Coupling\nEffects of Surfactant-Induced Interfacial\nTension Reduction and Wettability Alteration in CO<sub>2</sub> Storage

This\nstudy investigated the coupled effects of surfactant-induced\ninterfacial tension (IFT) reduction and wettability alteration on\nreducing residual brine saturation and its implications for CO<sub>2</sub> storage. Two surfactant types were compared: (i) a CO<sub>2</sub>-soluble surfactant (C-ss; isotridecyl ethoxylates TDA-11)\nthat reduces the IFT between CO<sub>2</sub> and the resident brine\nunder constant wettability and (ii) a brine-soluble surfactant (B-ss;\nSurfonic POA-25R2) that induces the coupled effects of IFT reduction\nand rock wettability alteration. The difference in the initial and\nmaximum capillary factors was assumed to represent the surfactant-modified\neffects of IFT and wettability alteration at the pore level. The relative\npermeability and capillary pressure were modulated in the reservoir\nsimulation through interpolation between the lower (nonsurfactant)\nand higher (surfactant) capillary factors, thereby representing surfactant\neffects at the field scale. Subsequently, a reservoir simulation was\nconducted using the MATLAB Reservoir Simulation Toolbox (MRSTaCO<sub>2</sub> lab) to evaluate brine and CO<sub>2</sub> saturation variations.\nC-ss addition resulted in a better surfactant distribution than B-ss,\nenhancing CO<sub>2</sub> storage. Surfactant injection reduced the\nresidual brine saturation near the injection well, increased the maximum\nCO<sub>2</sub> saturation, and improved the injectivity by reducing\nthe bottom-hole pressure (BHP). In addition, we introduced the pore\nspace utilization factor to evaluate the CO<sub>2</sub> storage variation.\nThe surfactant injection increased the level of CO<sub>2</sub> storage\nby optimizing pore space utilization and maximizing the storage efficiency;\nhowever, it reduced the level of CO<sub>2</sub> residual trapping,\npotentially affecting the level of CO<sub>2</sub> storage security.\nTypically, surfactant contact with the reservoir caprock poses a critical\nchallenge to caprock integrity, such as triggering capillary leakage;\ntherefore, surfactant injection should be performed below the reservoir\ncaprock to ensure its integrity.