A Newly Developed Chromium(III) Gel Technology

Abstract

Summary Laboratory testing of a recently developed chromium(III) [Cr(III)] gel technology is reported. The gels can be used in conjunction with a number of oilfield treatments. The single-fluid acrylaimide-polymer/Cr(III) carboxylate aqueous gels are formed by crosslinking acrylamide polymer with a Cr(III)-carboxylate-complex crosslinking agent. Representative gel compositions and associated gel properties are discussed. Introduction A single-fluid Cr(III) gel technology has been developed in which aqueous gels are formed by crosslinking acrylamide polymer with a Cr(III)-carboxylate-complex crosslinking agent. The gel technology has been field tested and/or used for a number of oilfield applications, including conformance-improvement treatments (CIT's), water and gas shutoff, zone abandonment, and gel squeeze cementing. The major thrust of our effort has been to apply the gel technology for CIT's, including treating both matrix (profilemodification treatments) and fracture conformance problems. This paper focuses on CIT use of the new Cr(III) gel technology. Most of the gels discussed were designed to treat fracture conformance problems in naturally fractured reservoirs. problems in naturally fractured reservoirs. Representative acrylaniide-polymer/Cr(III)carboxylate gel compositions and associated properties are reported. The laboratory studies reviewed show the dependence of gelation rate and gel strength on (1) the concentration, molecular weight (MW), and hydrolysis level of the polymer; (2) polymer-to-Cr(III) ratio; (3) temperature; (4) polymer solution pH; and (5) salinity. The gels of this technology involve a single-fluid system and do not involve sequentially injected fluids. Oils are produced by simply adding a single aqueous crosslinking-agent solution to the aqueous polymer solution. An entire family of gels, ranging from highly polymer solution. An entire family of gels, ranging from highly flowing to rigid, rubbery gels, can be produced by varying the formulation of the same chemical set. Thus, the gel technology is applicable to a wide range of oilfield problems and uses. Highly controllable gel times, ranging from minutes to weeks, are possible and can be preselected. Gels applicable to both injection and possible and can be preselected. Gels applicable to both injection and production well treatments (e.g., CIT's) can be produced. The gels production well treatments (e.g., CIT's) can be produced. The gels are relatively inexpensive because they typically contain 95 to 99.7% water, with the remainder being low-cost chemicals. Field use has shown the gel technology to be attractive from operational, environmental, and safety standpoints. Almost any water-soluble carboxylate containing polymer can be crosslinked with the Cr(III) carboxylate complex crosslinking agent. This includes carboxylate containing acrylamide terpolymers. Only gels formulated with polyacrylamide (PA) and partially hydrolyzed polyacrylarnide (PHPA) are discussed in this paper. polyacrylarnide (PHPA) are discussed in this paper. The crosslinking agent is a mixture of oligomeric Cr(III) coordinate covalent-bonded complexions containing low-MW carboxylate anions. when dissolved in water, a significant portion of the carboxylate ions are bound as ligands to the hexacoordinate Cr(III) ions. The hexacoordinate Cr(III) ions in the oligomers are bridged through coordinate covalent bonding involving a combination of carboxylate anions and oxygen (often olation). The molar ratio of total carboxylate to Cr(III) ions is less than 3.9:1, usually ranging between 2.3:1 and 3.5:1. The preferred carboxylate anion is acetate. The currently preferred Cr(III) carboxylate complex crosslinking agent is chromic acetate. The preferred crosslinking agent is not simply ionic chromic triacetate, as might be implied by the common nomenclature. Under conditions of our usage, chromic acetate is an oligomeric coordination complexion, believed to be predominantly trimeric in Cr(III). The crosslinking agent used in this study, chromic acetate, contained about a 3:1 molar ratio of acetate to Cr(III). A more detailed discussion of the crosslinking agent chemistry is presented in Ref. 3. Chromic acetate is highly water-soluble and readily mixes with acrylamide-polymer solutions. This commodity chemical is readily available and relatively inexpensive. Chromic acetate is commercially available in the form of a concentrated aqueous solution. The cost of the crosslinking agent during field use typically has been roughly 10 to 20% of the polymer cost. In contrast to highly toxic Cr(VI), reported to be carcinogenic, Cr(III) is relatively nontoxic and has been reported to be an "essential micronutrient" for humans. At low concentrations, Cr(III) is relatively nontoxic to aquatic life. The acrylarnide-polymer/Cr(III) carboxylate gel technology, when applied as CIT's for treating fracture conformance problems, has generated significant amounts of incremental oil profitably, even when oil prices have been depressed. Field use and testing of the gel technology have been implemented without any significant operational, safety, or environmental problems. Field application involves addition of the single crosslinking solution into the polymer solution either by in-line or batch mixing just before injection or use. Results have been reported for a nine-well (seven injection and two production wells) Wyoming Big Horn basin field test program for treating fracture conformance problems. The field test program for treating fracture conformance problems. The field test results appear encouraging. Much of the laboratory testing in this paper involved use of high-MW polymer gels designed for treating fracture conformance problems. These gels are designed to reduce or to eliminate the problems. These gels are designed to reduce or to eliminate the fluid flow capacity of fractures while simultaneously minimally invading the adjacent matrix reservoir rock. Most of the test results presented involved gel formulations for low- and intermediate temperature (i.e., 55 to 140 degrees F [13 to 60 degrees C]) applications. Only a limited amount of testing involved high-temperature (141 to 260 degrees F [61 to 127 degrees C]) gels of this technology. Two other gel technologies that involve Cr(III) crosslinking of acrylamide polymers have been reported. Acrylamide-polymer/Cr(VI) redox gels have been the most widely reported and used. These gels involve the chemical reduction of Cr(VI) to Cr(III) and are sensitive to oilfield interferences and environments. Especially troublesome is the interference by H2S. Cr(VI) redox gels are applicable only over a narrow pH range. Mumallah and Wu and Mumallah reported on gels for CIT's involving propionate-sequestered Cr(III) (chromium propionate) and acrylamide propionate-sequestered Cr(III) (chromium propionate) and acrylamide polymer. As subsequently reported in Refs. 15 and 16, Phillips polymer. As subsequently reported in Refs. 15 and 16, Phillips Petroleum Co. has patented processes for making propionate-sequestered Petroleum Co. has patented processes for making propionate-sequestered Cr(III), which is reported to be a crosslinking agent for CIT gels.