Electrostatic and Microstructural Insights into Carbonate Wettability under Activated Carbonated Brines with AOT Surfactant

Abstract This study introduces an integrated approach combining CO2, low-salinity carbonated brine, and sodium dioctyl sulfosuccinate (AOT) surfactant to systematically investigate their synergistic effects on the interfacial, electrostatic, and wettability characteristics of carbonate surfaces. The novelty lies in the concomitant investigation of interfacial, electrostatic, and microstructural analyses to clarify the underlying dominant mechanisms of wettability alteration in activated carbonated systems. The results showed that AOT significantly reduced IFT and accelerated relaxation time in the absence of CO2, particularly in seawater. However, CO2 dissolution increased IFT because of reduced interfacial activity, while simultaneously enhancing wettability alteration toward water-wet conditions. Contact angle and zeta potential analyses demonstrated that low-salinity carbonated brines with a low AOT concentration (50 ppm) achieved the strongest alteration in wettability (with a contact angle of approximately 24°) whereas no remarkable wettability reversal with contact angles of about 140° was observed at a high surfactant concentration (300 ppm) in seawater. Zeta potential analysis confirmed that low-salinity carbonated brines shifted the carbonate rock surface toward more negative values, enhancing electrostatic repulsion at the oil–brine–rock interface and promoting water-wet conditions. SEM and EDAX analyses further confirmed that surface morphology and elemental composition are strongly dependent on the brine–surfactant system. That is, low salinity promoted mineral dissolution and pore exposure, while high salinity and high surfactant concentrations caused surface coverage and reduced efficiency. The results provide a mechanistic basis for designing hybrid brine–surfactant–CO2 formulations for fractured carbonate reservoirs, particularly where wettability and interfacial properties are important controlling factors in fluid displacement.