Effect of Alkalinity on Oil Recovery During Polymer Floods in Sandstone

Abstract

Abstract Alkaline flooding has been purported as 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 allowing 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 k-partition method 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 significant impact on recovery factor, resistance factors and also residual resistance factors. Results of this study can be utilized to improve the engineering design of polymer flooding with the addition of alkali to improve the injectivity and reduce the loss of polymer due to adsorption on the rock surface.