ABSTRACT
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.
