GB2110744A - Method and compositions for acidizing subterranean formations - Google Patents

Abstract

Compositions consisting essentially of water, an acid, and a copolymer of acrylamide (about 5 to about 60 percent by weight) and dialkylaminoethylmethacrylate, trialkylaminoethylmethacrylate quaternary salts or acrylamido alkane sulfonic acid, are useful for acidizing or fracture-acidizing subterranean formations.

Description

SPECIFICATION Method and compositions for acidizing subterranean formations This invention relates to a method of acidizing or fracture-acidizing subterranean formations to enhance oil and gas production, and to certain compositions useful in the method.

Oil and gas accumulations usually occur in porous and permeable underground rock formations. In order to extract the oil and gas from the formation, a well is drilled into the formation. The oil and gas, in many instances, are contained in the pore spaces of the formation and are hydraulically connected by means of permeablility of interconnecting channels between the pore spaces. After the well is drilled into the formation, oil and gas are displaced to the well bore by means of fluid expansion, natural and artificial fluid displacement, gravity drainage, etc.

These various processes may work together or independently to drive the hydrocarbons into the well bore through existing flow channels. In many instances, however, production of the well may be impaired by insufficient channels leading into the bore hole. The formation, in many instances, is treated to increase the ability of the formation rock to conduct fluid into the well.

Acidizing of a subterranean formation penetrated by a well bore has been widely employed for increasing the production of fluids, e.g. crude oil, natural gas, etc. from the formation. 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 acidsoluble components of the formation. The technique is not limited to formations of high acid solubility such as limestone, dolomite, etc.. The technique is also applicable to other types of formation such as sandstone containing streaks or striations of acid-soluble components such as various carbonates.

During the acid treating operation, passageways for fluid flow are created in the formation or existing passageways therein are enlarged, thus stimulating the production of fluids from the formation. Acidizing operations wherein the acid is injected into the formation at a pressure or rate insufficient to create crakcs or fractures in the formation are usually referred to as matrix acidizing.

Hydraulic fracturing is also widely used for stimulating petroleum producing subterranean formations and comprises the injection of a suitable fracturing fluid down a well penetrating a formation and into the said formation under sufficient pressure to overcome the pressure exerted by the overburden. This results in creating a crack or fracture in the formation to provide a passageway which facilitates the flow of fluids through the formation and into the well. When the pressure of the fracturing fluid is reduced, a propping agent which is present in the fracturing fluid prevents the complete closure of the fracture. Combination fracture-acidizing processes are well known in the art.

Gelled acidic compositions containing crosslinked polymers have been utilized during the acidizing or fracture-acidizing of a subterranean formation. These compositions present a significant problem during the acidizing or fracture-acidizing of a formation where the bottom hole temperature or the enviromental temperature at the locus of the fracture which is to be propagated or enlarged exceeds about 200"F.

For instance, at such an elevated temperature, these compositions, when introduced into the formation for the purpose of fracturing the formation, undergo gel instability which is evidenced by precipitation and/or syneresis (bleeding or medium separation). The phenomenon of syneresis occurs because the space available for solvent molecules, i.e., water or other medium, in the amorphous network of the gel decreases as the fraction increases of molecular chains that participate in forming crystalline regions within the gel. Accordingly, as the "gel sets", it also "settles" and a supernatant layer of fluid becomes visible over the gel phase. This material, when settled, is, in many instances, damaging to the formation.

Other problems occur using gelled acidic compositions containing crosslinked polymers during the acidizing or fracture-acidizing of a suberranean formation with an elevated temperature. In many instances, these polymers undergo degeneration, e.g. decrease in viscosity, which results in poor penetration of the gelled acidic composition into the formation.

We have now devised certain gelled acidic compositions by which the above problem can be reduced or overcome in the acidizing or facture-acidizing substerranean formations.

According to the invention, there is provided a gelled acidic composition consisting essentially of: (a) water; (b) an acid; (c) a water-thickening amount of a water-dispersible copolymer of about 5-60 percent by weight acrylamide and the remainder selected from:

where R is methyl or ethyl; and X- is Cl, Br, I, or CH30SO3;

where R is ethyl or methyl; or

wherein R,R1,R2,R3 are each independently hydrogen, or an alkyl group having from 1 to 5 carbon atoms, and M is hydrogen, sodium, potassium or ammonium; and wherein the weight average molecular weight of said copolymer is from about 5,000,000 to about 20,000,000.

