Interfacial Tuning Controls CO2 Occupancy and Connectivity Hysteresis under Flow Reversal.

Geological carbon storage performance depends not only on how much CO2 is emplaced but also on how CO2 is organized at the pore scale. Here, we use synchrotron micro-CT imaging under reservoir conditions to quantify CO2 saturation, cluster connectivity, and interfacial geometry during steady-state fractional-flow drainage and imbibition in a sandstone core. Measurements compared a surfactant-free reference case with an interfacially tuned case using dilute, water-soluble nonionic surfactant in the brine. Interfacial tuning increases CO2 occupancy during co-injection, raising drainage saturation at high CO2 fractional flow from 5.8% to 14.9% and the CO2-only end point saturation from 18.9% to 26.2%. Importantly, topology responds in a regime-dependent, nonmonotonic manner: connectivity dominance increases during co-injection, whereas at the CO2-only end point, additional CO2 distributes among multiple clusters rather than reinforcing a single backbone, demonstrating decoupling between occupancy and connectivity dominance. Upon flow reversal, the two conditions exhibit contrasting connectivity-hysteresis pathways, indicating distinct CO2 reorganization during imbibition. Interfacial metrics support this interpretation, with the tuned case showing 90% larger CO2-brine interfacial area and 44% lower mean curvature. These results show that interfacial tuning can increase CO2 saturation while producing different connectivity outcomes, underscoring the need for topology-resolved metrics to assess storage efficiency and security.