GELLED AQUEOUS COMPOSITIONS

Abstract: Disclosed are gelled aqueous polymer compositions in brine solutions containing an amount of an anionic partially hydrolyzed polyacrylamide capable of gelling in the pre­ sence of a gelling agent characterized in that the gelling agent is a water-soluble cationic polyamide-epihalohydrin resin in an amount sufficient to cause a gel to form and is used, preferably, in brine solutions having a salt concentra­ tion from about 1% to about 10% at a pH of about 3 to about 11. The compositions are useful in the recovery of oil and gas by fracturing and in enhanced oil recovery.

Description: This invention relates to gelled aqueous polymer compositions comprising an anionic partially hydrolyzed acrylamide and a water-soluble cationic polyamide-epihalohydrin resin useful in the recovery of oil and gas by fracturing and in enhanced oil recovery. Oil and gas are generally found in porous or permeable subterranean formations. Once penetrated, oil and gas contained in the pore spaces are displaced into the wellbore by a variety of known methods. However, the production of the oil and gas may be impaired due to the plugging of the channels through which the oil and gas must flow to the wellbore by drilling fluids or as a result of insufficient flow channels to carry the oil and gas to the wellbore. Such well formations are said to have low permeability. An established method used today to stimulate the production of oil from a well having low permeability is fracturing with a hydraulic fracturing fluid, typically, containing propping agents, such as sand, walnut shell fragments and glass beads, to maintain the fracture in a propped condition when the pressure is relieved. This technique comprises the injection of a suitable fracturing fluid into the formation under sufficient pressure to fracture the formation. The resulting fracture provides passageways to facilitate the flow of the oil and gas into the wellbore. During the injection of fluid into the well formation, the fracturing fluid must have and must maintain the properties necessary to give fractures of reasonable lengths and widths. This is referred to as the fluid efficiency of the fracturing fluid. Such properties are related to the fluid loss characteristics and the viscosity of the particular fluid used. One way to minimize the loss of water to the walls of the wellbore and the fracture is to increase the viscosity of the fracturing fluid. The use of gelled polymer solutions to improve the fracturing fluid efficiency has been taught. In fact, water-insoluble gels are the state-of-the-art in fracturing fluids. Such gels are also formed by crosslinking the water-soluble polymers with a polyvalent metal, metal salt or complex which forms metal coordinate bonds with the polymer, such as are disclosed in U.S. Patents 4,018,286 and 4,043,921. This invention eliminates the need to use polyvalent metals, metal salts and complexes. Gelled polymer compositions can also be used as plugging agents in enhanced oil recovery. For example, the gelled polymer compositions can be placed in high permeability zones in the formation so that subsequent treatment is forced into the low permeability zones, thereby giving better sweep efficiency by increasing the contact with, and removal of, oil. Gelled polymer compositions can also be used as mobility control or "pusher" materials in post-primary oil recovery operations. For example, in surfactant flooding, gelled polymer compositions can be injected as slugs to push previously injected surfactant slugs to recover oil at a producing well. None of the known materials provide a gelled polymer composition having the gel strength and thermal stability necessary to withstand the varied conditions encountered in the use of fracturing fluids, plugging agents and mobility control materials. Various water soluble cationic polyamide-epichlorohydrin resins and water soluble polymers are known. Such are disclosed in particular in U.S. Patent 3,224,986 and British Patent 1,416,957. There has been no suggestion that such would be of any assistance in improving fracturing fluid technology. The present invention provides such improvement by the use as an oil and gas recovery fracturing fluid of a gelled aqueous polymer composition in a 1 to 10%, preferably 1% to 6%, brine solution containing an anionic partially hydrolysed polyacrylamide capable of gelling in the presence of a gelling agent, characterised in that the anionic partially hydrolysed polyacrylamide is a copolymer of acrylamide and sodium acrylate comprising 5 to 95 mole % sodium acrylate and the gelling agent is a water soluble cationic polyamide epihalohydrin resin (polyalkylene polyamine-dicarboxylic acid) comprising 0.9:1 to 1.2:1 polyalkylene polyamine: dicarboxylic acid and from 0.9 mole to 1.5 moles epichlorohydrin per mole of secondary amine