The invention also includes a method of acidizing or fracture-acidizing a subterranean formation comprising contacting said formation with a composition of the invention.

In the compositions, the amount of acid used is that which is capable of, and sufficient for, reacting with a significant amount of acid-soluble components of said formation. The waterdispersible polyer is a copolymer of about 5 to about 60% by weight acrylamide and dialkylaminoethylmethacrylate, a trialkylaminoethylmethacrylate quaternary salts, or acrylamido alkane sulfonic acid. When treating a formation, the composition is maintained in said formation in contact therewith for a period of time sufficient for the acid in said composition to react significantly with the acid-soluble components of said formation and stimulate the production of fluids therefrom.

In the method of the invention for acidizing or fracture-acidizing a subterranean formation, no special difficulty is encountered in handling and pumping the materials required to make the composition, or the composition itself. Surprisingly, the acidizing or fracture-acidizing composition does not exhibit the phenomenon of syneresis at temperatures over 200"F, and excellent penetration of the formation by the acid is achieved.

The preferred copolymer used in the present invention is a copolymer of 20% by weight acrylamide with the remainder from formula (1) wherein: each R is methyl, X is CH30SO3, and said polymer has a weight average molecular weight of about 7,000,000.

Generally, the amount of acidic gelling polymer useful in the composition of the invention can vary widely depending upon the viscosity of the composition of the invention desired in the formation to be treated and the bottom hole temperature of the formation. In general, the amount of thickening or gelling agent used will be in the range of from about 0.10 to about 20 percent weight. There is, however, really not limit on the amount of gelling agent which can be used as long as the gelled acidic composition can be pumped in accordance with the method of the invention.

Generally, the acidic polymer composition will contain an inhibitor to prevent or greatly reduce the corrosive attack of the acid on metal. Any of a wide variety of compounds known in the art and employed for this purpose can be used. The amount of the inhibitor utilized is not highly critical and may be varied widely. Usually this amount is defined as a small but effective amount, e.g., from about 0.10 percent to about 2.0 percent by weight or more of the acidic polymer solution.

When it is desirable to contact the subterranean formation with a foam containing the composition of the present invention, any foaming agent compatible with the composition of the invention and capable of producing foam in an acidic environment may be used. Many of the foaming agents are disclosed in U.S. Patent No. 4,044,083.

Various methods are known in the art for preparing monomers utilized in the composition of the invention. For instance, see U.S. Patent No. 3,573,263 for the preparation of the monomers of formulae (1) and (2) and U.S. Patent No. 3,507,707 for the preparation of monomer of formula (3).

The weight average molecular weight of the polymers useful in the practice in the invention range from about 5,000,000 to about 20,000,000. The most preferred copolymer weight average molecular weight ranges from about 5,000,000 to about 15,000,000.

As used herein and in the claims, weight average molecular weight is defined by the following formula: weight average molecular weight =

where Ci is the concentration of molecular species i, M; is the molecular weight of species i, and the summation is of all species i.

Acids useful in the practice of the invention include any acid which is effective in increasing the flow of fluids, e.g. hydrocarbons, through the formation and into the well. Thus, under proper conditions of use, examples of such acids can include inorganic acids such as hydrochloric acids, hydrofluoric acid, and phophoric acid; C,-C4 organic acids such as formic acid, acetic acid, citric acid, propionic acid, butyric acid and mixtures thereof and combinations of inorganic and organic acids. The concentration or strength of the acid can vary depending upon the type of acid, the type of formation being treated, and the above-stated compatibility requirements, and the results desired in the particular treating operation.Generally speaking, the concentration can vary from 0.4 to about 60 weight percent, depending upon the type of acid, with concentrations within the range of 10 to 50 weight percent usually preferred, based upon the total weight of the gelled acidic compositions. When an inorganic acid such as hydrochloric acid is used, it is presently preferred to use an amount which is sufficient to provide an amount of hydrochloric acid within the range of from about 0.4 to about 35, more preferably at least about 10 percent, weight percent based on the total weight of the gelled acidic composition.

