Mixture of Hydroxypropylcellulose and Poly(maleic Anhydride/Alkene-1) as a Hydrocolloid Gelling Agent

The productivity of oil and gas wells can be improved by increasing the area of communication within a selected producing zone. The drainage area can be increased by hydraulic fracturing of the producing zone to provide fractures and channels emanating from the well base area into the contiguous subterranean formations.

The hydraulic fracturing process is accomplished by rapid pumping of an aqueous fluid medium down a well which penetrates the subterranean formation where fracturing is desired. The rapid pumping of the aqueous fluid creates a hydrostatic pressure which energizes splitting forces in the confined zone. Pressures as high as 10,000 psi are employed to effect formation fracturing.

As cracks and channels are formed, a propping agent which is suspended in the high viscosity hydraulic fluid penetrates the newly created fissures and becomes lodged therein. The function of the propping agent is to support the fractures in an open position as a conduit for the flow of fluids such as oil, gas or water through the fractured zone. Various noncompressible materials are employed as proppants. These include sand, rounded walnut shells, glass beads, aluminum spheres, and the like. U.S. Pat. No. 3,708,560 proposes polyester resin prills as a proppant in hydraulic fracturing fluid systems.

After a fracturing operation has been completed and the propping agent has been deposited, the hydrostatic pressure is released, the flow of fluid is reversed, and the hydraulic fracturing fluid is withdrawn.

Hence, the hydraulic fracturing fluid composition functions to force fracturing under hydrostatic pressure, and it serves to transport the suspension of propping agent into the porous subterranean formations. The hydraulic well-treating fluid medium must exhibit advantageous viscosity and particulate solids transport properties.

Well-treating hydraulic fluids commonly employ a viscosity increasing agent such as a water-soluble polymer to improve the flow characteristics and the suspending ability of the fluids. Carboxymethylcellulose, polyacrylamide, polyvinylpyrrolidone, guar, tragacanth, Irish moss, modified starch, and the like, have been employed as water-soluble or water-dispersible additives to provide pumpable gels having solids suspending properties. U.S. Pat. Nos. 3,483,121; 3,757,864; 3,779,914; 3,818,998; 3,900,069; 3,971,440; and references cited therein, describe well-treating hydraulic fluid compositions which contain novel polymeric friction-reducing gelling components.

U.S. Pat. No. 3,516,496 describes an improved well-completion and workover hydraulic fluid composition comprising an aqueous solution containing a synergistic additive mixture of a polyoxyethylene polymer and powdered marble. The composition is injected into permeable subterranean formations to plug portions thereof temporarily by means of an impermeable filter cake. The filter cake is subsequently removed by backflowing and/or dissolving the solid particles with a pH-adjusting fluid such as hydrochloric acid.

U.S. Pat. No. 3,836,465 describes a viscoelastic hydraulic fluid composition comprising an aqueous medium containing a mixture of polyalkylene oxide and lignosulfonate salt. The composition is recommended as a fluid loss control agent in drilling fluids and as a diverting agent in subterranean strata.

Other well-treating hydraulic fluid compositions are described in U.S. Pat. Nos. 3,765,918; 3,882,029; 3,953,336; 3,988,246; 4,048,079; and 4,038,206. These particular prior art hydraulic fluids have in common the use of a xanthomonas gum as a gelling agent, particularly in combination with another gelling agent. When xanthomonas gum is blended with another hydrocolloid in an aqueous medium, the resultant solution normally exhibits a viscosity which is consistent with the additive viscosities of the two components.

Hydraulic fracturing fluids which contain a natural resin, such as xanthomonas gum or hydroxyethylcellulose, commonly include a breaker additive to effect a delayed action thinning out of the fluid medium. This facilitates the subsequent removal of the hydraulic fracturing fluid composition from the fractured formation.

Breaker additives include enzymes which under the well-fracturing conditions in a delayed manner autonomously reduce the hydraulic fluid viscosity by degrading the natural gum incorporated as a gelling agent. U.S. Pat. Nos. 2,681,704; 2,801,218; 3,615,794; 3,684,710; and references cited therein, describe new enzyme compositions, such as mannan depolymerase which is derived from a microbiologic source.

