Molecular dynamics simulations of shale wettability alteration and implications for CO 2 sequestration: A comparative study

Geological CO 2 sequestration (GCS) in shale reservoirs is considered an imperative approach to reducing CO 2 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 H 2 O on the organic matter (graphene) and inorganic matter (hydroxylated quartz) surfaces of shale in a CO 2 atmosphere utilizing molecular dynamics (MD) simulations and focused on elucidating the effects of CO 2 pressure, temperature, and salinity on wettability from the perspectives of the H 2 O droplet contact angle (CA), gas-water density distributions and interaction energy. The results demonstrate that the H 2 O droplet is gradually stripped from the graphene surface by CO 2 with increasing CO 2 pressure, inducing a wetting transition from intermediate-wet to strongly CO 2-wet, while the hydroxylated quartz surface consistently exhibits a strongly H 2 O-wet state. This significant discrepancy in wetting behaviors can be attributed to the differential adsorption capabilities of CO 2 . The hydrophilicity of graphene and quartz surfaces is enhanced with increasing temperature for the given CO 2 pressure condition, and the wettability alteration is more pronounced above the CO 2 critical temperature for graphene, while the opposite is observed for quartz. The H 2 O CA in the CO 2-H 2 O-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 CO 2-wet feature of shale organic matter during GCS is favorable for the adsorption trapping of CO 2 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 CO 2 injection, and the results gained are expected to furnish new insights into CO 2 sequestration-enhanced gas recovery (CS-EGR) and gas-water two-phase flow at the nanoscale.