Carbonated Water Injection in Oil-Wet Carbonate Rock Samples: A Pore-Scale Experimental Investigation of the Effect of Brine Composition

Abstract

Research into the application of carbonated water injection (CWI) for oil recovery has primarily focused on sandstone reservoirs. Therefore, characteristic pore-scale mechanisms that principally depend on the injected brine composition in oil-wet carbonates are still poorly understood. This study uses a pore-scale approach that incorporates core flooding with microcomputed tomography (micro-CT) imaging to investigate underlying mechanisms during CWI with different brine compositions and salinities in oil-wet carbonate rocks. The results reveal that mere carbonation of brine deficient in potential determining ions (PDIs)─Ca2+, Mg2+, and SO42–─does not improve the oil displacement efficiency in oil-wet carbonates. Particularly, certain pore-scale phenomena─wettability reversal and interfacial tension (IFT) reduction─that control the oil displacement efficiency do not significantly change toward desirable states. Extant oil-wet conditions were maintained as we observed a marginal reduction in the average in situ contact angle. In addition, the equilibrium oil–brine IFT was similar to that of oil–brine systems characterized by high brine salinity. Contrarily, enriching low-salinity seawater, containing definite amounts of PDIs, with CO2 promoted superior oil recovery within all pore-size groups with an overall incremental value as high as 24%. This enhanced performance was evidenced by a dominance of wettability reversal to near-neutral states, which created a more favorable capillary pressure required for pore-level displacements. Wettability alteration originated from the reduction in electrostatic attraction between oil–brine and brine–rock interfaces through surface adsorption of SO42– ions in low-pH environments. These observations establish wettability reversal aided by PDIs as the overriding pore-scale oil-recovery mechanism in oil-wet carbonates during CWI.