CA1253679A - Hydraulic fracturing process and compositions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Improved fracturing processes are described which use novel aqueous hydraulic fracturing fluids.
The fluids comprise: (a) an aqueous medium, and (b) a thickening amount of a thickener composition com-prising (i) a water-soluble or water-dispersible interpolymer having pendant hydrophobic groups chemically bonded thereto, (ii) a nonionic surfactant having a hydrophobic group(s) that is capable of associating with the hydrophobic groups on said organic polymer, and (iii) a water-soluble electrolyte. Addi-tionally, the fluids preferably contain a stabilizing amount of a thiosulfate salt. As an example, an inter-polymer of acrylamide and dodecyl acrylate was used in combination with a nonionic surfactant (HLB of from 10 to 14) to thicken a dilute aqueous solution of KCl and sodium thiosulfate; the aqueous solution had excellent properties for use as a high temperature hydraulic fracturing fluid.

29, 991-F -30-

Description

ti'79 HYDRAULIC FRACTURING PROCESS AND COMPOSITIONS

This invention pertains to novel compositions of mat-ter and methods of using same in fracturing sub-terranean formations penetrated by a wellbore.

Hydraulic fracturing is a term that has been applied to a variety of methods used to stimulate the production of fluids (e.g. oil, natural gas, brines, etc.) from subterranean formations. In hydraulic fracturing, a fracturing fluid is injected down a wellbore and against the face of the formation at a pressure and flow rate at least sufficient to overcome the overburden pressure and to initiate and/or extend a racture(s) into the formation. The fracturing fluid usually carries a proppant (e.g. 20-40 mesh sand, bauxite, glass beads, etc.) into a fracture which keeps the formation from closing back down upon itself when the pressure is released. The proppant-filled fractures provide permeable channels through which the formation ~fluids can flow to the wellbore and thereafter be with--,:
:~.

~ 29,991-F -1-1~5~6'7~3 drawn. Hydraulic fracturing has been used for many years as stimulation technigue and extensive work has been done to solve problems present at each stage of the process. For example) the fracturing fluid is exposed to high temperatures and/or high pump rates and shear which can cause the fluids to degrade and to prematurely l'drop" the proppant before the fracturing operation is completed. Considerable effort has, therefore, been spent trying to design fluids that will satisfactorily meet these rigorous conditions.

A wide variety of fluids has been developed, but most of the fracturing fluids used today are aqueous-based liquids which have been either gelled or foamed.

Aqueous gels are usually prepared by blending a polymeric gelling agent with an aqueous medium. Most frequently, the polymeric gelling agent of choice is a solvatable polysaccharide. These solvatable poly-saccharides form a known class of compounds which include a variety of natural gums as well as certain cellulosic derivatives which have been rendered hydra-table by virtue of hydrophilic substituents chemically attached to the cellulose backbone. The solvatable polysaccharides therefore include galactomannan gums, glucomannan gums, cellulose derivatives, and the like.

The solvatable polysaccharides have a remark-able capacity to thicken aqueous liquids. Even small amounts are sufficient to increase the viscosity of such aqueous liquids from lO to lO0 times or more. In some instances, the thickened aqueous liquid has suf-ficient viscosity to carry the proppant during the course of the fracturing process and represents a 29,991-F -2-~,, 6'79 satisfactory fracturing fluid. In other instances, however, it is necessary to crosslink the polysac-charide in order to form a gel havi~g sufficient strength and viscosity to carry the proppant. A variety of crosslinkers have been developed to achieve this result at different pH ranges.

The borate ion has been used extensively as a crosslinking agent for hydrated guar gums and othe~
galactomannans to form aqueous gels used in fracturing and other areas. For example, Kern described a cross-linked system in U.S. Patent No. 3,058,909 which was used extensively in the oil and gas industry as a fracturing fluid. A fracturing process which comprised crosslinking, guar-containing compositions on-the-fly with a borate crosslinker was described by Free in U.S.
Patent No. 3,974,077. The borate-crosslinked systems require a basic pH (e.g. 8.5 to lO) for crosslinking to occur.
:~
Other crosslinXing agents were developed ` 20 using certain transition metals. Chrisp described certain of these crosslinked systems in U.S. Patent ~ No. 3,202,556 and U.S. Patent No. 3,301,723. In U.S.
; Patent No. 3,202,556, aqueous solutions of galacto-mannan gums were crosslinked at a pH of from about 5 to 13 with antimony or hismuth crosslinkers. In U.S.
Patent No. 3,301,723 Chrisp described the use of certain titanium, zirconium, and other transition metals as crosslinking agents for galactomannan gums at a pH also in the range from 6 to 13. In both Chrisp patents, a basic pH was used to prepare crosslinked materials having utility in the explosive industry.
,~
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2 9 , 9 9 1- F -3-. ' ,.

