agent and/or a chelating agent in any chosen crosslinked gelled acid ofthe present invention.

Claims (19)

1. A method for acid treating a porous subterra nean formation susceptible of attack by an acid and penetrated by a well bore, which method comprises: injecting into theformation via the well bore a gelled acidic composition comprising water, as a water-thickener, a water-dispersible polymer selected from polyacrylamides and polymethacrylamides, partially hydrolyzed polyacrylamides and partially hydrolyzed polymethacrylamides wherein a portion ofthe carboxamide groups are initially hydrolyzedto carboxyl groups, cross-linked polyacrylamides and cross-linked polymethacrylamides, partially hydrolyzed cross-linked polyacrylamides and partially hydrolyzed cross-linked polymethacrylamides wherein a portion ofthe carboxamide groups are initially hydrolyzed to carboxyl groups, copolymers of acrylamides or methacrylamides with another ethylenically unsaturated monomer copoly merizabletherewith, and mixturesthereof, an acid to react with the acid-soluble components of the formation, a small amount of at least one water-dispersible aldehyde to cause gelation of an aqueous dispersion ofthe polymer, the acid, and the aldehyde, at least one compatible reductantand/orchelantas an iron control agentto preventorsignificantly prevent the formation of an insoluble residue as the gelled acid reacts with the acid-soluble components oftheformation in the presence of dissolved ferric iron, the polymer, the acid, and the aldehyde(s), in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereofto permit the gelation and thus form a said composition having sufficient stability to degeneration by the heat of the formation to permit penetration of the composition intotheformation; and maintaining the composition in the formation for the acid therein to reactwith the acid-soluble components ofthe formation andstimulatethe production of fluids therefrom.

2. A method as claimed in Claim 1 wherein the water-dispersible polymer is a polyacrylamide or a polymethacrylamide wherein, optionally, up to 45 percent ofthe carboxamide groups are initially hydrolyzed to carboxyl groups.

3. A method as claimed in Claim 1 wherein the water-dispersible polymer is a random copolymer represented bytheformula

wherein: R is hydrogen ora lower alkyl radical containing from 1 to 6 carbon atoms; R' is an alkylene radical containing from 1 to 24 carbon catoms or is an arylene radical containing from 6to 10 carbon atoms; R', is hydrogen ora methyl radical; M is hydrogen, ammonium, oran alkali metal; Z is either -NH2 or -Om, with the proviso thatthe copolymer contains at least 10 mol percent ofthe above Type I monomer units in which Z is -NH2; and x andy are the mole percent values of the respective individual monomer units land II, with x being in the range of from 10 to 99 and withy being in the range of from 1 to 90.

4. A method as claimed in Claim 3 wherein: R is a hydrogen atom ora methyl radical; R' is -C(CH3)2-CH2-; M is hydrogen, sodium or potassium; and xis in the range of from 20 to 95 andy is in the range offrom 5 to 80.

5. A method as claimed in any one of the preceding claims wherein the aldehyde is an aliphatic monoaldehyde having from 1 to 10 carbon atoms, glyoxal, glutaraldehyde orterephthaldehyde.

6. A method as claimed in Claim 5 wherein the aldehyde is formaldehyde, acetaldehyde, or a mixture offormaldehyde and acetaldehyde and the acid is hydrochloric acid.

7. A method as claimed in any one of the preceding claims wherein the iron control agent comprises erythorbic acid, ascorbic acid, ethylenediaminetetraacetic acid, N - 2 - hydroxyethyl - ethylenediaminetriacetic acid, or a soluble salt thereof.

8. A method as claimed in any one of the preceding claims wherein the amount of the iron control agent is at least 51b/l 000 gallons of composition (0.6kg/1000 I).

9. A method as claimed in Claim 8 wherein the iron control agent is in an amount of 10 to 201b/ 1000gal (1.2to2.4kg/1000l).

10. A method as claimed in any one of the preceding claims wherein: the amount ofthe polymer is within the range of from 0.2 to 3 weight percent based upon the total weight of the composition; the amount ofthe aldehyde(s) is within the range of from 0.001 to 5 weight percent, based upon the total weight of the composition; and the amount ofthe acid is within the range of from 0.4 to 60 weight percent, based on the total weight ofthe composition.

11. A method as claimed in Claim 10 wherein: the amountofthe polymer is within the range offrom 0.75 to 2 weight percent; the acid is hydrochloric acid, and the amountthereof is sufficientto provide an amount of HCI within the range offrom 0.4to 35 weight percent; and the aldehyde is a C1 to C10 aliphatic monoaldehyde, ora mixture thereof, and the amount ofthe aldehyde is within the range of from 0.004to 2weightpercent.

12. A method of matrix-acidizing orfractureacidizing a porous subterranean formation using a composition comprising an iron control agent and substantially as hereinbefore described.

13. A method of recovering a formation fluid from a subterranean formation which comprises a method as claimed in any one of the preceding claims.

14. Aformation fluid whenever recovered bya method as claimed in Claim 13.

15. A gelled acidic composition, suitable for matrix-acidizing orfractu re-acidizing of a porous subterranean formation susceptible of attack by an acid, comprising: water, as a water-thickener, a water-dispersible polymer selected from polyacrylamides and polymethacrylamides, partially hydrolyzed polyacrylamides and partially hydrolyzed polymethacrylamides wherein a portion ofthe carboxamide groups are initially hydrolyzed to carboxyl groups, crosslinked polyacrylamides and crosslinked polymethacrylamides, partially hydrolyzed crosslinked polyacrylamides and partially hydrolyzed crosslinked polymethacrylamides wherein a portion ofthe carboxamide groups are initially hydrolyzed to ca rboxyl groups, copolymers of acrylamides or methacrylamides with another ethylenically unsaturated monomercopolymerizable therewith and mixtures thereof, an acid to react with the acid-soluble components ofthe formation, a small amount of at least one water-dispersible aldehydeto cause gelation of an aqueous dispersion of the polymer, the acid, and the aldehyde, and at least one compatible reductantand/orchelantas an iron control agentto prevent or significantly preventthe formation of an insoluble residue as the gelled acid reacts with the acid-soluble components oftheformation in the presence of dissolved ferric iron, the polymer, the acid, and the aldehyde(s), in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit gelation and thus form a said composition having sufficient stability to degeneration by the heat ofthe formation to permit penetration of the composition into theformation when injected thereinto.

16. A composition as claimed in Claim 15 which is as defined in any one of Claims 2 to 11.

17. A gelled acidic composition comprising an iron control agent and substantially as hereinbefore described.

18. An iron control agent intended for a method as claimed in any one of Claims 1 to 12 orfor a composition as claimed in any one of Claims 15 to 17.

19. The use of a reductantand/orchelantasan iron control agentfora method as claimed in any one of Claims 1 to 12 for a method as claimed in any one of Claims 15to 17.