group wherein the fracturing fluid is used at a pH of from 5 to 9. The length of time which a polymer gel can maintain its integrity depends on the depth of the well and on the thermal stability of the gel at that depth. The temperature of the well formation varies as a function of depth. The deeper the well, the higher the temperature at the bottom of the well. For example, the typical temperature range of the bottom of a well 610m (2,000 feet) deep is 32-33°C (89-91°F) whereas a well 3050m (10,000 feet) deep generally has a bottom hole temperature range of 60-93°C (141-200°F), and a well 5490m (18,000 feet) deep a bottom hole temperature range of 93-196°C (199-385°F). At the present time, most wells drilled are under 3050m (10,000 feet) in depth. The thermal stability of a polymer gel depends on the strength of the polymer-polymer bonds and on the stability of the polymer backbone to degradation. The superior gel strength of the gelled compositions used in this invention is believed to result from the electrostatic interaction of the cationic polyamide-epihalohydrin resin and the anionic partially hydrolyzed polyacrylamide. The anionic partially hydrolysed polyacrylamide is a copolymer of acrylamide with sodium acrylate containing 5 to 95 mole % sodium acrylate. Molecular weights can range from 100,000 to 10 million or more, preferably 2-10 million or more. The most preferred anionic polyacrylamides are copolymers of acrylamide with sodium acrylate containing 10 to 70 mole % sodium acrylate. The polymers can be prepared by any method practised in the art. Thus, polymerization can be conducted with chemical initiators, such as redox initiators; with thermally-active initiators, such as persulfates, peroxides and azo compounds; with ionizing irradiation, such as ultraviolet light, X-ray, gamma ray, or electron beam. Typical polymerization methods employed include solution polymerization, precipitation polymerization, water-in-oil emulsion polymerization, and water-in-oil suspension polymerization. Typically, the molecular weight of such copolymers ranges from 50,000 to 30,000,000. The preparation of copolymers of acrylamide and sodium acrylate is well known in the art. The amount of anionic polyacrylamide used in the practice of this invention can vary depending on the viscosity and purity of the polymer. In general, a water thickening amount, i.e. that amount which will significantly thicken the water to which it is added, is used. Typically, this amount is from 0.1% to 1.0%, preferably 0.1% to 0.6%. The polyamide-epichlorohydrin resins are generally prepared by the reaction of a dicarboxylic acid with a polyalkylene polyamine to form a water-soluble, long-chain polyamide containing -NH(C n H 2n NH x -CORCO- recurring units, where n and x are each 2 or more, preferably 2 to 4, and R is the divalent organic radical of the dicarboxylic acid. This long-chain polyamide is then reacted with epichlorohydrin to form a water-soluble cationic thermosetting resin. Typical dicarboxylic acids include saturated aliphatic dicarboxylic acids preferably containing 3-8 carbon atoms, such as malonic, succinic, glutaric, adipic, azelaic, and sebacic acid, or blends of 2 or more. Suitable polyalkylene polyamines contain two primary amine groups and at least one secondary amine group and include polyethylene polyamines, polypropylene polyamines, and polybutylene polyamines. Typical polyamines include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, propylenediamine, hexamethylenediamine, and piperazine, or mixtures thereof. The mole ratios of polyalkylene polyamine to dicarboxylic acid are from 0.9:1 to 1.2:1. The aqueous polyamide formed above is converted to a cationic thermosetting resin by reaction with epichlorohydrin. This is generally carried out at 45-100°C until the viscosity of a 25% solids solution at 25°C has reached at least B, and preferably at least D, on the Gardner-Holt scale. It is preferred that all secondary amine groups be converted to tertiary and/or quaternary ammonium groups. There is utilized from 0.9 mole to 1.5 moles of epichlorohydrin per mole of secondary amine group. Polyamide-epihalohydrin resins are prepared in aqueous solution at a pH of from 7 to 9.5. At this pH they have a tendency to gel on standing. The resin solution may be stabilised against premature gelation by adding to the aqueous solution thereof sufficient acid, such as hydrochloric acid, to obtain and maintain the pH at about 2. Such resins are referred to in the art as acid-stabilized resins, and are usually marketed in this form. In forming the gel, the anionic polyacrylamide is generally used at concentrations of 500 ppm to 10,000 ppm polymer solids, based on the total weight of the composition. Preferably 1,000 ppm to 6,000 ppm polymer solids is used. Expressed as a percent, this is usually from 0.05% to 1.0% polymer solids, by weight of the total composition, preferably 0.1% to 0.6%. The polyamide-epihalohydrin resin, preferably polyamide-epichlorohydrin resin is generally used at concentrations of 100 ppm to 5,000 ppm resin solids, preferably 500 ppm to 2,500 ppm. As a percent, this is from 0.01% to 0.5% resin solids, preferably 0.05% to 0. 