Molecular dynamics simulations of shale wettability alteration and implications for CO2 sequestration: A comparative study

Geological CO2 sequestration (GCS) in shale reservoirs is considered an imperative approach to reducing CO2 emissions and mitigating greenhouse gas effects, with the wettability of reservoir rocks playing a critical role in this process. In this study, we comparatively investigated the wetting characteristics of H2O on the organic matter (graphene) and inorganic matter (hydroxylated quartz) surfaces of shale in a CO2 atmosphere utilizing molecular dynamics (MD) simulations and focused on elucidating the effects of CO2 pressure, temperature, and salinity on wettability from the perspectives of the H2O droplet contact angle (CA), gas-water density distributions and interaction energy. The results demonstrate that the H2O droplet is gradually stripped from the graphene surface by CO2 with increasing CO2 pressure, inducing a wetting transition from intermediate-wet to strongly CO2-wet, while the hydroxylated quartz surface consistently exhibits a strongly H2O-wet state. This significant discrepancy in wetting behaviors can be attributed to the differential adsorption capabilities of CO2. The hydrophilicity of graphene and quartz surfaces is enhanced with increasing temperature for the given CO2 pressure condition, and the wettability alteration is more pronounced above the CO2 critical temperature for graphene, while the opposite is observed for quartz. The H2O CA in the CO2-H2O-shale systems only slightly increases with increasing salinity, which is related to the spatial effect of ions in the brine. The results of this work indicate that the CO2-wet feature of shale organic matter during GCS is favorable for the adsorption trapping of CO2 but unfavorable for capillary trapping, while the opposite is true for inorganic matter. This study sheds light on the phenomena and regularity of the wettability alterations of shale organic and inorganic matter by CO2 injection, and the results gained are expected to furnish new insights into CO2 sequestration-enhanced gas recovery (CS-EGR) and gas-water two-phase flow at the nanoscale.