CO2 Storage Efficiency Improvement Using Interfacially Switchable Surfactant: A Two-Stage Capillary Flip for Greater Residual Trapping

Abstract Saline aquifers are among the promising options for large-scale CO2 sequestration, but their storage efficiency is typically limited to ~5% due to physical constraints governed by gravity, viscous, and mobility during multiphase flow. These factors are sensitive to relative permeability (kr), which is strongly influenced by capillary pressure at the pore scale. Capillary pressure can be adjusted by altering the interfacial tension (σ) between fluids. Residual trapping, a key mechanism for improving long-term storage security, involves two main processes: drainage—where non-wetting CO2 displaces brine and reaches the maximum CO2 saturation (Smax), and imbibition—where brine re-occupies into the pore space, leaving immobile CO2 as residual saturation (St). Lower σ during drainage promotes CO2 invasion by reducing capillary resistance, while higher σ during imbibition promotes the formation of larger, disconnected CO2 clusters, enhancing CO2 trapping. In the current study, we introduce a switchable surfactant system using lauric acid (LA), which reversibly increases σ upon CO2 exposure. Core-flooding experiments are conducted with sandstone cores under conditions (37.5 °C, 1600 psi) to compare this switchable system to a non-switchable baseline (deionized water). The results show that Smax in the switchable system was greater than that of non-switchable deionized water by 25% and the relative permeability of CO2 (krg) is also greater. During imbibition, higher St and a greater trapping ratio (Smax/St) were observed. These findings suggest that switchable surfactants can improve residual CO2 trapping via controlled capillary manipulation.