~ ` 11. ~536'79 `
- ~ - 71456-27 Another patent whieh deseribed the use of titanium erosslinkers for solvatable polysaeeharides was Tiner et al. in U.S. Patent No. 3,888,312. The erosslinked gels formed by Tiner were said to be useful as fraeturing fluids. The use of sueh erosslinked gells was alleged to overeome the high friction loss experieneed during the pumping of many high viscosity fracturing fluids previously known. This observation eorroborated the diselosure b~ Chrisp in U.S. Patent ~o. 3,301,723 a-t eolumn 10 that erosslinked gels formed using titanium,ehromium, iron, and zirconium crosslinkers had a high surface tension (i.e. stickiness and tackiness are absent), ready workabillty and other desirable physical characteristics.
A class of thickenerswas recently deseribed by 5yamalarao Evani in U.S. Patent No. 4,432,881, entitled "Water-Dispersible Hydrophobic Thiekening Agent". Evani indicated that suehthickeners would be useful in a variety of fluids, ineluding fraeturing fluids.
Reference is also made to the "Handbook of Water-Soluble Gums and Resins" by Robert L. Davidson, Editor as published by MeGraw-Hill, Inc. (1980) for an exeellent treatise on water-soluble polymers whieh includes a diseussion on hydratable (or solvatable) polysaceharides. Reference is also made to "Hydraulic Fracturing"
by G.C. Howard and C.R. Fast, Monograph Volume 2, Henry L. Doherty ` Series, published by the Soeiety of Petroleum Engineers (1970) whieh is an exeellent introduetion to the subjeet of hydraulie fraeturing, even though it is now somewhat dated~

....

~L~S3~7~

According to one aspect of the present invention there is provided in the process of fracturing a subterranean formation pene-trated by a wellbore by injecting a hydraulic fracturing fluid through said wellbore and against said subterranean formation at a flow rate and pressure at least sufficient to initiate and/or extend a fracture into said formation, the improvement comprising using as said hydraulic fracturing fluid an aqueous composition having chemical and physical properties sufficient to render it useful as a hydraulic fracturing fluid at 275F, said composition comprising:
A. an aqueous medium, and B. a thickener composition in an amount sufficient to increase the viscosity of said aqueous medium, said thickener composition comprising:
: ~1) a water soluble or water dispersible . interpolymer having pendant hydrophobic groups chemically bonded thereto and containing, in interpolymerized form, from about 99.0 to 99.4 mole percent of a water soluble monomer or mixture of such monomers and from about 1.0 to about 0.6 mole percent of a water insoluble monomer or mixture of such monomers.
(2) a water soluble or water dispersible nonionic surfactant having an ~ILB of from ~: about 10 to about 14 and having a hydrophobic '~

~L2~36'79 "` -5a- 71456-27 group capable of associating with the hydrophobic groups on said interpolymer, and

(3) a water soluble monovalent inorganic salt, wherein said water-soluble monomer or mixture of monomers is represented by the formula:
: ,R
~CH2-C~
C=O
z wherein R is hydrogen or methyl and Z is NH2 or NH-R"-S03M, where R" is an alkylene group of from 1 to 4 carbon : atoms and M is hydrogen, an ammonium or ` alkali metal ion and said water :: : insoluble monomer or mixture of monomers is represented by the formula:
~: R
i~;
, ~
~CH2-C~
~, C=O
X-R"
wherein X is -O- and R" is an alkyl group of Erom about 8 to about 24 carbon : ~ atoms and R is hydrogen or ethyl.