The disadvantage of employing natural gum as a gelling agent in hydraulic fracturing fluids is the lessening of fluid loss control caused by plugging of apertures which reduces the permeability of a fractured formation. In the case where a natural resin is employed as a gelling agent in combination with an enzyme breaker additive in a hydraulic fracturing fluid, the hydraulic gel system is often unsatisfactory because the hydrolysis residue of the natural gum is present in sufficient quantity to create a plugging effect and reduces the permeability of a fractured formation.

There remains a need for an efficient and economic hydraulic fluid medium which exhibits improved properties in well-treating applications.

The use of hydrocolloids and mixtures of hydrocolloids is also common in various other formulations developed for specific applications in diverse industries.

U.S. Pat. No. 2,868,741 describes a water base stencil duplicating ink composition which includes a mixture of hydrocolloids such as polyacrylic acid and hydroxyethylcellulose.

U.S. Pat. No. 3,325,425 describes a carboxy-containing acrylic interpolymer latex paint formation which includes a hydrocolloid thickener mixture such as methylcellulose and sodium salt of polycarboxylated condensed naphthalene.

U.S. Pat. No. 3,793,272 describes a synthetic resin composition which consists of an emulsion or latex of a vinyl or acrylic resin such as polyacrylic acid in admixture with a hydroxybutylhydroxyethylcellulose derivative.

As is evident from the prior art exemplified hereinabove, hydrocolloid thickened fluid media find important technical application in a broad variety of manufacturing industries. There is continuing research effort to develop novel high viscosity aqueous systems which exhibit improved properties in diverse applications.

Accordingly, it is a main object of this invention to provide a thixotropic fluid composition comprising an aqueous medium and a synergistic mixture of hydrocolloid components, which aqueous composition exhibits a greater viscosity than the sum of the calculated additive viscosity values of the said hydrocolloid components.

It is another object of this invention to provide a partially-synthetic hydrocolloid thickened aqueous medium having improved viscosity and suspending properties for application as a well-treating hydraulic fluid composition.

It is a further object of this invention to provide a well-fracturing hydraulic fluid composition containing a partially synthetic hydrocolloid gelling agent in combination with a breaker additive, which is characterized by a low yield of hydrolysis residue.

Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.

One or more objects of the present invention are accomplished by the provision of a fluid composition comprising an aqueous medium which is thickened with a synergistic viscosity inducing mixture of hydroxypropylcellulose and maleic anhydride/alkene-1 copolymer.

The present invention is based on the discovery that unlike most combinations of natural and synthetic hydrocolloid gums and resins employed for thickening aqueous solutions, a mixture of hydroxypropylcellulose and maleic anhydride/alkene-1 copolymer is capable of inducing an extraordinary viscosity enhancement effect, which usually is at least 100 percent greater than the calculated additive viscosity contributions of the respective hydrocolloid components. It was found that the specific combination of hydroxypropylcellulose with maleic anhydride/alkene-1 copolymer unexpectedly provides a substantial synergistic effect on the viscosity of an aqueous medium, while to the contrary various other combinations of structurally related hydrocolloid mixtures exhibited less viscosity synergism or no viscosity synergism. In some cases, a combination of hydrocolloids as a gelling agent exhibited an observed viscosity value which was less than the sum of the calculated additive viscosity contributions of the respective hydrocolloid components of the gelling agent.

In the practice of the present invention, a synergistic effect can be observed when a mixture of hydroxypropylcellulose and maleic anhydride/alkene-1 copolymer is employed as a gelling agent in an aqueous medium in a quantity between about 0.05 and 5 weight percent, based on the weight of water.

The relative ratio between the hydroxypropylcellulose and maleic anhydride/alkene-1 copolymer components in the gelling agent can vary over a broad range between about 95:5 and 5:95 weight percent of hydroxypropylcellulose to maleic anhydride/alkene-1 copolymer. In order to achieve an incremental synergistic viscosity increase of at least 50 percent over the calculated additive viscosities of the individual gelling agent components as measured in centipoises at 25.degree. C., it is preferred to maintain the relative ratio of the hydroxypropylcellulose to maleic anhydride/alkene-1 copolymer in the gelling agent in the range between about 70:30 and 30:70 weight percent. An incremental synergistic viscosity increase of at least 100 percent can be obtained when the said gelling agent components are in the range between about 60:40 and 40:60 weight percent.

