The Effects of Shear History on the Gelation of Polyacrylamide/Chromium VI Thiourea Solutions

Abstract

Summary The influence of shearing during gelation on the gelation rate of a polyacrylamide/chromium(VI)/thiourea solution was studied. Gelations were carried out under steady-state shear. oscillatory shear, and programed shear. When shear rate was constant throughout the experiment, gels formed more slowly at high than at low shear rates. This behavior is opposite to that observed with polyacrylamide/chromium(VI)/bisulfite solutions, where increased shear rates led to faster gelations. The gelation mechanism is believed to involve at least two chemical reaction steps: reduction of chromium(VI) to chromium(III) followed by crosslinking of the polymer by chromium(III). In gelation with bisulfite as the reducing agent, chromium(III) is generated rapidly, and the rate-determining step (which may be polymer-diffusion controlled) is probably the crosslinking of polymer by chromium(III). In contrast, chromium(III) is generated relatively slowly in gelation with thiourea as the reducing agent, and the rate-determining step appears to be the redox reaction. The conclusion that the rate of gelation and strength of the gel formed for the gelation of polyacrylamide by chromium(III) are strongly influenced by the total shear history of the solution has important implications for design of field applications of gelled polymers in EOR. The evaluation of gel characteristics in quiescent bottle tests, a common practice, may not be sufficient to determine whether a gel of the desired strength can be obtained in situ because all gelling solutions must be subjected to considerable shearing during injection. Introduction In-situ gelation of polymers is one of several methods of modifying the permeability of porous rocks to increase volumetric sweep efficiency of waterfloods or other EOR processes. In these processes, a polymer/metal-ion system is injected into the reservoir, where the metal ion interacts with the polymer to form a three-dimensional gel structure. When the polymer is polyacrylamide the metal ion is usually Cr(VI). Gelation is initiated by reducing Cr(VI) to Cr(III) by adding a suitable reducing agent in the injected mixture. Commonly used reducing agents include sodium bisulfite, sodium thiosulfate. and thiourea. H2S, present in some injection and reservoir waters, also acts as a reducing agent. The redox reaction is quite rapid with sodium bisulfite and H2S as reducing agents. Longest gelation times are obtained with thiourea. The redox reaction rate when sodium thiosulfate is used is slower than with sodium bisulfite but faster than with thiourea. Gelation appears to occur by a crosslinking reaction involving Cr(III) that forms polymer aggregates of increasing size as the gel point is approached. Because rheological properties of polymer solutions are quite sensitive to average molecular weight, the rheology of gelling solutions should provide information on kinetics of gelation and properties of the gels. Previous research in our laboratory established correlations between gel times (determined by Brookfield viscometry) and polymer and metal-ion concentrations. Vossoughi et al. studied gelation of a polyacrylamide in the presence of Cr(VI) and bisulfite during continuous steady shear using a Weissenberg rheogoniometer equipped with a come and plate. In this system, the Cr(VI)/bisulfite redox reaction is rapid and gel times on the order of 15 to 20 minutes were obtained. Separate redox reaction rate studies indicated that the amount of chromium released in the first minute of the reaction is an order of magnitude larger than that required for gelation. Thus gelation is controlled by the rate of crosslinking between Cr(III) and polymer molecules. Vossoughi et al. observed that gelation under steady shear followed the same trends observed in Brookfield experiments. Because the gelation occurred in a cone-and-plate viscometer, the entire sample was subjected to about the same shear rate throughout gelation. The gelation rate was found to vary with shear rate. Gels formed more quickly under higher shear rates. Post-gelation studies indicated that gels behaved as Bingham plastics, exhibiting a yield stress under zero shear. Gels that formed under high shear rates were less viscous than those formed under low shear rates. Although stronger gels appeared to be formed at low shear rates, they were more susceptible to shear degradation during post-gelation tests, in which shear rate was varied, than gels-formed at high shear rates. The gelation of polyacrylamide by the bisulfite redox system was also studied by Prudhomme et al. using low-amplitude oscillatory shearing. They determined the effects of polymer and dichromate concentration on the rate of change of the storage modulus. This rate was constant over a long portion of the gelation experiment, indicating that crosslinking of the polymer proceeds at a constant rate after a short induction period. They demonstrated that the oscillatory shearing technique is a useful means of monitoring the buildup of structure in a gelling solution. While the studies of polyacrylamide gelation by chromium with bisulfite as a reducing agent were important, the particular systems studied were ones that gelled very rapidly and were subject to synergists within a few hours of gelation. Gel treatments for in-depth treatment of porous media should gel more slowly to treat areas away from the wellbore and must be stable over a much longer time span. This paper describes rheological studies of polyacrylamide gelation by chromium/thiourea systems. These systems are of interest because the redox reaction is much slower; thus the rate at which Cr(III) is made available for crosslinking is at least an order of magnitude slower than in the bisulfite system. Gelation times on the order of weeks to months can be observed by judicious selection of chemical composition. This paper also presents the rheological data obtained during poly-acrylamide gelation in the laboratory under well-defined shearing conditions. Results are presented for gelation under steady shear, variable (programed) shear, and low-amplitude oscillatory shear. These techniques provide information about gel structure in reacting solutions under various shear conditions.