~: According to a further aspect of the present invention there is :~

, , ', ':, ' , :`

': ~ . ; . .
', ~S~'7~
-5b- 71456-27 provided an aqueous composition having chemical and physical properties sufficient to render it useful as a hydraulic frac-turing fluid at 275F, said eomposition comprising A. an aqueous medium; and B. a thickener composition in an amount sufficient to increase the viscosity of said aqueous medium, said thiekener composition : comprising:
(1) a water soluble or water dispersible interpolymer having pendant hydrophobic groups ehemieally bonded thereto and eontaining, in interpolymerized form, from about 99.0 to 99.4 mole pereent of a water soluble monomer or mixture of such monomers and from about 1.0 to about 0.6 mole pereent of a water in-soluble monomer or mixture of such monomers.
: (2) a water soluble or water dispersible nonionie surfaetant having an HLB
~: of from about 10 to about 14 and having a hydrophobie group eapable of assoeiating with the hydrophobie groups on said inter-polymer, and (3) a water soluble monovalent inorganie salt, wherein said water soluble monomer , ... .; :

~L~53~i~9 or mixture of monomers is represented by the formula:
R

2-C~
C=O
Z
wherein R is hydrogen or methyl and Z is NH2 or NH-R"-S03M, where R" is an alkylene group of from 1 to 4 carbon atoms and M is hydrogen, an ammonium or alkali metal ion and said water insoluble monomer or mixture of monomers is represented by the formula:

R
~CH2-C~
C=O
X-R"
wherein X is -0- and R" is an alkyl group of ~rom about 8-to about 24 carbon atoms and R is hydrogen or methyl.
The fluids are superior to the commercial fracturing fluids which contain organo-metallic crosslinked guar or hydroxy-propylguar. The fluids of this invention can be easily formulated :~ :
~ to achieve an acceptable initial viscosity, and the initial ; viscosity will be retained by the fluid for an extended time even i, ~

lZ536'~9 ~- _7_ 71456-27 under conditions of high temperature and/or shear. The stable rheology of the present fluids will result in better fracturing treatment design and control of the fracture geometry and pro-ppant placement.
The new fluid compositions are formulated by blending the aqueous medium with the thickener compositions.
The aqueous medium is usually water, dilute acid (e.g.
up to about 10 percent hydrochloric acid), aqueous alkanols (e.g. aqueous Cl to C3 alkanols), and the like. Water is pre-ferred.
The thickener composition comprises a water-soluble or water-dispersible interpolymer having pendant hydrophobic groups chemically bonded thereto. The interpolymer contains, in inter-polymerized form, from 99.0 to 99.4 mole percent of a water-soluble monomer or mixture of such monomers and from 1.0 to 0.6 mole percent of a water-insoluble monomer or mixture of such monomers. Evani, supra, generically described these interpoly~
mersor(copolymers, as they are sometimes referred to) and a ,~
method(s) for preparing such interpolymers. However only certain of the interpolymers described by Evani can be used herein which meet the above-stated criteria. These are polymers containing at least one of the water-soluble monomers represented by formula R
CH2- ,C ~
C-O (I) Z

~2S3~'79 where R and Z are as defined above.
The water-insoluble monomers are represented by formula II;

R
- ~CH2-C'~
C=O (II) X=R"I

where R, X and R"'are as defined above.
The interpolymers are~sually solid polymers having a number average molecular weight of about one million or more. It has been found that such po1ymer-~ ' ,' a~
~ e~

`' ' `:

6~7~
g are more readily dispersed/ dissolved into the aqueousmedium when ground to a mesh size of at least about 60.
E.g. a mesh size of about 60 to 80 works quite well.

The thickener composition also comprises a nonionic surfactant having a hydrophilic-lipophilic balance (H~B) of from 10 to 14, and preferably from 11 to 12. Such nonionic surfactants constitute a known class of compounds having many~members, any of which can be used herein. Th~s class of compounds is illus-trated, for example, in the handbook of McCutcheons,Combined Edition (published by McCutcheons' Division, MC Publishing Company, Glen Rock, NJ). Blends of non-ionic surfactants can be used, if desired. Blends of nonionic surfactants and anionic surfactants can also be used, and generally are when the interpolymer is a vinyl addition polymer prepared by the preferred process ; in Evani. In Evani's process, an anionic surfactant ~;~ (e.g. sodium lauryl sulfate) is used as an emulsifying agent during the emulsion polymerization and it is present in the dried polymer product.

~- The nonionic surfactants are prepared in many instances by reacting ethylene oxide with a compo~md having active hydrogen (i.e. active in the Zerewitinoff - reaction) and are referred to as "ethoxylated" compounds.
For example, nonionic surfactants have been prepared by reacting ethylene oxide with aIcohols, amides, alkylated--phenols, etc. Preferred nonionic surfactants are ethoxylated aliphatic alcohols, and most preferred are ethoxylated alkanols having from 8 to 24 carhon atoms in the alkanol moiety.
..