In the practice of the present invention, the mixture of hydroxypropylcellulose and poly(maleic anhydride/alkene-1) can be dry-blended to produce a convenient form of the invention gelling agent. The said gelling agent can then be incorporated into an aqueous medium by the simple expediency of mixing the gelling agent in the medium with stirring. Hydration of the two hydrocolloid components of the gelling agent is usually complete within a period of several hours at room temperature.

In order to achieve solubilization of the poly(maleic anhydride/alkene-1) component of the thickener composition in an aqueous medium, it is advantageous to add an alkaline reagent to the aqueous medium to convert the carboxylic acid anhydride groups of the copolymer into the form of neutral salts. Hence, it is desirable that the ultimate pH of the thickened aqueous composition be in the range between about 4 and 13, and preferably in the range between about 6 and 11. The adjustment of pH, and the hydration of the copolymer component, are facilitated by the addition of an appropriate quantity of alkali metal hydroxide such as sodium hydroxide. If the hydroxypropylcellulose hydrocolloid component of the gelling agent contains an alkaline residue as an impurity from the synthesis reaction of cellulose with propylene oxide in the presence of an alkaline catalyst to produce the hydroxypropylcellulose, then it is an economic advantage to permit the said alkaline residue to aid and effect adjustment of pH of the thickened aqueous composition into the desired range.

As an alternative method of formulating a thickened aqueous composition of the present invention, the respective hydroxypropylcellulose and poly(maleic anhydride/alkene-1) components are first dissolved individually to form separate aqueous solutions, and then the two solutions are blended together.

Hydroxypropylcellulose is commercially produced by reacting alkali cellulose with propylene oxide at elevated temperatures and pressures. Methods of producing hydroxyalkylcellulose are described in U.S. Pat. Nos. 2,572,039; 3,131,196; 3,485,915; and references cited therein.

Scientific literature suggests that etherification proceeds by a substitution reaction mechanism that yields hydroxypropyl substituents containing secondary hydroxyl groups almost exclusively. The secondary hydroxyl groups in the hydroxypropyl substituents in turn are available for further reaction with propylene oxide, which results in the formation of side-chains containing more than one mole of combined propylene oxide.

An idealized structural model for hydroxypropylcellulose with a molar substitution (M.S.) of 3 can be represented by the chemical formula: ##STR1##

The term "molecular substitution" refers to the average number of moles of hydroxypropyl substituted in the cellulose per anhydroglucose unit. For the purposes of the present invention, the average M.S. in the hydroxypropylcellulose hydrocolloid component preferably will vary in the range between about 2 and 5, and most preferably in the range between about 2.5 and 4.

The synergistic viscosity enhancement observed in the high viscosity aqueous fluid media of the present invention appears to increase as the molecular weight of each hydrocolloid component increases. Hence, in the practice of the present invention it is preferred that the molecular weight of the hydroxypropylcellulose component is at least 300,000, and most preferably is at least 800,000. Klucel H (Hercules) is a commercial hydroxypropylcellulose product which is characterized as having a nominal molecular weight of about 1,000,000.

The term "maleic anhydride" as employed herein is meant to include .alpha.,.beta.-olefinically unsaturated dicarboxylic acid anhydride comonomers represented by the structural formula: ##STR2## wherein R.sub.1 and R.sub.2 are independently selected from the group consisting of hydrogen, halogen, cyano, and aliphatic and aromatic substituents such as alkyl, aryl, alkaryl, aralkyl, cycloaliphatic, and the like, containing between one and about ten carbon atoms.

Compounds corresponding to the above formula include maleic anhydride, chloromaleic anhydride, 2,3-dichloromaleic anhydride, cyanomaleic anhydride, 2,3-dicyanomaleic anhydride, methylmaleic anhydride, 2,3-dimethylmaleic anhydride, ethylmaleic anhydride, propylmaleic anhydride, butylmaleic anhydride, 2,3-di-n-butylmaleic anhydride, phenylmaleic anhydride, benzylmaleic anhydride, cyclohexylmaleic anhydride, and the like.

The term "alkene-1" is meant to include alphaolefinically unsaturated hydrocarbons containing between 2 and about 6 carbon atoms. Illustrative of preferred alkene-1 comonomers are ethylene, Propylene, isobutylene, butene-1, pentene-1 and hexene-1.

