Rheological Monitoring of the Formation of Polyacrylamide/Cr+3 Gels

The formation of polyacrylamide/chromium-ion gels has been followed rheologically. The time dependence of the storage modulus has been used to analyze the kinetics of the gelation process. The kinetic theory of rubber elasticity has been used to determine the crosslink density in the gel from the measured value of the storage modulus. The effects of changing polymer, chromium ion, and reducing agent concentrations have been studied. polymer, chromium ion, and reducing agent concentrations have been studied. Introduction Water-soluble polymers of high molecular weight, such as polyacrylamides, polysaccharides, and hydroxyl ethyl celluloses, have been studied as polysaccharides, and hydroxyl ethyl celluloses, have been studied as water flooding additives since the late 1950's. These polymers are added to control fluid movement in reservoirs to improve sweep efficiencies. In addition to enhanced fluid mobility control in porous media by increased viscosities of polymer solutions, the injection of dilute (250 ppm) polyacrylamide solutions causes permeability reductions that persist after polyacrylamide solutions causes permeability reductions that persist after the mobile polymer is flushed from the pore space by water. This reduction in permeability to water is a result of the retention of polyacrylamide in the porous rock by adsorption and mechanical entrapment. Rock permeability also can be reduced deliberately by crosslinking a polyacrylamide solution in situ to form a three-dimensional (3D) gel. The polyacrylamide solution in situ to form a three-dimensional (3D) gel. The gelled polymer is capable of shutting off fractures and zones of high permeability. The rate at which this 3D gel is formed determines how far permeability. The rate at which this 3D gel is formed determines how far the solution can be pushed into the rock formation away from the injection well before gelation occurs. Polyacrylamides are known to form gels in the presence of Cr+3 ions. The process involves the reduction of Cr+6 to Cr+3 with a reducing agent such as sodium bisulfite or thiourea. When Cr+6 is reduced to Cr+3, the trivalent chromium ion and polymer react slowly to form a 3D gel structure. The mechanism by which polyacrylamide or partially hydrolyzed polyacrylamide forms gels in the presence of metal ions is not well polyacrylamide forms gels in the presence of metal ions is not well understood. One idea is that Cr+3 serves as a crosslinking agent between the polyacrylamide molecules. Another suggestion is that Cr+3 forms a stable dispersion in the polymer solution, resulting in either a highly viscous liquid or a gel. Only a limited amount of data has been published on the kinetics of the polyacrylamide/chromium ion gelation process. Terry et al. followed the increase of the steady shear viscosity with time after the introduction of a reducing agent to a polyacrylamide/Cr+6 solution. Gelation time was defined as the time required for the shear viscosity to reach an arbitrary value. The effects of varying polymer type and concentration, Cr+6 concentration, and reducing agent type and concentration were investigated. A linear relationship was found between the reciprocal of the gelation time and the reciprocal of the polymer concentration for a given polymer reducing agent system. The gelation time decreases both with increasing polymer concentration and with increasing Cr+6 and reducing agent polymer concentration and with increasing Cr+6 and reducing agent concentrations. An Arrhenius-type relationship was shown between gelation time and temperature by Willhite and Jordan. During the buildup of a 3D gel network, the shear viscosity increases, but the shearing motion imposed on the sample also tends to break down the network being formed. SPEJ p. 804