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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)Ca 2+ , Mg 2+ , and SO 4 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 CO 2 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 SO 4 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.

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