Phase Stability of Mixed Carbon Dioxide/Methane Hydrates Confined in Nanoporous Carbon.

Beyond confinement and surface effects, the phase stability of gas hydrates in natural environments (e.g., porous rocks, marine sediments) also depends on their composition. Here, we use molecular modeling tools to determine the hydrate-liquid-vapor coexistence of mixed carbon dioxide/methane hydrates under both bulk and confinement conditions. In agreement with available experimental data, gas hydrates in confinement are found to be stable in a narrower temperature/pressure domain compared to their bulk counterpart. We also identify that the methane/carbon dioxide composition change upon hydrate formation plays an important role in the melting temperature depression of the nanoconfined gas hydrate. Moreover, this melting point depression in confinement is found to be quantitatively predicted by the Gibbs-Thomson equation. In this article, we also discuss the impact of system size and of the gas composition in the vapor phase on the melting point of the gas hydrate. In particular, the carbon dioxide-to-methane ratio in the vapor phase is found to have a negligible impact on the phase stability of mixed hydrates. Finally, the water density distribution shows that the formation of the water condensed film at the pore surface in hydrate dissociation and formation depends on temperature. As a result, the competition between the wall/hydrate interface and the wall/liquid interface leads to two scenarios for hydrate-liquid coexistence near surfaces or in confinement.