Molecular Dynamics Characterization of Gas Diffusivity of CO2-CH4 in sI Clathrate Hydrates

Clathrate hydrates are cage-like water structures entrapping a gaseous guest molecule in their cavities. The need for alternative fuel sources has spurred an interest in methane hydrates as estimates predict that they account for two thirds of the fossil fuel global reserves. However, their exploitation is not without fault as methane hydrates occupy a crucial role in stabilizing the soil as well as being a potential major source of greenhouse gases. Fortunately, methane found in clathrate hydrates can be swapped with carbon dioxide (CO2), thus allowing for carbon sequestration, and rendering the production of methane effectively carbon neutral. The process has been shown to be feasible and resulting in the formation of stable CO2 hydrates, but the mechanism surrounding the exchange remains to be elucidated.<br/>This presentation focuses on a system where pure methane sI hydrates are exposed to a front of CO2 dissolved in water. The system conditions are set to be favorable to the melting of methane hydrates and to the formation of CO2 hydrates. As the system evolves with time, methane escapes the lattice and CO2 diffuses into the latter, replacing methane in partially melted cages as well as forming new cages at the interface in a phenomenon known as co-growth. An analysis of the impact of temperature, pressure, and CO2 concentration on the diffusivity of CO2 and methane is conducted. It is followed by an analysis on how co-growth at the interface hinders diffusion and prohibits CO2 from diffusing deeper in the lattice. Previous work found in the literature has quantitatively described the impact of such impediment on the ultimate replacement ratio, but the presented research is the first quantify the effect on mass diffusivity. This work will also discuss the impact of water vacancies and of gas occupancy on mass diffusivity. To understand the effects of vacancies and gas occupancy on the thermodynamics of the system, the Gibbs free energy, the enthalpy, and the entropy are calculated. Our previous work quantified diffusivity in a CO2 filled hydrate system and the impact of such vacancies and serves as a basis for comparison. The investigation also looks into how diffusion evolves from a non-Fickian process to a Fickian one. Finally, this work also tackles how the swapping mechanism between methane and CO2 differs near the interface from deeper strata with the hole-in-cage model dominating the latter.<br/>The methodology is based on molecular dynamics. The Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) is used to perform simulations on a 3x3x3 methane hydrate lattice juxtaposing a CO2 front dissolved in water. The force fields used for methane, carbon dioxide, and water are OPLS-AA, EPM2, and TIP4P/ICE respectively. The LAMMPS software allows to quantify the mean squared displacement (MSD) which can be correlated to the mass diffusivity. Furthermore, LAMMPS computes primordial thermodynamics parameters such as the Free Gibbs energy, the entropy, and the enthalpy.