25%. Exact amounts of each will depend on the molecular weight and mole % anionic groups in the anionic polyacrylamide, ionic strength, temperature, pH, and the strength of gel desired. Some mineral salts must be present in the aqueous solution, since a precipitate will form between the anionic polyacrylamide and the cationic polyamide-epichlorohydrin resin in the absence of salts or ionic strength. The salt concentration is from 1% to 10%, preferably 1% to 6%, most preferably 1% to 4%. Any water-soluble salt may be employed, but generally salts of alkali metals are preferred, such as NaCl, KCl, KBr, Na₂SO₄ and NaNO₃. KCl is preferred. Generally there is an optimum amount of salt that will provide the best gel strength for a given anionic polyacrylamide. For example, for anionic polyacrylamides containing 7 to 8 mole % sodium acrylate groups, about 1 wt. % KCl is best for anionic polyacrylamides containing about 24 mole % sodium acrylate groups, about 2 wt. % KCl is best, and for anionic polyacrylamides containing about 50 mole % sodium acrylate groups 2 to 4 wt.% KC1 is best. As the mole % of sodium acrylate or mole % carboxylate groups to amide groups increases, the optimum salt concentration increases. In use, the pH of the solution is from 5-9. Often a pH near 7 gives the best gel strength. Oilfield brines containing mineral salts with greater than 1,000 ppm solids content (0.1%) may be employed, although sea water and hard brines containing 1,000 ppm (0.1%) or more divalent cations, such as Ca⁺⁺, and having a total dissolved solids (TDS) above 30,000 ppm (3.0%),are unsatisfactory. Gels are formed at about 25°C (75°F) to about 85°C (185°F) or higher. The temperature can govern the rate of gel formation, the maximum gel strength, and the rate of gel decay. Thus, it is often necessary to vary the anionic polyacrylamide and cationic polyamide resin concentrations, the type and amount of salt, and the pH to obtain the rate of gel formation and gel strength desired for a particular oil reservoir at a given temperature. The compositions for use in this invention can be prepared for use as fracturing fluids at the surface in a suitable tank equipped with a suitable mixing means, and pumped down the wellbore into the formation using conventional equipment for such compositions. The compositions can also be prepared "on the fly", i.e. while being pumped down the well. In this case the anionic polyacrylamide solution containing the proppant is prepared in a tank near the well head and pumping of the solution into the wellbore through a conduit is commenced. The solution of the cationic resin in its active form is prepared in a separate tank and is introduced into the aqueous solution stream of the anionic polyacrylamide containing the proppant by a suitable connection a few feet downstream from the anionic polyacrylamide mixing tank. The rate at which the components are introduced into the conduit will depend on the pumping rate of the particular composition of this invention and will be within the skill of those skilled in the art. Alternative methods for combining such fracturing fluid ingredients will also be known to those skilled in the art. The compositions of this invention can also be mixed in solution at the surface to form dilute gels which are readily pumpable. The heat of the wellbore, particularly at the bottom of the wellbore, will activate the polymer-polymer bonding reaction. For shallow fracturing, the composition or the separate polymer and resin solutions can be heated before pumping. The following examples are illustrative of the effect of variation of certain parameters such as pH, acrylate content, temperature etc. Some are outside the scope of the invention with respect, e.g. to pH. All parts and percentages used in this specification are by weight unless otherwise specified. Example 1 This example illustrates an embodiment of the gelled aqueous polymer composition of this invention and how to prepare it. A mixing vessel equipped with a stirrer is charged with 1.2 g of an acrylamide-sodium acrylate copolymer, having a sodium acrylate content of 24 mole %, in 198.8 g of water. Stirring is continued until the polymer is completely dissolved to give a 6,000 ppm polymer solids solution. An acid-stabilized solution of the (diethylenetriamine-adipic acid) polyamide-epichlorohydrin resin is diluted in a separate container to give a 5% active resin solids solution. Another mixing vessel is charged with 41.66 g of the 6,000 ppm polymer solids solution, 51 g of water and 1.0 g KC1 and hand