Coupling Effects of Surfactant-Induced Interfacial Tension Reduction and Wettability Alteration in CO₂ Storage

This study investigated the coupled effects of surfactant-induced interfacial tension (IFT) reduction and wettability alteration on reducing residual brine saturation and its implications for CO2 storage. Two surfactant types were compared: (i) a CO2-soluble surfactant (C-ss; isotridecyl ethoxylates TDA-11) that reduces the IFT between CO2 and the resident brine under constant wettability and (ii) a brine-soluble surfactant (B-ss; Surfonic POA-25R2) that induces the coupled effects of IFT reduction and rock wettability alteration. The difference in the initial and maximum capillary factors was assumed to represent the surfactant-modified effects of IFT and wettability alteration at the pore level. The relative permeability and capillary pressure were modulated in the reservoir simulation through interpolation between the lower (nonsurfactant) and higher (surfactant) capillary factors, thereby representing surfactant effects at the field scale. Subsequently, a reservoir simulation was conducted using the MATLAB Reservoir Simulation Toolbox (MRSTaCO2 lab) to evaluate brine and CO2 saturation variations. C-ss addition resulted in a better surfactant distribution than B-ss, enhancing CO2 storage. Surfactant injection reduced the residual brine saturation near the injection well, increased the maximum CO2 saturation, and improved the injectivity by reducing the bottom-hole pressure (BHP). In addition, we introduced the pore space utilization factor to evaluate the CO2 storage variation. The surfactant injection increased the level of CO2 storage by optimizing pore space utilization and maximizing the storage efficiency; however, it reduced the level of CO2 residual trapping, potentially affecting the level of CO2 storage security. Typically, surfactant contact with the reservoir caprock poses a critical challenge to caprock integrity, such as triggering capillary leakage; therefore, surfactant injection should be performed below the reservoir caprock to ensure its integrity.