CA1067686A - Self-breaking viscous aqueous solutions and the use thereof - Google Patents

Abstract

ABSTRACT

Viscous aqueous solutions are prepared which maintain a certain viscosity range over a predetermined period of time (i.e., induction period) and which there-after break, i.e., decrease in viscosity over a very short period of time. The viscous aqueous solution comprises a polysaccharide having an ability to thicken water and dispersed or dissolved in the aqueous solution an effec-tive quantity of an organic ester which, at a temperature at which it is desired to break the viscous solution, hy-drolyzes over a delayed period of time to form an acid which degrades the polysaccharide, thus causing a split-ting of the polysaccharide chain and a decrease in the viscosity of the solution.
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Description

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The present invention resides in a viscous aque-ous solution which may be employed as a fracturing fluid, as a workover fluid, as a drilling fluid and in other pro-cesses where such characteristics are desired.
S More specifically, the present invention resides in a viscous aqueous solution comprising a thickening quan-tity of a polysaccharide and as a breaker an organic ester which hydrolyzes over a certain period of time to release an acid which causes the acid hydrolysis of the polysac-charide with a corresponding splitting of the polysaccharide chain and a decrease in the viscosity of the solution at a sufficient pressure to fracture said formation.
Viscous aqueous solutions are employed in many processes. For example, they are employed to fracture subterranean formations penetrated by a borehole for in-creasing the production of petroleum fluids, e.g. crude oil and natural gas. Viscous aqueous solutions are also employed in the secondary recovery of oil from oil bearing subterranean formations by fluid drive processes. Also, various drilling fluids are based upon viscous aqueous liquids.
In all of these processes it is oftentimes de-sirable to decrease the viscosity of the viscous aqueous solution after a period of time to permit clean up of the formation, disposal of the aqueous solution and the like.
Many "breakers" have been developed to cause a degradation of various thickening agents in such processes. However, most of these breakers immediately start to degrade the polymer thereby lowering the efficiency of the process.
Therefore, these breakers in many instances can only be 17,263-F -1- -10676~16 employed below critical elevated temperature levels or must be injected in a secondary fluid to contact the vis-cous fluid in its subterranean location. There is a need for a viscous aqueous system which can be employed at an elevated temperature and which will maintain a certain viscosity range for a certain period of time and then thereafter break to permit easy recovery and clean up of a formation. The present invention concerns such a discovery.
The invention resides in a viscous aqueous well treating composition which has a certain viscosity for a predetermined period of time at a certain temperature and which subsequently breaks to form a lower viscosity fluid within a short period of time, which comprises a viscous aqueous solution containing a sufficient quantity of a polysaccharide thickening agent, containlng acetal link-ages and which in an aqueous solution is degraded by an acid to provide a solution having a desired viscosity at the temperature of use and an effective quantity of a Cl-C12 organic ester of a carboxylic acid which hydro-lyzes in said a~ueous solution at said temperature of use to produce an acid which degrades the polysaccharide and causes the viscosity of the aqueous composition to be lowered to a desired lower level.
The inven~ion further resides in a method of fracturing a subterranean formation penetrated by a borehole, said formation being substantially free of acid soluble minerals which comprises injecting through said borehole and in contact with said formation a viscous 17,263-F -2-: -7t;E~6 aqueous solution comprising a thickening quantity of a polysaccharide and as a breaker an organic ester which hydrolyzes over a certain period of time to release an acid which causes the acid hydrolysis of the polysac-charide with a corresponding splitting of the polysac-charide chain and a decrease in the viscosity of the solution at a sufficient pressure to fracture said for-mation.
It is known that acetal linkages, i.e. dialkoxy groups attached to the same carbon group, are susceptible to acid hydrolysis and splitting. Polysaccharides, i.e.
high molecular weight carbohydrates, contain many acetal linkages. Polysaccharides are sometimes viewed as con-densation polymers in which monosaccharides or their deri-vatives, e.g. uranic acids or amino sugars, have glycosi-dically joined with the elimination of elements of water.
Polysaccharides which may be employed in the practice of the present invention include natural occurring polysac-charides which are dispersible in cold or hot water to produce viscous solutions. Also included are polysac-charides in water soluble or water swellable forms, which are the derivatives or modifications of natural occurring polysaccharides, e.g. celluloses and various gums, which in their natural form are substantially insoluble in water.
One group of polysaccharides which is within the scope of the invention are the industrial gums such as those generally classified as exudate gums, seaweed gu~s, seed gums, microbial polysaccharides; and hemicellu-loses (cell wall polysaccharides found in land plants) 17,263-F -3-~.