stirred until the ingredients are dissolved and a homogeneous mixture is obtained. Five (5.0) g of the 5% cationic resin solids solution is added and hand stirred until a homogeneous mixture is obtained. The pH is adjusted to pH 7.0 with 10% NaOH and 10% H₂SO₄. Additional water is then added to give exactly 100.0 g total solution containing 2500 ppm of the acrylamide-sodium acrylate copolymer and 2500 ppm of the (diethylenetriamine-adipic acid) polyamide-epichlorohydrin resin. The solution is then stored at a temperature of 40°C (104°F). A medium gel is formed in one day and a medium-heavy gel in 2 days. The gel is still medium-heavy after 3 days. Examples 2-38 These examples illustrate other embodiments of the gelled aqueous polymer compositions of this invention. The procedure and formulation of Example 1 are used to prepare the compositions except that the formulation is changed as set forth in Table 1 below. Examples 39-44 Theses examples illustrate other embodiments of the gelled aqueous polymer compositions of this invention. The procedure and formulation of Example 1 are used to prepare the compositions except that the formulation is changed as set forth In Table II below. The compositions were tested under near optimum conditions, i.e., at pH 7 and at a salt concentration of from about 1% to about 2%. Examples 45-53 Theses examples illustrate other embodiments of the gelled aqueous compositions of this invention. The procedure and formulation of Example 1 are used to prepare the compositions except that the formulation is changed as set forth in Table III below. The compositions were tested under near optimum conditions at high polyacrylamide concentrations and at varying polyamide-epichlorohydrin resin concentrations. The test results in Table III show that gels develop at room temperature (25°C or 77°F) at high anionic polyacrylamide concentrations. Further, the test shows the buildup in gel strength that occurs in the presence of cationic polyamide-epichlorohydrin resins. The aqueous gelled polymer compositions have good thermal stability as indicated by their high thermal breakdown temperatures, T B , and by their storage modulus, G'. G' is a measure of gel strength. A strong gel has a storage modulus of about 100·10⁻⁵ N/cm² (100 dynes/cm²). A weak gel has a storage modulus of about 10·10⁻⁵ N/cm² (10 dynes/cm²). For example, the T B and G', as measured by a Rheometrics pressure rheometer, for the gel of Example 54 formed with 5600 ppm acrylamide-sodium acrylate (24 mole % sodium acrylate), 2500 ppm (diethylene triamine-adipic acid) polyamide-epichlorohydrin resin, and 1.5% KCl at pH 7 is 177°C (350°F) (T B ) and 120·10⁻⁵ N/cm² (120 dynes/cm²)(G'). Thus, this invention provides aqueous gelled polymer compositions having high temperature stability useful in fracturing fluids for the recovery of oil and gas from well formations. The gels are also useful as plugging agents in enhanced oil recovery and in other gas and oil recovery operations. Features, advantages and other specific embodiments of this invention will become readily apparent to those exercising ordinary skill in the art after reading the foregoing disclosures. In this regard, while specific embodiments of this invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as disclosed and claimed.

Claims: Use as an oil and gas recovery fracturing fluid of a gelled aqueous polymer composition in a 1 to 10% brine solution containing an anionic partially hydrolysed polyacrylamide capable of gelling in the presence of a gelling agent, characterised in that the anionic partially hydrolysed polyacrylamide is a copolymer of acrylamide and sodium acrylate comprising 5 to 95 mole % sodium acrylate and the gelling agent is a water soluble cationic polyamide epihalohydrin resin (polyalkylene polyamine-dicarboxylic acid) comprising 0.9:1 to 1.2:1 polyalkylene polyamine: dicarboxylic acid and from 0.9 mole to 1.5 moles epichlorohydrin per mole of secondary amine group wherein the fracturing fluid is used at a pH of from 5 to 9. Use as an oil and gas recovery fracturing fluid in accordance with Claim 1, wherein the fracturing fluid is used at a pH near 7. Use as an oil and gas fracturing fluid in accordance with claim 1 or 2, wherein the brine solution has a salt concentration of from 1% to 6%. Use as an oil and gas fracturing fluid in accordance with any of Claims 1-3, wherein the water-soluble cationic polyamide-epihalohydrin resin is present in an amount from 0.01% to 0.50% resin solids. Use as an oil and gas fracturing fluid in accordance with any of Claims 1-4, wherein the water-soluble cationic polyamide-epihalohydrin resin is a (diethylene triamineadipic acid) polyamide-epichlorohydrin resin.

Inventors: ECHT, ELLIOTT, LEES, RONALD DOUGLAS
Applicants: HERCULES INCORPORATED
Application year: 1985
Main classification: C08L33
IPC codes: C09K8, C08L77, C08L33