UC Irvine

Natural gas remains a critical fuel used both as a power source and a stabilizing component for sustainable energy alternatives of an intermittent nature. Gas hydrates, located in sediments of permafrost and in submarine outer continental margins, contain large amounts of methane, the main constituent of natural gas. Conventional methane extraction methods involve high energy costs and pose environmental risks. One strategy enabling methane extraction and simultaneous CO2 sequestration in a carbon-neutral process is injecting CO2 into a methane hydrate bed. This method forms a new CO2 hydrate that remains on the seabed, and the exothermic process stimulates methane hydrate dissociation. To investigate this mechanism, a comparative analysis of CO2 hydrate and CH4 hydrate was performed using molecular dynamics simulations (MD), focusing on diffusion, structural evolution, and interface dynamics under hydrate formation and semi-dissociation conditions. This research study also analyzed methane and carbon dioxide interactions at hydrate formation conditions and the structural evolution of semi-dissociated methane hydrates in order to distinguish diffusive behavior that might affect spontaneous CO2 sequestration in CH4 hydrates. The work also includes examining the role of implementing the TIP4P and TIP4P/Ice water force fields. The results show that formation was not observed in any of the cases. TIP4P/Ice water is generally more diffusive than TIP4P water, and water interacting with CH4 is more mobile than with CO2. Comparative studies of semi-dissociated hydrate systems show that TIP4P/Ice better captures ice structure and phase transitions. Still, its temperature sensitivity can limit its use in high-temperature or non-equilibrium simulations, like in the semi-dissociated hydrate state. The interface simulation shows that a purely solid-state substitution of methane by carbon dioxide is unlikely, and it is more probable that the exchange includes the dissociation of methane hydrate first to produce a liquid water phase into which the carbon dioxide migrates. The semi-dissociated hydrate interface simulation is unable to capture the reformation of a CO2 hydrate, suggesting that MD is better suited to dissociation studies of hydrates compared to their formation. Future work is suggested to include examining the role of additional species and surfactants in the interface dynamics of hydrates.