1067~;86 other than cellulose and pectins. Included by way of spe-cific example are xylan, mannan, galactan, L-arabino-xylans, L-arabino-D-glucurono-D-xylans; 4-O-methyl-D-glucurono-D--xylans;L-arabino(4-O-methyl-D-glucurono)-D-xylans; D-gluco--D-mannans; D-galacto-D-mannans and arabino D-galactans, algin, such as sodium alginate, carrageenin, fucordan, laminaran, agar, gum arabic, gum ghatti, gum karaya, tama-rind gum, tragacanth gum, guar gum, locust bean gums and the like. Modified gums such as carboxyalkyl derivatives (e.g. carboxymethyl guar and hydroxyalkyl derivatives, e.g.
hydroxypropyl guar) can also be employed. Modified cellu-loses and starches and derivatives thereof can also be em-ployed. Hereinafter these are referred to as water solu-ble cellulose and starch. There are literally thousands of such materials which have varying properties that can be employed in the practice of the present invention, for example, cellulose ethers, esters and the like.
In general, any of the water-soluble cellulose ethers can be used in the practice of the invention. Those cellulose ethers which can be used include among others, the various carboxyalkyl cellulose ethers, e.g. carboxy-ethyl cellulose and carboxymethyl cellulose (CMC); mixed ethers such as carboxyalkyl hydxoxyalkyl ethers, e.g.
- carboxymethyl hydroxyethylcellulose (CMHEC); hydroxy-alkyl celluloses such as hydroxyethyl cellulose, and hy-droxypropyl cellulose; alkylhydroxyalkyl celluloses such as methylhydroxypropylcellulose; alkyl celluloses such as methyl cellulose, ethyl cellulose and propyl cellulose;
alkylcarboxyalkyl celluloses such as ethylcarboxymethyl 17,263-F -4-1~;76~36 cellulose; alkylalkyl celluloses such as methylethyl cellulose;
and hydroxyalkylalkyl celluloses such as hydroxypropylmethyl cellulose; and the like. Many of the cellulose ethers are com-mercially available in various grades. The carboxy-substituted cellulose ethers are available as the alkali metal salt, usu-ally the sodium salt. However, the metal is seldom referred to and they are commonly referred to as CMC, Cl~XEC, etc. For ex-ample, water-soluble CMC is available in various degrees of carboxylate substitution ranging from about 0.3 up to the maxi-mum degree of substitution of 3Ø In general, CMC having a degree of substitution in the range of 0.4 to 1.2 is preferred.
Frequently, CMC having a degree of substitution in the range of 0.65 to 0.85 is a more preferred cellulose ether. CMC hav-ing a degree of substitution less than the above-preferred ranges is usually less uniform in properties and thus less de-sirable for use in the practice of the invention. CMC having a degree of substitution greater than the above-preferred ranges usually has a lower viscosity ar.d more i9 required in the practice of the invention. The degree of substitution of CMC is commonly designated in practice as CMC-7, CMC-9, CMC-12, etc., where the 7, 9 and 12 refer to a degree of substitution of 0.7, 0.9 and 1.2, respectively.
Organic ester breakers are employed which, at an ele-vated temperature, hydrolyze in an aqueous solution to form an acid or acids which in turn cause the hydrolysis and the break-ing of the polysaccharide. Esters which may be employed in-clude low molecular weight (Cl-C12) esters of organic carboxy-lic acids or mixtures thereof. Both mono and polycarboxylic acid esters can be employed including those which contain chloride substituents which upon hydrolysi~ form both a car-boxylic acid and hydrochloric acid. Diarboxylic acid esters 17,263-F ~5~

