CO ₂ Sequestration and CH ₄ –CO ₂ 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.