Non-equilibrium decomposition dynamics and fluctuation-dissipation analysis of structure I methane hydrate in confined space

Abstract

In this work, molecular dynamics simulations are used to study the decomposition process of methane hydrates in confined space. The characteristics at different decomposition moments of methane hydrates in confined space are analyzed from the aspects of the density distribution of water molecules, the distribution of F3 contour, the diffusion of water and methane molecules and the variation of F3 at different positions. The influences of different initial temperatures, different solid-solid interfacial contact areas and different crystalline planes at hydrate-quartz interface on the decomposition are estimated. The fluctuation and dissipation characteristics of hydrate-like methane molecules for methane hydrate in confined space are investigated. The results show that the equilibrium temperature of methane hydrate will be shifted downward with spatial confinement, thereby accelerating the decomposition of methane hydrate. The decomposition rate of methane hydrate along the solid-liquid interface in confined space is higher than that along the solid-solid interface. It’s noticed that when the solid-solid contact area decreases, the decomposition rate of methane hydrate decreases. However, the effect of solid-solid contact area on the decomposition rate of methane hydrate is obviously weaker than that of solid-liquid contact area. When hydrate contact with different crystalline planes of quartz, there will be a difference in the decomposition rate. The fluctuation period and relaxation time of hydrate-like methane molecules in confined space are smaller than those for bulk methane hydrate, while it’s just the opposite for the apparent decomposition rate constant k’, which indicates that the existence of confined space is conducive to enhancing the non-equilibrium mass transfer characteristics of methane hydrate decomposition.