Nanoscale Insights into CO₂ Enhanced Shale Gas Recovery in Gas-Water Coexisting Kerogen Nanopores

The presence of water clusters in kerogen nanopores reduces the occurrence and migration of methane (CH₄) and thus affects shale gas extraction. CO₂ injection, as an effective approach to enhance shale gas recovery, still presents challenges in its ability to mitigate the impact of immobile water clusters within the kerogen. In this work, molecular dynamics simulations were employed to investigate the microscopic transport process of water clusters and CH₄ following CO₂ injection in the gas-water coexisting kerogen nanopores. The results demonstrate that CO₂ can desorb irreducible water clusters to dredge the pores while extracting CH₄, enhancing gas-water mobility, and shale gas recovery by transitioning the wettability of the kerogen nanopore surface from weakly water-wet to CO₂-wet. The impact of CO₂ on the wettability of kerogen surfaces is primarily manifested in two aspects: CO₂ can intrude the interface between water clusters and kerogen to reduce the number of hydrogen bonds between them, resulting in the detachment of water clusters; and the surface of kerogen nanopores can form a layer of CO₂ gas film, which prevents desorbed water clusters and CH₄ from readsorbing onto the wall surface. This study provides important insights in enhancing the understanding of the microscopic mechanisms in nanoscale flow, as well as for the development of an unconventional gas reservoir.