Amounts within the range of about 10 to about 30 weight percent will frequently be practical amounts of use. The acids used in the practice of the invention can contain any of the known corrosion inhibitors, de-emulsifying agents, sequestering agents, surfactants, friction reducers, etc., known in the art. The preferred acids for carrying out the invention are hydrochloric acid, acetic acid, formic acid and mixtures thereof.

The gelled acidic compositions of the invention are aqueous compositions. They normally contain a significant amount of water. The amount of water can vary depending upon the concentrations of the other components in the composition, particularly the concentration of the acid. For example, when an organic acid such as acetic acid is used in a concentration of 60 weight percent, the amount of water present in the composition clearly will be less than when an inorganic acid such as HCI is used in the concentration of about 35 weight percent. Thus no precise overall range of water content can be set forth. Based on the above-stated overall ranges for the concentrations of the other components, the water content of the compositions can be in the range of from about 30 to 99, frequently about 60 to 90, weight percent. However, amounts of water outside the ranges can be used.

Propping agents which can be used in the practice of the invention include any of the known in the art, e.g., sand grains, walnut shell fragments, sintered bauxite, tempered glass beads, nylon pellets or any mixture of two or more thereof and similar materials. Such agents can be used in concentrations of about 0.1 to 10 pounds per U.S. gallon of fracturing fluid. In general, propping agents with particle sizes of 6 mesh to about 400 mesh, more preferably 8 to 100, and most preferably 1 2 to 60 mesh are employed.

Any suitable method can be employed for preparing the gelled acidic composition of the invention. Thus, any suitable mixing technique or order of addition of the components of the composition to each other can be employed and will provide a composition having sufficient stability to degeneration by the heat of the formation (to which the composition is to be used) and to provide good penetration of the composition into, and significant etching of, said formation. It is ordinarily preferred, however, to first disperse the polymer in a non-hydrating compound such as an alcohol or an oil before contacting the polymer with water or acid.Thus, it is within the scope of the invention to moisten or slurry the polymer with a small amount of a low molecular weight alcohol, e.g., C, to C3 alcohols, or a hydrocarbon such as diesel oil or mineral oil as a dispersion aid to disperse the polymer in the water or acid solution. The gelled acidic composition of the invention may be held at ambient temperatures for several days prior to its introduction into the formation.

The gelled acidic composition of the invention can be prepared on the surface and a suitable tank equipped with suitable mixing means. The composition is then pumped down the well and into the formation employing conventional equipment for pumping acidic compositions. It is, however, within the scope of the invention to prepare the compositions while they are being pumped down the well. Thus, a dispersion of the polymer and water can be prepared in a tank adjacent to the well head and then, a few feet down stream from the tank, a connection can be provided for introducing the acid into the polymer dispersion.

The composition is next introduced into the subterranean formation. The acid is introduced into the subterranean formation whereby the calcareous rock in the formation is dissolved thereby increasing the permeability, and permitting better flow of fluids through the formation.

The pumping rate and pressure utilized will obivously depend upon the characteristics of the formation and whether fracturing of the formation is desired. After the gelled acid composition has been injected in this manner, the well will normally be shut in and allowed to stand for a period ranging from several hours to a day or more. If there is pressure on the well, pressure is then released and the spent acid composition containing salts formed by the reaction of the acid composition, is permitted to flow back into the well bore and is pumped or flowed to the surface. The well may thereafter be placed on production or used for other purposes.

In order that the invention may be more fully understood, the following Example is given by way of illustration only.

EXAMPLE The effect of temperature on the viscosity of various polymers in a 20 percent hydrochloric acid was determined by first homogeneously mixing the components with water and heating the resulting fluid from 1 00 F to 250"F in a Fann Model 50 Viscometer while taking the 50 rpm dial reading, using a number 1 spring and sleeve at a temperature indicated on Table I. This reading is reported in Table I as the shear stress extered by the fluid in Ib/ft2. All samples contained 1 percent by weight of the polymer.

Sample A contained a copolymer made up of 20 percent by weight acrylamide and 80 percent by weight of trimethylaminoethyl methacrylate quaternary salt. Samples B and C contained a copolymer containing 60 percent by weight acrylamide and 40 percent by weight 2-acrylamido-2-methylpropane sulfonic acid. Samples D, E and F contained 60 percent by weight acrylamide and 40 percent by weight 2-acrylamido-2-methylpropane sulfonic acid. The weight average molecular weight of samples A, B, C, D, E, and F was greater than 5,000,000 but less than 10,000,000. In addition, samples D, E and F were crosslinked using formaldehyde. The results of these tests are shown in Table I.

