Molecular dynamics simulation on CO2 hydrate growth and CH4-CO2 replacement in various clay nanopores

Microscopic knowledge of CO2 hydrate growth and CH4-CO2 replacement in clay nanopores is crucial for hydrate resources exploitation and geological CO2 sequestration. Herein, massive molecular dynamics simulations are performed to investigate CO2 hydrate growth and CH4-CO2 replacement from freshwater and brine in various clay nanopores. Simulation results indicate that surface properties of various clay (montmorillonite, illite and kaolinite) exhibit different effects on CO2 hydrate growth and CH4-CO2 replacement via surface adsorption. For the specific clay models used in this study, the kaolinite surface is most favorable for CO2 hydrate growth, followed by the illite surface, and finally the montmorillonite surface. The strong affinity of kaolinite siloxane surface for CO2 molecules may stabilize CO2 nanobubble, which is unfavorable for CO2 nanobubble decomposition. On the other hand, only a few CH4 hydrates are replaced by CO2 molecules owing to the difficulty of mass transfer of CO2 molecules. Interestingly, the synergy of the kaolinite siloxane surface and interfacial water layer enhances the mass transfer of CO2 molecules, allowing the CO2 molecules in the interfacial water layer to further replace CH4 hydrate. These microscopic insights into CO2 hydrate growth and CH4-CO2 replacement in various clay nanopores help the exploitation of hydrate resources and the development of hydrate-based CO2 sequestration.