~ 29,991-F 9 ., .~ ~i 1~'53~'7~3 The thickener composition additionally com-prises a water-soluble electrolyte. The electrolyte - can be any of the known class of water-soluble electro-lytes. This class includes simple salts of inorganic S and organic acids where the cation and/or anion are monovalent or polyvalent (e.g. NaCl, CaCl2, Na acetate, etc.). Preferably, however, the electrolyte is a monovalent inorganic salt (i.e. both the cation and anion are monovalent); and among these, the sodium, potassium and ammonium halides are a preferred sub group. Pota~sium chloride is the electrolyte of choice in most instances where the fluid composition is to be - used as a fracturing fluid.

The relative amounts of the above-named components in the thickener composition can be varied, generally, however, the interpolymer is included in amounts of from 0.3 to 1.5 weight percent, based on weight of aqueous medium; a preferred amount is from 0.4 to 1.0 weight percent. The nonionic surfactant is normally included in amounts of from 0.06 to 0.3 weight percent, and preferably from 0.08 to 0.2 weight percent, based on weight of aqueous medium. The water-soluble electrolyte is normally used in amounts of from 0.5 weight percent to 4 weight percent, based on weight of aqueous medium and preferably in amounts of rom 0.75 to 1.5 weight percent. From Evani's disclosure, it was a surprise to discover that elevated viscosity fluids could be obtained with the above thickener compositions which utilize substantially lower surfactant levels and that the initial viscosity level could be increased or decreased by adjusting the electrolyte content and/or , the hydrophobe content rather than the surfactant concent~ation. The result of this discovery is increased 29,991-F -10-~a~ 9 polymer efficiency and thermal/shear stability at electrolyte (e.g. salt) concentrations normally associated with fracturing fluids.

The p~ of the fracturing fluids can be varied, but is usually selected in the range of from 6 to 10.
The fluids tend to be more stable at higher temperatures (e.g. 300 to 375 F) where the pH is alkaline. The pH
is preferably from 8 to 10 for high temperature applica-tions.

A wide range of additives can be included, if desired, into the present fracturing fluids. For example, one can include a proppant material (e.g. 20 to 40 mesh sand, bauxite, glass beads, etc.), fluid loss additives (e.g. silica flour, kerosene, diesel, etc.), "energizing" gases (e.g. nitrogen, carbon dioxide, air, etc.) which are comingled with the fluids, breakers ~e.g. persulfate salts, etc.) which reduces the viscosity of the fluid after a period of time, foaming agents (all foaming agents are surfactants, but not vice versa), crosslinking agents (e.g. aldehydes, polyvalent metal ions, etc.), stabilizers (e.g. methanol, an alkali metal or ammonium thiosulfate, etc.), and the like. The amount of additive(s) included can be varied to it the particular need so long as a minimum amount is included to perform the desired function. It is easily within the expertise o one skilled in the fracturing art to determine such quantities.

The skilled artisan will also know that some additives (e.g. an alkali metal or ammonium thiosulfate) are also water-soluble electrolytes and, as such, will influence the fluid rheology as noted above. As a 29,991-E' -11-1~536~79 related aside, the water-soluble thiosulfate salts were particularly efficient in sta~ilizing the present fracturing fluids against thermal degradation and subsequent loss of viscosity. Hence, it is advantageous to include the thiosulfate salts in small but stabilizing amounts. The alkali metal thiosulfates and ammonium thiosulfate are preferred, based on commercial availability and cost effectiveness.

The method of formulating the fracturing fluid can also be varied to convenience. The com-ponents can be "batch mixed" or blended in a "con-tinous" mannex. For example, the thickener composition or components thereof can be added to the aqueous medium in a fracturing tank or similar vessel and the contents of the tank circulated with a pump until thorough blending is achieved. Preferably, however, the fracturing fluids are prepared by a continuous process in which at least one of the components of the thickener (preferably, the interpolymer and/or the soluble electrolyte) is added "on-the-fly" while the fluid is being pumped into the wellbore. The reason for this preference is the lower viscosity of the fluid without one of the key ingredients in the thickener.
This makes it easier to pump the fluid out of its reservoir with lower hydraulic horsepower requirements.
After the material is flowing in the pipeline, it is convenient to add the ingredient(s) and additives to the flowing stream by conventional means; e.g. through a "T" joint of a "Y" joint in the conduit, usually on the discharge side of the high pressure pump(s).
Proppant and fluid loss additives, such as kerosene or diesel, are usually added downstream after the aqueous medium and thickener composition have been blended together.

