The Effect of Ba2+ and Sr2+ on EOR Efficiency in Carbonate Reservoir during Low-Salinity Waterflooding

Abstract

When low-salinity water containing SO42- is injected into carbonate oil reservoirs, rock dissolution and in-situ precipitation occur as chemical equilibria are progressed, altering both permeability and wettability. The Ba2+/Sr2+ ions present in the formation water as impurities chemically react with the SO42- ions, and BaSO4 and SrSO4 are precipitated. In addition, when injected low-salinity water including SO42-, either dissolution of Ca2+-containing minerals or CaSO4 precipitates are occurred. These reactions can cause a serious impact on the efficiency of enhanced oil recovery (EOR). Therefore, the main purpose of this study was to identify EOR efficiency induced by low-salinity waterflooding (LSWF) when Ba2+/Sr2+ were present in a carbonate oil reservoir. From the results of the effluent analysis and material balance calculation with the produced Ba2+, Sr2+, and Ca2+ concentrations, when Ba2+/Sr2+ concentrations were low, permeability was improved because of rock dissolution predominating over in-situ precipitation. These results concurred to the permeability change which was calculated with the measured pressure data. Also, in the analysis of wettability alteration through the measurements of contact angles and relative permeabilities before and after LSWF, higher concentrations of impurities consumed more SO42- in precipitating BaSO4 and SrSO4, resulting in weaker wettability alteration due to the reduction of sulfate activity. These phenomena ultimately influenced EOR efficiency, i.e., the oil recovery was greater as Ba2+/Sr2+ concentration were lower. Therefore, applying LSWF containing SO42- ion to carbonate oil reservoirs is not always a desirable EOR method when Ba2+ or Sr2+ is present, as an impurity, in the formation water