Viscometric Measurement of Chromium(ill)-Polyacrylamide Gels by Weissenberg Rheogoniometer

Abstract

Abstract Gelled polymers are being used increasingly to modify the movement of injected fluids in secondary and enhanced oil recovery processes. A common gelation process involves the reduction processes. A common gelation process involves the reduction of Cr(VI) to Cr(III) in the presence of polyacrylamide. The Cr(III) reacts or interacts with the polymer to form a gel network. Although correlations of gelation time with principal process variables have been obtained, viscometric data have not process variables have been obtained, viscometric data have not been reported during or after gelation. These data are needed for fluid flow calculations in surface equipment and estimation of flow behavior in reservoir rocks. A Weissenberg Rheogoniometer, with cone and plate geometry, was used to obtain viscometric data for the gelation of polyacrylamide and chromium (III). Solutions consisting of polyacrylamide polymer, sodium dichromate-dihydrate and sodium polymer, sodium dichromate-dihydrate and sodium bisulfite were gelled under a steady shear field at constant temperature. The shear stress vs. time profile for the gelation process was interpreted to define a gelation time and to determine process was interpreted to define a gelation time and to determine the apparent viscosity of the gelled fluid. The gelation time decreased as the applied shear rate increased up to about 14.25 sec and was affected by shear rate history. Viscometric properties of the gelled solutions were determined. Apparent viscosity of the gelled solutions decreased as the shear rate under which they were formed increased. Post gelation studies indicated that gels exhibited a residual or yield stress at zero shear rate and behaved as Bingham plastics under steady shear. Gels formed at low shear rates were plastics under steady shear. Gels formed at low shear rates were more viscous than gels formed at high shear rates. However, the structure of these gels was susceptible to shear degradation. Introduction Reservoir heterogeneity is a major factor affecting the performance of waterfloods. The main source of reservoir performance of waterfloods. The main source of reservoir heterogeneity is usually permeability variations in the vertical direction. When there are large differences in permeability, injected fluids tend to flow through regions of high permeability bypassing oil that exists in regions of low permeability. Altering the fluid distribution by reducing the fraction of the water flowing in high permeability zones, termed permeability modification, has potential to increase volumetric sweep efficiency of a waterflood and thus oil recovery. The use of gelled polymers to alter per ability is a rapidly developing technology Gelation of polymer solutions can be achieved by mixing a polymer solution with multi-valent metal components such as chromium and a polymer solution with multi-valent metal components such as chromium and a reducing agent. In this process, Cr(VI) is reduced to Cr(III) by the reducing agent. The trivalent chromium ion and polymer react or interact to form a three-dimensional gel structure. The polymer solution may be injected as a mixture containing metal ion and reducing agent. Another procedure involves injection of the polymer and metal ion followed by a slug containing the reducing agent. Insitu mixing initiates the gelation process. Design of gelled polymer systems requires information about the chemistry of the process, properties of the reacting solution and the gel. Design parameters of interest include:rate of gelation;effects of polymer, metal ton, and reducing agent concentrations on gelation rate;polymer, metal ton, and reducing agent concentrations on gelation rate;temperature effects;viscometric properties during formation of the gel and after gelation; andchemical and mechanical stability of the gel over long time periods. Terry, et al. studied the gelation of polyacrylamide by reducing Cr(VI) to Cr(III) with bisulfite and thiourea. The rate of gelation was inferred from the reading indicated by a Brookfield viscometer with spindle turning at a constant rate in the center of a beaker containing the reacting solution. Gelation time was defined as the time required to reach an arbitrary reading on the viscometer following a steep rise in the viscometer reading. P. 113