A Newly Developed Chromium(lll) Gel Technology

Abstract

Laboratory testing of a recently developed chromium(III) [Cr(lll)] gel technology is reported. The gels can be used in conjunction with a number of oilfield treatments. The single-fluid acrylamide-polymer/Cr(lll)-carboxylate aqueous gels are formed by crosslinking acrylamide polymer with a Cr(lII)-carboxylate-complex crosslinking agent. Representative gel compositions and associated gel properties are discussed. IntroductionA single-fluid Cr(Ill) gel technology has been developed in which aqueous gels are formed by crosslinking acrylamide polymer with a Cr(III)-carboxylate-complex crosslinking agent. 1,2 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(IlI) gel technology. Most of the gels discussed were designed to treat fracture conformance problems in naturally fractured reservoirs. Representative acrylamide-polymer/Cr(lII)-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. Gels are produced by simply adding a single aqueous crosslinking-agent solution to the aqueous 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 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(lIl)-carboxylate-complex crosslinking agent. This includes carboxylate-containing acrylamide terpolymers. Only gels formulated with polyacrylamide (PA) and partially hydrolyzed polyacrylamide (PHPA) are discussed in this paper.The crosslinking agent is a mixture of oligomeric Cr(III) coordinate-covalent-bonded complex ions containing low-MW carboxylate anions. When dissolved in water, a significant portion of the carboxylate anions are bound as ligands to the hexacoordinate Cr(lll) ions. The hexacoordinate Cr(III) ions in the o...

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