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are found generally to be preferred in the practice of the invention because of their initial slow hydrolysis and ac-celerated hydrolysis at elevated temperatures of 150F or higher. Examples of esters which ~ay be employed include methyl formate, ethyl formate, eth~lchloro acetate, triethyl phosphate, butyl formate, diethyl oxalate, dimethyl phthalate, dibutyl phthalate, triglycerol acetate, ethyl trifluoro ace-tate, ethyl trichloro acetate, diethyl maleate, ethyl lactate and diethyl tartrate.
The amount of polysaccharide to be employed in the viscous aqueous solution depends on such factors as the desired maximum viscosity, the intended use of the viscous aqueous solution and the like. For example, if the aqueous solution is to be employed as a fracturing fluid, viscosities ranging from about 20 to about 1000 centipoise measured at 100 RPM on a Fann Model 35 Viscometer at a temperature ranging from about 60 to 200F are desirable. In flooding techniques aqueous solutions having viscosities ranging from slightly greater than water to about 100 centipoise at 60 to 200F
are desirable, and for drilling fluids viscositie in the range of about 5 to about 1000 centipoise at tempera-tures ranging from 60 to 200F are desirable. The exact amount of polysaccharide to be employed will depend on the particular polysaccharide, its ability to increase the viscosity of the fluid, the tempera~ure of the intended use of the fluid and the like. All of these parameters are easily ascertained by simple laboratory experiments and well-known characteristics of the various polysaccharides available on the commercial 17,263-F -6-market. Generally the polysaccharide is employed in an amount ranging from about 10 to about 200 pounds of poly-saccharide per 1000 gallons of aqueous solution.
The amount of organic ester breaker to be em-ployed will vary depending on the particular organic ester employed, the temperature of the viscous aqueous solution when it is to be broken, the time delay that is desired and the like.
The organic ester breaker is employed in an amount ranging from about .02 to about 2.0 equivalent weights of ester per equivalent weight of the polysac-charide. The equivalent weight of the polysaccharide for the purpose of calculating the amount of breaker to be employed is the molecular weight of the monosac-lS charide which makes up the polysaccharide molecule. For example, the equivalent weight of guar gum is 162 (i.e., the molecular weight of a single sugar unit). As another example the equivalent weight of hydroxyethyl cellulose (with a hydroxyethyl group molar substitution of 2.5) is 272. The equivalent weight of the ester is the molecular weight of the ester divided by the number of carboxyl groups in the corresponding acid. For example the equiva-lent weight of diethyl oxala,te is 73.
The following examples indicate how these vari-ous parameters may effect the breaking time and the degree of breakage of various examples of suitable breakers. For any particular use simple laboratory procedures may be employed to ascertain the breaker-polysaccharide system most desired.

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As indicated, the viscous aqueous solution may be employed for various purposes. For example, in frac-turing subterranean formations penetrated by a borehole to increase the recovery of petroleum fluids, viscous aqueous solutions are pumped through the borehole at sufficient pressure to cause the formation to fracture.
Well-known fracturing techniques and equipment may be employed in the practice of the present invention. The following examples will facilitate a more complete under-standing of the practice of the present invention.
Example 1 Various viscous aqueous solutions were prepared by mixing a certain breaker and a thickening agent into water using a Waring Blender on low speed for one minute.
The samples were then placed in a constant temperature bath and the viscosities measured periodically with a Fann Model 35 Viscometer at 100 rpm. In the first series of test~ the aqueous solution contained an equivalent of 60 pounds of a fast hydrating hydroxyethyl cellulose per 1000 gallons of water. These tests are reported in the following Table I. In a second series of tests an aqu-eous solution was prepared containing an equivalent of 60 pounds of a fast hydrating hydroxyethyl cellulose and 40 pounds per 1000 gallons of water of a delayed hydrating 2S glyoxal treated hydroxyethyl cellulose. These tests are reported in the following Table II. In a third series of tests a viscous aqueous solution was prepared containing an equivalent of 60 pounds of the fast hydrating hydroxy-ethyl cellulose and 90 pounds per 1000 gallons of the 17,263-F -8-101~7~

delayed hydrating hydroxyethyl cellulose. These tests are reported in the following Table III.
As a comparison hydrazine sulfate was employed to break a viscous fluid prepared as set forth above for S the second series of tests. Hydrazine sulfate is a well--known oxidizer breaker for cellulose thickening agents.
The results employing hydrazine sulfate are reported in Table IV. A comparison of the results reported in Table II with those of Table IV demonstrate the unique proper-ties of the present invention.

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~0671~86 Example 2 In this example the effect of various esters on the viscosity of an aqueous solution thickened with an equivalent of 60 pounds of guar gum per 1000 gallons of S water at 176F was determined. The viscosity was deter-mined according to the same procedure described in Example l. The amount of breaker is expressed as equivalent of ester per equivalent of guar gum. The equivalent weight of guar was taken as 162 (mol. wt. of the single sugar ur.it). The equivalent weight of the ester was taken as the molecular weight of the ester divided by the number of carboxylic groups in the corresponding acid. The re-sults of these tests are reported in the following Table ~. .

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17, 263-F -17--: , , . . . ~ , . , ~:,. . ` :- ' ' , 7~86 Example 3 , The effect of different concentrations of breaker on the induction period (period over which vi~c08ity remains within a desired range) and breaking time of a viscous aque-ous solution containing an equivalent of 60 pounds of guar gum per 1000 gallons of solution was determined as follows:
Certain amounts of diethyl oxalate were added to different samples of the indicated viscous aqueous solution in a container. The container was then placed in a bath maintained at 176F, and the viscosity of the viscous solu-tion was determined periodically with a Fann viscometer at 100 rpm. The results of these tests are set forth in the following Table VI.