; 29,991-F -12-~253~i'7~

After the fracturing fluid is formulated, it is injected through the wellbore against the face of ~` the formation at a flow rate and pressure sufficient to initiate and/or extend a fracture(s) into the subterranean formation. This is conventional practice and the methods/hardware of implementing the fracturing process are known. In most fracturing processes, i~ is routine practice to inject a pad fluid of the same co~position (or compatible composition) to establish injectivity and/or to initiate the fracture ahead of the fluid bearing the proppant. Examples of such compatible pad fluids include, aqueous ammonium chloride, dilute hydrochloric acid (e.g. 1 to 5 percent), stiff stable foam having a Mitchel foam quality of 0.60 to 0.85, etc. The use of a fracture pad fluid is good technique and is recommended in conjunction with the use of the present fracturi.ng fluids. Overflush fluids are also normally used and are recommended. Overflush fluids are also of the same composition (or compatible composi-tion) but do not contain proppant. Such overflush1uids are used to clear the conduit and piping in the borehole of proppant-laden fluid and to force the proppant as far into the fracture as possible. The overflush fluid can be the same as the pad fluid or different.

Experimental The following experiments will further illus trate the invention.

Preparation of the Interpolymers The method of preparation described by Evani was used to prepare interpolymers of acrylamide and dodecyl methacrylate. In -this procedure, dodecyl 29,991-F -13-S~'7~

methacrylate is emulsified in deionized water containing sodium laurylsulfate as an emulsifier and a nonionic surfactant C13H27 O ~CH2CH2O~6H, i.e. the condensation product of 6 moles of ethylene oxide onto tridecanol (HLB 11.4). Acrylamide monomer is~added to this emulsion, followed by a che~ant (Versenex 80), a free radical catalyst (Vazo 6 ~, and sufficient dilution-water to increase the volume of the reaction mixture to 250 milliliters. The citrate bottle containing the reaction mixture was then purged several times with nitrogen, sealed, and placed in a hot water bath (69C) for periods of from 4 to 16 hours~ After this heating period, the reaction mixture was removed from the citrate bottle and volatiles were stripped from the polymerization product using a heated drum drier. The resulting dried interpolymer, i.e. the reaction product obtained after the volatiles were removed, was thus obtained as a tough thin film which was later shredded to a desired "particle" size or small flakes. In some instances, the dried interpolymer film was ground to a smaller and more definite particle size which passed through a 60 mesh sieve. To urther illustrate this method of preparation, one such reaction mixture con-tained acrylamide ~46.8 parts of a 51.9 weight percent solution of acrylamide monomer in water), dodecyl methacrylate (O.7 parts), sodium laurylsulfate (25 parts of a 10 weight percent solution of sodium lauryl-sulfate in water), nonionic surfactant C13H27 O
- ~C2H5O~6H (5 parts; Trycol TDA 6 by Emery Chemical Co.), chelant (2.5 parts of a 2 weight percent solution of Versenex in an aqueous acetic acid solvent), Catalyst (1 part of Vazo 64; azobisisobutyronitrile in t-butanol solvent) and water of dilution. This reaction mixture was purged with nitrogen and processed as noted above to give the interpolymer used in Example 1.
~ T~ ar~< .
~ 29,991-F -14-7~

The relative amounts of acrylamide and dodecyl methacrylate were varied in other instances to give interpolymers having different mole percents of the two monomers. Otherwise, the methods of preparation were maintained essentially the same.

