Experimental and modeling study on the transport of chromium(III) acetate solutions through carbonate rocks

Gelled polymer systems are applied to oil reservoirs to increase sweep efficiencies in displacement processes. A common gel system consists of partially hydrolyzed polyacrylamide crosslinked by chromium(III) acetate. Transport of polymer and chromium through the reservoir rock are essential for a successful treatment. In carbonate reservoirs, dissolution of carbonate raises the pH of the gelant to levels where chromium precipitates, robbing the gelant of crosslinker.; The transport of chromium(III) acetate solutions through dolomite rock material was studied by injecting various solutions through dolomite cores and measuring Cr, Ca, and Mg concentrations and pH in the effluent. The effects of residence time and injected chromium(III) acetate concentration on chromium(III) retention were investigated. Retention of chromium(III) was a kinetic process, in that lower flow rates in dolomite cores, or longer residence times, resulted in a larger amount of retained chromium(III). The percentage of chromium precipitation increased with a decrease in the injected chromium(III) acetate concentration. Additional experiments of flowing brine solutions through a dolomite core, and bottle tests studying coprecipitation of calcium and/or magnesium with chromium(III) precipitation and chromium(III) precipitation in solutions with added ground dolomite were conducted to investigate the discrepancy between the simulated and experimental values of calcium and magnesium concentrations.; A mathematical model simulating the transport of brine and chromium(III) acetate solutions through carbonate-containing rocks was developed by fitting parameters of the rate equations for carbonate dissolution and chromium precipitation to experimental data. The mathematical model simulated the steady-state and equilibrium data well for dolomite dissolution in the presence of brine solutions with different pH values and acetate ions. Retention of chromium from several investigators was matched by one set of parameters in the rate equation for chromium precipitation. The mathematical model simulated the steady-state values of the measured Cr(III) concentrations in the effluent for various experimental conditions. The presence of chromium in fluids injected into Baker dolomite cores significantly reduced the dissolution of dolomite, releasing Ca and Mg, as compared to similar injected fluids that contained no chromium. The mathematical model did not simulate dolomite dissolution when in contact with chromium acetate solutions well.