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Example 4 The effect of various quantities of diethyl-oxalate on the induction and breaking time for an aqueous solution thickened with a fast hydrating hydroxyethyl cel-lulose at a fluid temperature of 176F was determined.
The thickened solution was prepared from an aqueous solu-tion having an initial pH of 5.8. The cellulose was em-- ployed in an amount equal to 2.88 gms per 400 ml of water.
The viscosities were determined with a Fann Viscometer at 100 rpm. The results of these tests are reported in the following Table VII.

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7tit36 Example 5 In this example the effect of CaCO3 and NaHCO3 on the breaking of aqueous solutions thickened with hy-droxyethyl cellulose (HEC) or guar gum and containing as a breaker diethyloxalate was determined. The te~ts were conducted at 176F or 200F with aqueous solutions which were thickened with 2.88 grams of the gum per 400 ml of solution. Viscosities were determined periodically with a Fann Viscometer at 100 rpm. The results of the tests are set forth in the following Table VIII.

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This data and Test No. 3 of Example 4 clearly demonstrate the adverse effect of neutralizing ions on the breaking ability of esters. Because of this the be-havior of the system is unpredictable in acid soluble for-mations, e.g., limestone, etc., and therefore it is pre-ferred to employ the viscous aqueous solution of the pres-ent invention in the treatment, e.g.~ fracturing, flooding, etc., of formations which contain substantially no acid soluble minerals. The viscous aqueous solution can be successfully employed to treat sandstone and other simi-lar siliceous formations.

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Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A viscous aqueous well treating composition which has a certain viscosity for a predetermined period of time at a certain temperature and which subsequently breaks to form 2 lower viscosity fluid within a short period of time, which comprises:
a viscous aqueous solution containing a suffi-cient quantity of a polysaccharide thickening agent, con-taining acetal linkages and which in an aqueous solution is degraded by an acid to provide a solution having a de-sired viscosity at the temperature of use and an effective quantity of a C1-C12 organic ester of a carboxylic acid, which hydrolyzes in said aqueous solution at said tempera-ture of use to produce an acid which degrades the poly-saccharide and causes the viscosity of the aqueous compo-sition to be lowered to a desired lower level.

2. The composition of Claim 1, wherein the organic ester consists of a low molecular weight ester of an organic carboxylic acid containing from one to twelve carbon atoms.

3. The composition of Claim 1, wherein the organic ester consists of a dicarboxylic acid ester which has an ac-celerated rate of hydrolysis at an elevated temperature of at least about 150°F.

4. The composition of Claim 1, wherein the poly-saccharide is a cellulose ether.

5. The composition of Claim 4, wherein the organic ester consists of a dicarboxylic acid ester.

6. The composition of Claim 5, wherein the organic ester is present in the composition in an amount ranging from about .02 to about 2.0 equivalent weight of said organic es-ter per equivalent weight of said polysaccharide.

7. The composition of Claim 1, wherein a suffi-cient quantity of polysaccharide is present to provide a viscous aqueous composition having a viscosity ranging from about 20 to about 1000 centipoise at a temperature ranging from about 60° to about 200°F.

8. The composition of Claim 7, wherein the poly-saccharide is a cellulose ether.

9. The composition of Claim 8, wherein the organic ester is a dicarboxylic acid ester.

10. The composition of Claim 9, wherein the organic ester is present in said composition in an amount ranging from about .02 to about 2.0 equivalent weight of said organic ester per equivalent weight of said polysaccharide.

11. A method of fracturing a subterranean formation penetrated by a borehole, said formation being substantially free of acid soluble minerals which comprises:
injecting through said borehole and in contact with said formation the composition of Claim 1 at a sufficient pres-sure to fracture said formation.

12. The method of Claim 11, wherein the organic ester consists of a low molecular weight ester of an organic carboxylic acid containing from 1 to 12 carbon atoms.

13. The method of Claim 11, wherein the organic ester consists of a dicarboxylic acid ester which has an accelerated rate of hydrolysis at an elevated temperature of at least about 150°F.

14. The method of Claim 11, wherein the poly-saccharide is a cellulose ether.

15. The method of Claim 14, wherein the organic ester consists of a dicarboxylic acid ester.

16. The method of Claim 15, wherein the organic ester is present in the composition in an amount ranging from about .02 to about 2.0 equivalent weight of said or-ganic ester per equivalent weight of said polysaccharide.

17. The method of Claim 11, wherein a sufficient quantity of polysaccharide is present to provide a viscous aqueous composition having a viscosity ranging from about 20 to about 1000 centipoise at a temperature ranging from about 60° to about 200°F.

18. The method of Claim 17, wherein the poly-saccharide is a cellulose ether.

19. The method of Claim 18, wherein the organic ester is a dicarboxylic acid ester.

20. The method of Claim 19, wherein the organic ester is present in said composition in an amount ranging from about .02 to about 2.0 equivalent weight of said or-ganic ester per equivalent weight of said polysaccharide.