Rheology-Testlng Procedures The rheology of the fracturing fluid formula-tions was determined on a Fan ~50C viscometer. Such viscometers are commercial instruments and are widely used in the industry. In this instrument, the sample (50 cc) is loaded into a chamber (cup), a bob eguipped with torque-recording means is immersed in the sample, the chamber containing the sample is pressurized, (400 psi), and the chamber is then rotated at a constant rate of rotation for a pre-determined period of time.
An electronic reading is then recorded showing the amount of torque on the bob which results from the fluid being sheared by the rotating sample cup. Usually the temperature of the sample is increased at a constant rate to the desired test temperature as well. The experimental data from the Fann 50C runs are mathematically treated using the classical Power r.aw Fluid Model equations. In this mathematical treatment the data are plotted on a graph of log (shear stress) as the Y-axis vs log (shear rate) as the X-axis. The experimental points approximate a straight line in this graphical representation; the slope of this straight line is identified as n (the "behavior index") and the inter-cept with the Y-axis is identified as Kv or KViscometer (the 'lconsistency inde~") Kviscometer is K i fl ) by the equatin K(pipe flow) v Once the Power Law constants n and ~ 4n .:
~ r~ k . 29,991-F -15-;~, ~2,536'7~

K' have been determined, the apparent viscosity at any given shear rate is calculated using the equation:
Viscosity (cps) - 47,800 K _. The constant n (shear rate) 1 n (or n' some literature reports) is related to the ; Newtonian behavior of the fluid. If n is 1, then the fluid is called a Newtonian fluidi if n is less than 1, - then the fluid is not Newtonian in its ~ehavior. The constant K (or K' in some literature reports) is a measure of initial viscosity at one (1) reciprocal second shear rate. From the above equation it is seen that K is directly proportional and is considered an important factor when assessing the proppant transport capacity of the fluid, for example.

An "ideal" fracturing fluid is presently envisioned as one having a constant, high viscosity (n _ 0; K~ 0) until the fluid has created the fracture and/or positioned the proppant in the fracture, and then instantaneously "breaks" to form a li~uid having a viscosity the same as water, or less (n = 1; K O).
Such "ideal" fluids are not presently available, but the fracturing fluids of this invention are surprisingly temperature and shear stable and can be tailored to meet various viscosity requirements.
i Preparation of the Fracturing Fluids The test fluids were formulated by adding, with stirring and low heat, the dried and shredded interpolymer to a water solution usually containing small amount of KCl (about 0.1 to 0.2 weight percent) 30 and a nonionic surfactant C12H250 ~CH2CH20~1oH to ; assist in dispersion and hydration. Stirring was 29,991-F -16-'79 continued until a smooth homogeneous mixture was obtained.
Ali~uots of this masterbatch solution were then diluted with water containing various levels of KCl and other additives, such as sodium thiosulfate, to give the fracturing fluid formulations tested below, each of which contained 0.12 weight percent sodium thiosulfate.

In all of the following examples, the test formulations were prepared as noted above and the rheology was determined at 275~F at a shear rate of 170 and/or 511 reciprocal seconds. The test formulations and rheology data are summarized in Table I. All percentages are weight percentages. To further illustrate the preparation of the test formulation, in Example 1 the interpol~mer (2 grams, g) containing a mole ratio of acrylamide/dodecyl methacrylate of 99.4/0.6 was dissolved in 98 milliliters (mL) of an aqueous medium containing 96 mL of deionized water and 2 mL of a 1 weight percent solution of a nonionic surfactant C12H25 0 ~C2H5Otlo~ in water, and 0.2 g of KCl using slow stirring and mild heat; this gave the solution masterbatch.
. Fifty ~50) mL of this masterbatch was diluted with 50 mL of a solution containing 2 weight percent of KCl and 0.24 weight percent of sodium thiosulfate in water;
this gave the test formulation as a liquid solution containing 1.0 weight percent interpolymer, 2.1 weight percent KCl, 0.01 weight percent nonionic surfactant C12H25 0 ~C2H50~1oH, and 0.12 weight percent sodium thiosulfate. The rheology in this test formulation was then determined as indicated above and reported as Example 1 in Table I. All test formulations had a pH
of about ~ to 10.

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~536'7'3 The typical laboratory test for determining the usefulness of a fracturing fluid is by determining the apparent viscosity of the fluid at temperature and at constant shear rate of 170 sec 1 over a period of S several hours. The fluid must maintain a minimum viscosity for the successful creation of the fracture and placement of pr~ppant into the created fracture.
Excessive viscosity is not desirable because it could lead to high friction pressures which can limit treat-ment designs due to hydraulic horsepower or pressurelimitations. ~lso, some control of the geometry of the fracture may be achieved by carefully selecting fluid viscosity and pump rates. The minimim and maximum viscosities allowable for the fluid will vary depending on the individual treatment design which takes into consideration such things as formation properties, size of the desired fracture, amount of proppant to be placed, well temperature, and mechanical limitations.
It is apparent that the minimum and maximum viscosity requirements for fluids can vary considerably; however, for laboratory evaluations some broad useful ranges can be ide~tified. For the purposes of definition, we consider a fluid to be useful, as a fracturing fluid in this invention if it maintains apparent viscosity of at least about 30 centipoise (CPS~ for at least 2 hours at 275F at a shear rate of 170 reciprocal seconds as measured on a Fann 50c viscometer. A11 of the above fluids in Examples 1 to 31 meet this standard.
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,

