Effect of Alkalinity on Oil Recovery During Polymer Floods in Sandstone

Alkaline flooding has been purported to be a promising process for enhancing heavy-oil recovery, while alkaline/polymer (AP) and alkaline/surfactant/polymer (ASP) injection represent commercial flooding strategies for lighter oils. The alkali in an ASP flood can reduce adsorption of surfactants and react with acids in the oil to form soaps. Polymers increase the viscosity of water and control mobility ratio. The addition of an alkali to a straight polymer flood can further increase the efficiency in polymer flooding. The alkali can react with the rock and polymer to reduce polymer adsorption and decrease polymer-solution viscosity to allow higher injectivity. We report results of core experiments for polymer, alkali, and AP tertiary floods. The conditions tested correspond to Wyoming's Minnelusa sandstone reservoirs. Berea cores were waterflooded to residual-oil saturation, and then a tertiary injection of a polymer, alkali, or AP solution was run, followed by waterflooding. We also show results of polymer-solution viscosity with varying alkali concentration. Polymer-adsorption results from dynamic and static experiments with and without alkali are reported. Numerical history match of coreflooding results was performed using CMG-STARS. Results show that a tertiary alkali injection produces negligible oil recovery and pressure-drop increase. Straight polymer injection produces considerable oil recovery with a significant increase in pressure drop that may not be favorable for field designs. The injection of the AP solution also produced considerable oil recovery, but the increase in pressure drop was less than that of the straight polymer flood. The effects of alkali on polymer and rock surface lead to a significant impact on recovery factor, resistance factors, and also residual resistance factors. Results of this study demonstrate one of the benefits of adding alkalis in polymer flooding-namely, the improvement in injectivity-in addition to the known reduction in polymer losses because of adsorption on the rock surface. The modeling strategy should help with alkali-enhanced polymer-flooding designs.