The Effect of Temperature on Gelation Time for Polyacrylamide/Chromium (III) Systems

Gelled polymers are being applied to modify the movement of injected fluids in the vicinity of injection and production wells in secondary and enhanced oil-recovery projects. One approach to gelation is to form a bulk gel in situ by injecting a slug of a polyacrylamide polymer solution containing chromium (VI) followed by a polymer slug containing a reducing agent such as sodium bisulfite. Upon mixing, CR(VI) is reduced to Cr(III), and in the subsequent reaction a gel is formed. The gelation time controls the volume of fluid that can be injected in the treatment and thus is an important variable in the process. Gelation time is known to be a function of the concentration of the reactants (chromium ion, reducing agent, and polymer) as well as the polymer type, and some data relating these variables to gelation time have been reported. Another variable affecting the reaction rate is temperature, but no data relating gelation time and temperature have been published. The purposes of the work described in this paper were to obtain experimental data on the effect of temperature on gelation time for typical polyacrylamide/Cr(III) gel systems over the range of temperatures commonly encountered in reservoirs and to develop a method of correlating the data. Gelation times were measured for five different polymers, including polymers with various degrees of hydrolysis and polymers with nonionic, anionic, and cationic character. The temperature range was 25 to 80 deg. C. Polymer, metal ion, and redox system concentrations and salinity also were varied. It was determined that, for a given polymer-reducing agent system at a specified concentration, the gelation time decreases as temperature is increased. The data were correlated in a manner analogous to the Arrhenius method of correlating chemical reaction rate data. That is, plots of the logarithm of gelation time vs. the reciprocal of the absolute reaction temperature were linear over the temperature range studied. By use of a simple nth-order reaction rate model, the slope of the Arrhenius-type plot was related to activation energy. These activation energies were found to vary only slightly for the polymer systems and concentrations investigated. The results have direct application in the design of gel treatments for injection or production wells. The correlation method provides a way of predicting the effect of temperature on the time required for a given system to gel. It is recognized that in field applications factors beyond the scope of data taken in this paper may affect the gelation process. Introduction The volumetric sweep efficiency of a secondary or enhanced oil-recovery process is a major factor in determining the amount of oil recoverable by that process. In waterflooding, low efficiency results in high WOR's that lead to high operating costs in handling produced water relatively early in a project. When the WOR becomes high enough that the project is no longer economically justified, the process is terminated, and a significant amount of oil may be left unrecovered. In enhanced oilrecovery methods that involve the injection of expensive chemicals, low efficiency is even more costly. Economics may not justify the initiation of such a treatment if the expected efficiency is not sufficiently high. Reservoir heterogeneity is the primary reason for poor sweep efficiency. Particularly common are permeability variations in the vertical direction. The injected fluids tend to flow in the zones of higher permeability, bypassing the oil in the tighter zones if the permeability differences are significant. The resulting low sweep efficiency could be improved if the high permeabilities could be reduced. SPEJ P. 463^