The ratio of maleic anhydride and alkene-1 comonomers in the copolymer hydrocolloid usually is approximately equimolar (1:1), and in some cases the ratio will vary between about 5:4 and 4:5.

The molecular weight of the copolymer may vary over a range between several thousand and several million. In terms of viscosity as defined in U.S. Pat. No. 3,781,203, the specific viscosity values can vary between about 0.1 and 10.

A particularly preferred type of poly(maleic anhydride/alkene-1) hydrocolloid is a high molecular weight maleic anhydride/isobutylene copolymer having a specific viscosity of at least 2.5 as determined in a solution of one gram of copolymer in one hundred milliliters of methyl ethyl ketone (MEK) at 25.degree. C.

In another of its embodiments this invention accomplishes an important objective by the provision of a hydraulic fluid composition of high viscosity which comprises an aqueous solution having incorporated therein (1) a hydrated mixture of hydroxypropylcellulose and poly(maleic anhydride/alkene-1) hydrocolloids as a gelling agent, and (2) a breaker additive for subsequent reduction of solution viscosity. For the purpose of well-fracturing applications, the preferred hydraulic fluid compositions contain additionally a propping agent such as sand or comminuted walnut shells.

The quantity of gelling agent incorporated in an invention well-treating hydraulic composition can vary in the range between about 0.05 and 5 weight percent based on the weight of the water component. A preferred range is between about 0.1 and 2 weight percent of gelling agent, based on the weight of water.

The breaker additive in the invention hydraulic fracturing fluids is preferably a compound which under formation fracturing conditions autonomously degrades the hydroxypropylcellulose and poly(maleic anhydride/alkene-1) gelling agent components so as to reduce the viscosity of hydraulic fluid which is under hydrostatic pressure. Although the effect of the breaker additive commences immediately upon intimate admixture of the hydroxypropylcellulose/poly(maleic anhydride/alkene-1) gelling agent and the breaker additive, the time required to reduce the solution viscosity by 50 percent can range over a period between about one half hour and two hours. The rate of gelling agent degradation is affected by pH, temperature, and salt content of the hydraulic fluid system.

The breaker additive can be employed in a quantity between about 0.01 and 25 weight percent, based on the weight of gelling agent in an invention hydraulic fluid composition.

One type of breaker additive compounds which can be employed are those which provide an acidic pH to an invention well-treating hydraulic fluid composition. Such breaker additives include inorganic and organic acids, and compounds such as esters which convert to acidic derivatives under well-treating conditions. Illustrative of suitable breaker additives of this type are sulfuric acid, hydrochloric acid, p-toluene-sulfonic acid, acetic acid, triethyl phosphate, methyl formate, ethyl propionate, butyl lactate, and the like. This type of breaker additive can be employed in a quantity between about 0.5 and 20 weight percent, based on the weight of hydroxypropylcellulose/poly(maleic anhydride/alkene-1) gelling agent in a hydraulic fluid composition.

Another type of breaker additive compounds which can be employed are oxidizing agents. Illustrative of suitable breaker additives of this type are ammonium persulfate, potassium dichromate, potassium permanganate, peracetic acid, tertiary-butyl hydroperoxide, and the like. This class of breaker additive can be employed in a quantity between about 0.5 and 20 weight percent, based on the combined weight of hydroxypropylcellulose and poly(maleic anhydride/alkene-1) in a hydraulic fluid composition.

The above-described classes of breaker additive compounds are known in the art. The selection and application of breaker additives in well-treating hydraulic fluid compositions is described in U.S. Pat. Nos. 3,922,173; 3,816,151; 3,960,736; 4,021,355; and references cited therein; and in Hydraulic Fracturing, by G. C. Howard and C. R. Fast, Monograph Series, Volume 2, Society of Petroleum Engineers, Dallas, Texas (1970).

It is to be noted that an enzyme type of breaker additive may be employed in combination with one of the breaker additives described above. Hemicellulase enzyme, for example, can hydrolyze hydroxypropylcellulose at a convenient rate, but it is ineffective for degradation of a poly(maleic anhydride/alkene-1) type of synthetic hydrocolloid resin.

