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

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 SO4 2-???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 SO4 2- 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.