A New Conformance-Improvement-Treatment Chromium(III) Gel Technology

Abstract

Abstract Laboratory testing and evaluation of a new chromium(III) [Cr III] acrylamide-polymer gel technology for conformance-improvement-treatment (CIT) use are reported. This paper is primarily limited to discussing the gel technology as applicable to fracture conformance problems. Several notable features of the gel technology are as follows. The gels, as injected in the field, are a single fluid system. Gels are made by simply adding a single aqueous crosslinking-agent solution to the aqueous polymer solution. The base chemical of the cross-linking agent is a readily available and relatively inexpensive CrIII chemical. An entire family of CIT gels, ranging from highly flowing to rigid rubbery gels, can be produced by varying the formulation of the same chemical set. Thus, the new gel technology is applicable to a wide range of conformance problems. Highly controllable gel times, ranging from minutes to weeks, are possible and can be preselected. Gels have been shown to be stable for extended periods of time when aged at temperatures ranging from 55 to 255F (13 to 124C]. Over the same temperature range, the gels have been shown to possess exceptional yield strengths (resistance to flow) and to be effective plugging agents. The gels are relatively inexpensive because they typically contain 98 to 99.7% water. with the remainder being low cost chemicals. Gels of the new technology are insensitive to oilfield interferences and environments, including H2S. They are compatible with all tested oilfield fluids and equipment and with all tested reservoir rocks and minerals. The gels can be made over a polymer solution pH range of at least 4.0 to 12.5. Gels can be formulated with low-molecular-weight polyacrylamide polymers when low viscosity (watery) treatment fluids are required. The gels can be chemically degraded (reversed). Laboratory studies are described which show the dependence of gelation rate and gel strength on the following parameters:polymer type, concentration, molecular weight, and hydrolysis level;polymer-to-chromium ratio;temperature;polymer solution pH; andsalinity. Gels are shown to exhibit favorable phase-stability, shear, and leakoff properties. A new bottle-testing scheme is described. Bottle testing is used to effectively, rapidly, and inexpensively monitor in a semi-quantitative manner gelation rate and gel strength as a function of time over a broad range of temperatures and gel parameters. Gel viscosities, as determined by dynamic oscillatory measurements, are used to substantiate bottle-testing findings and trends. For selected rigid gels, gel breakdown pressures in porous media and yield pressures are reported. Introduction New Gel Technology The intent of this paper is to report on laboratory testing and evaluation of a newly developed, promising, and patented gel conformance-improvement-treatment (CIT) technology where aqueous gels are formed by crosslinking acrylamide polymers (polyacrylamide or partially hydrolyzed polyacrylamide) with a chromium(III) [CrIII] crosslinking agent. The cross-linking agent is a mixture of complex CrIII ions containing low-molecular-weight carboxylate anions, in association with other anions. The preferred carboxylate anion is acetate. The crosslinking agent, as added to the polymer solution, is predominantly an oligomeric chromium complex ion. The crosslinking agent is highly water soluble and readily mixes with acrylamide-polymer solutions. The preferred CrIII crosslinking-agent base chemical is relatively inexpensive and readily available in the form of a concentrated aqueous solution. Typically, the cost of the crosslinking agent during our field tests has run about 15 to 20% of the polymer costs. In contrast to highly toxic CrVI2-5, CrIII is relatively non-toxic and has been reported to be an "essential micronutrient" for humans. CrIII, in low concentrations, is relatively non-toxic to aquatic life. P. 99^