Effect of Fluid–Rock Interactions on In Situ Bacterial Alteration of Interfacial Properties and Wettability of CO₂–Brine–Mineral Systems for Geologic Carbon Storage

This study explored the feasibility of biosurfactant amendment in modifying the interfacial characteristics of carbon dioxide (CO₂) with rock minerals under high-pressure conditions for GCS. In particular, while varying the CO₂ phase and the rock mineral, we quantitatively examined the production of biosurfactants by Bacillus subtilis and their effects on interfacial tension (IFT) and wettability in CO₂–brine–mineral systems. The results demonstrated that surfactin produced by B. subtilis caused the reduction of CO₂–brine IFT and modified the wettability of both quartz and calcite minerals to be more CO₂-wet. The production yield of surfactin was substantially greater with the calcite mineral than with the quartz mineral. The calcite played the role of a pH buffer, consistently maintaining the brine pH above 6. By contrast, an acidic condition in CO₂–brine–quartz systems caused the precipitation of surfactin, and hence surfactin lost its ability as a surface-active agent. Meanwhile, the CO₂-driven mineral dissolution and precipitation in CO₂–brine–calcite systems under a non-equilibrium system altered the solid substrates, produced surface roughness, and caused contact angle variations. These results provide unique experimental data on biosurfactant-mediated interfacial properties and wettability in GCS-relevant conditions, which support the exploitation of in situ biosurfactant production for biosurfactant-aided CO₂ injection.