CO2 Sequestration and CH4-CO2 Replacement in Methane Hydrate Reservoirs under Associated Gas Conditions: A Molecular Dynamics Perspective.

Methane hydrate possesses both high energy density and low carbon emission potential. Replacing CH4 with CO2 (CH4-CO2 replacement) is considered as a sustainable approach to simultaneously recover energy and achieve carbon sequestration. However, the influence mechanism of associated gases such as H2S on the replacement process under clay confinement and marine conditions remains poorly understood. This study employs molecular dynamics (MD) simulations to systematically investigate gas behavior evolution, hydrate structural responses, and microscale regulatory effects on the sequestration mechanism during CH4-CO2 replacement in the presence of H2S. Results show that CO2 hydrate nucleation mainly occurs on the surface of pre-existing CH4 hydrate. Due to pore structure and the adsorption layer on clay surfaces, hydrate tends to grow preferentially at the nanopore center. The presence of H2S does not significantly disturb the adsorption layer but enhances the early aggregation of gas bubbles. H2S also shows a higher occupancy capability for 512 and 51262 cages and can form topologically continuous hydrate frameworks by edge-sharing with cages occupied by other gases, promoting the formation of a closed hydrate shell. This structure improves CO2 sequestration efficiency and CH4 hydrate stability but hinders mass transfer and direct replacement pathways. These findings reveal the critical regulatory role of associated gases in marine hydrate reservoirs and provide theoretical support for methane hydrate exploitation and CO2 sequestration strategy design.