Mineralogical and Surface Responses During CO2–Synthetic Formation Water–Granitic Gneiss Interaction in Bohai Buried-Hill Reservoirs

Offshore buried-hill reservoirs are potential targets for integrated CO2-enhanced oil recovery and geological storage. However, the early-stage response of clay-rich granitic gneiss under reservoir conditions remains poorly constrained. We conducted 7-day static experiments with an illite-rich mineral powder assemblage at 125 °C and 35 or 50 MPa using synthetic formation water. Complementary granitic gneiss discs were reacted at 50 MPa for surface observations. The powders were characterized by XRD, FTIR, and XPS and the discs by SEM–EDS. XRD indicated lower relative abundances of illite after reaction and more pronounced changes in several carbonate and clay phases at 50 MPa. FTIR showed the attenuation of structural O–H, carbonate, and aluminosilicate-related bands, while XPS indicated surface elemental redistribution. SEM–EDS revealed mineral-selective surface alteration and localized particle accumulation. Quartz appeared less reactive than clay minerals and calcite over the experimental duration. The particles may include redeposited fines or secondary products, but their phase identity remains unresolved. Together, the observations characterize early-stage mineralogical and surface responses relevant to evaluating CO2–rock interactions in clay-rich buried-hill reservoirs. Their consequences for injectivity and long-term storage require direct petrophysical and geomechanical evaluation.