Effect of CO2/Brine/Shale Interaction on Shale Water Wettability and Spontaneous Imbibition

Wettability and spontaneous imbibition characteristics play pivotal roles in fluid distribution and migration within reservoirs, profoundly impacting shale oil and gas extraction as well as methane and CO2 occurrence in pore systems and the selection and recovery of fracturing fluids. In this study, Yanchang shale in the Ordos Basin underwent investigation, focusing on ScCO2–brine–rock interactions under varied pressures. X-ray diffraction, low-temperature N2 adsorption, spontaneous imbibition, and contact angle tests were employed to analyze changes in the shale wettability and pore structure pre- and post-ScCO2 exposure. The results indicate significant alterations post-CO2–brine–rock interactions: dissolution of carbonate and select clay minerals, alongside reductions in the shale pore volume (PV), and specific surface area. Expansion of clay mineral water absorption predominantly contributes to PV decreases. The interaction's effects yield differing mass accretions: Mdodecane > Mbrine > Mdistilled water. These changes mitigate water-blocking reactions, enhancing fluid migration and thereby boosting shale gas recovery rates. However, escalating CO2 pressures correlate with heightened shale imbibition masses, potentially diminishing the fracturing fluid recovery efficiency. Furthermore, diminished shale water wettability reduces capillary forces, hindering the CO2 geological storage feasibility. This research underscores foundational insights into advancing integrated CO2-enhanced shale gas extraction and geological storage projects in the Ordos Basin.