TABLE I Shear Stress lb/ft Formaldehyde Sample GRAM/100 cc 100 F 125 F 150 F 175 F 200 F 225 F 250 F Observations A 0 7.@ 7.5 7.3 6.5 6.0 5.0 4.8 The gel remained elastic and clear at over 200 F B 0 14.3 14.0 13.0 11.2 10.5 8.5 7.0 The gel remained elastic and clear at over 200 F C 0 15.3 15.0 14.3 13.0 11.5 10.3 8.8 The gel remained elastic and clear at over 200 F D 0.185 16.7 16.2 14.2 12.7 13.0 39.0 90.0 The gel became cloudly with substantial syneresis at over 200 F E 0.093 18.0 18.0 17.2 15.5 15.0 18.0 27.5 The gel became cloudly with substantial syneresis at over 200 F F 0.037 19.2 18.5 16.2 13.2 13.1 13.2 31.5 The gel became cloudly with substantial syneresis at over 200 F The results of the Table show that the composition of the invention, Samples A, B, and C, were very stable at high temperatures including 250"F while samples D, E, and F, which were crosslinked using formaldehyde, displayed syneresis at over 200"F.

The metric equivalents of the units used herein are as follows: "F "C 200 93 100 38 250 121 125 52 150 66 175 79 225 107 Ibf/ft2 kgf/m2 7.8 38.1 7.5 36.6 7.3 35.6 6.5 31.7 6.0 29.3 5.0 24.4 4.8 23.4 14.3 69.8 14.0 68.3 13.0 63.4 11.2 54.7 10.5 51.2 8.5 41.5 7.0 34.2 15.3 74.7 15.0 73.2 14.3 69.8 13.0 63.4 11.5 56.1 10.3 50.3 8.8 42.9 16.7 81.5 16.2 79.1 14.2 69.3 12.7 62.0 13.0 63.4 39.0 190.3 90.0 439.2 18.0 87.8 17.2 83.9 15.5 75.6 15.0 73.2 27.5 134.2 19.2 93.7 18.5 90.3 16.2 79.1 13.2 64.4 13.1 63.9 31.5 153.7

Claims (13)

1. A gelled acidic composition consisting essentially of: (a) water; (b) an acid; (c) a water-thickening amount of a water-dispersible copolymer of about 5-60 percent by weight acrylamide and the remainder selected from:

where R is methyl or ethyl; and X- is Cl, Br, I or CH30SO3;

where R is ethyl or methyl; or

wherein R,R1,R2,R3 are each independently hydrogen, or an alkyl group having from 1 to 5 carbon atoms, and M is hydrogen, sodium, potassium or ammoninum; and wherein the weight average molecular weight of said copolymer is from about 5,000,000 to about 20,000,000.

2. A composition according to claim 1, wherein the acid is hydrochloric acid, hydrofluoric acid, citric acid, acetic acid or formic acid, or any mixtures of two or more thereof.

3. A composition according to claim 1 or 2, wherein the copolymer is a copolymer of acrylamide and the compound of formula:

4. A composition according to claim 1, 2 or 3, wheren the copolymer has a weight average molecular weight of from about 5,000,000 to about 15,000,000.

5. A composition according to any of claims 1 to 4, wherein the copolymer contains about 20 weight percent acrylamide.

6. A composition according to any of claims 1 to 5, wherein the acid is hydrochloric acid, which is present in the composition in an amount of from about 10 to about 30 weight percent.

7. A composition according to any of claims 1 to 6, which also includes a propping agent.

8. A composition according to claim 1 or 2, wheren the copolymer is a copolymer of acrylamide and a compound of formula:

9. A composition according to any preceding claim, wherein the composition is foamed.

1 0. A composition as defined in claim 1 substantially as herein described in the Example.

11. A method of acidizing a subterranean formation, which comprises contacting said formation with a gelled acidic composition as claimed in any preceding claim.

1 2. A method according to claim 11, wherein said composition is introduced into said formation at sufficient pressure to fracture said formation.

13. A method of acidizing a subterranean formation substantially as herein described.