Claims (11)

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

1. In the process of fracturing a subterranean formation penetrated by a wellbore by injecting a hydraulic fracturing fluid through said wellbore and against said subterranean formation at a flow rate and pressure at least sufficient to initiate and/or extend a fracture into said formation, the improvement comprising using as said hydraulic fracturing fluid an aqueous composition having chemical and physical properties sufficient to render it useful as a hydraulic fracturing fluid at 275°F, said composition comprising:
A. an aqueous medium; and B. a thickener composition in an amount sufficient to increase the viscosity of said aqueous medium, said thickener composition comprising:
(1) a water soluble or water dispersible interpolymer having pendant hydrophobic groups chemically bonded thereto and containing, in interpolymerized form, from about 99.0 to about 99.4 mole percent of a water soluble monomer or mixture of such monomers and from about 1.0 to about 0.6 mole percent of a water insoluble monomer or mixture of such monomers.
(2) a water soluble or water dispersible nonionic surfactant having an HLB of from about 10 to 14 and having a hydrophobic group capable of associating with the hydrophobic groups on said interpolymer, and (3) a water soluble monovalent inorganic salt, wherein said water soluble monomer or mixture of monomers is represented by the formula wherein R is hydrogen or methyl and Z is NH2 or NH-R"-SO3M, where R"
is an alkylene group of from 1 to 4 carbon atoms and M is hydrogen, an ammonium or alkali metal ion and said water insoluble monomer or mixture of monomers is respresented by the formula:

wherein X is -O- and R" is an alkyl group of from about 8 to about 24 carbon atoms and R is hydrogen or methyl.

2. The process defined by claim 1 wherein said nonionic surfactant is an ethoxylated aliphatic alcohol.

3. The process defined by claim 2 wherein said nonionic surfacant is an ethoxylated alkanol; said alkanol having from about 8 to about 24 carbon atoms.

4. The process defined by claim 1 wherein said salt is an ammonium, sodium and/or potassium halide.

5. The process defined by claim 4 wherein said salt is KCl.

6. The process defined by claim 1 which additionally com-prises a stabilizing amount of a water soluble thiosulfate.

7. The process defined by claim 6 wherein said thiosulfate is ammonium thiosulfate and/or an alkali metal thiosulfate.

8. The process defined by claim 1 wherein said fracturing fluid additionally comprises a normally liquid hydrocarbon as a fluid loss additive.

9. The process defined by claim 8 wherein said normally liquid hydrocarbon is kerosene or diesel oil.

10. The process defined by claim 1 wherein said fracturing fluid additionally comprises a particulate solid proppant.

11. An aqueous composition having chemical and physical properties sufficient to render it useful as a hydraulic fract-uring fluid at 275°F, said composition comprising A. an aqueous medium; and B. a thickener composition in an amount sufficient to increase the viscosity of said aqueous medium, said thickener composition comprising:
(1) a water soluble or water dispersible interpolymer having pendant hydrophobic groups chemically bonded thereto and containing, in interpolymerized form, from about 99.0 to about 99.4 mole percent of a water soluble monomer or mixture of such monomers and from about 1.0 to about 0.6 mole percent of a water insoluble monomer or mixture of such monomers.

(2) a water soluble or water dispersible nonionic surfactant having an HLB of from about 10 to about 14 and having a hydrophobic group capable of associating with the hydrophobic groups on said interpolymer, and (3) a water soluble monovalent inorganic salt, wherein said water soluble monomer or mixture of monomers is represented by the formula:

wherein R is hydrogen or methyl and Z is NH2 or NH-R''-SO3M, where R'' is an alkylene group of from 1 to 4 carbon atoms and M is hydrogen, an ammonium or alkali metal ion and said water insoluble monomer or mixture of monomers is represented by the formula.

wherein X is -0- and R'' is an alkyl group of from about 8 to about 24 carbon atoms and R is hydrogen or methyl.