The present invention well-treating hydraulic fluid compositions consisting essentially of (1) an aqueous medium, (2) hydroxypropylcellulose/poly(maleic anhydride/alkene-1) gelling agent, (3) a breaker additive, and (4) a propping agent, are eminently suitable for application as well-fracturing fluid media.

The hydraulic fluid compositions of the present invention exhibit excellent solution stability and heat stability in comparison with the corresponding hydraulic fluid compositions containing as a gelling agent any of the conventional industrial gums or cellulosic derivatives of the types illustrated in U.S. Pat. No. 3,960,736. The invention hydraulic fluid compositions have superior ability to hydrate and develop high viscosity in the presence of salts. Further, a breaker additive can degrade the hydroxypropylcellulose/poly(maleic anhydride/alkene-1) gelling agent at a convenient rate and with a resultant low yield of residue, e.g., a yield of less than about 2 weight percent residue, based on the original weight of gelling agent.

The following examples are further illustrative of the present invention. The reactants and other specific ingredients are presented as being typical, and various modifications can be derived in view of the foregoing disclosure within the scope of the invention.

This Example illustrates a procedure for the production of maleic anhydride/alkene-1 copolymer.

To a closed reactor there is charged one part of maleic anhydride, one part of isobutylene and 5 parts of benzene. To the resultant reaction mixture there is added 0.3 weight percent benzoyl peroxide, based on the weight of monomer charge.

The reaction system is heated at 70.degree. C. for a period up to about five hours until a substantially quantitative yield of maleic anhydride/isobutylene copolymer is produced.

This Example illustrates a general procedure for the production of hydroxypropylcellulose in accordance with U.S. Pat. Nos. 2,572,039 and 3,131,176.

To a slurry of 1 part of finely divided cotton linters, 1.5 parts water and 10 parts of tertiary-butanol is added 0.8 part 50% sodium hydroxide. The mixture is stirred for about 20 minutes, after which time 1.7 parts propylene oxide in 1.5 parts tertiary-butanol are added and the reaction medium is heated at a temperature of about 50.degree.-60.degree. C. for a period of several hours.

Optionally, the resultant product mixture is neutralized with an organic or inorganic acid, and then the product mixture solids are separated from the liquid reaction medium by filtration. The filter cake is stirred with 80% tertiary-butanol aliquots as a purification procedure. The hydroxypropylcellulose product so produced has a nominal hydroxypropyl M.S. of 2.5.

It is to be noted that for purposes of the present invention it is not necessary to neutralize the alkalinity of the above proposed product mixture, since the presence of a basic reagent such as an alkali metal hydroxide is advantageous for promoting the solubilization of poly(alkyl vinyl ether) component when one of the synergistic viscosity fluid media of the present invention is prepared in a subsequent procedure.

This Example illustrates the large viscosity increase in an aqueous medium provided by a synergistic mixture of hydroxypropylcellulose and maleic anhydride/alkene-1 copolymer as a gelling agent.

A 1% aqueous solution of each of hydroxypropylcellulose (Klucel H; Hercules) and maleic anhydride/isobutylene copolymer (Isobam HH; Kuraray Co. Ltd.) are prepared. The viscosity of each solution is determined in centipoises with a Brookfield Viscometer Model RVF, spindle No. 4 at 20 rpm.

In the preparation of the solutions, dissolution of the maleic anhydride/isobutylene copolymer is aided by adjusting the pH of the aqueous medium into the range of about 6-11 with sodium hydroxide.

An equal volume of each solution are blended together and the viscosity of the blend solution is measured.

Under comparable experimental conditions, the following viscosity data are obtained for the indicated hydrocolloid mixtures:

The calculated viscosity of a blend is determined by means of the following equation: ##EQU1## where

.eta.S=viscosity of blend

X=weight percent of first component of blend

.eta..sup.1 =viscosity of first component of blend

.eta..sup.2 =viscosity of second component of blend

As it is apparent from a comparison of the above reported data, a present invention gelling agent consisting of a mixture of hydroxypropylcellulose and poly(maleic anhydride/alkene-1) imparts an unexpectedly large incremental synergistic viscosity increase to an aqueous medium. Similar advantages are achieved when in the copolymer hydrocolloid component of the gelling agent the maleic anhydride comonomer contains a substituent such as methyl or chloro, and the alkene-1 comonomer is ethylene, propylene, butene-1, pentene-1, hexene-1, and the like.