Evolution of methane during gas hydrate dissociation

Abstract

We simulated decomposition of structure I methane hydrate (H) with all cages filled in contact with two reservoirs (pools) of liquid water (W) which in turn are in contact with either two methane gas reservoirs (G), or with vacuum (V), under constant volume–constant energy conditions. By adding gas or empty spaces to the simulation box we allow the released methane to diffuse out of the liquid phase and into the gas phase similar to what happens during methane hydrate dissociation. The evolution of the released methane molecules during the hydrate dissociation process was carefully monitored. We found that some of the released methane gas reaches the gas phase and contributes to the increase of gas pressure on the hydrate phase. As the hydrate dissociates, liquid water phase becomes supersaturated with methane, methane molecules aggregate, and spherical regions of high concentration of methane form which we identify as “nano-bubbles”. These nano-bubbles grew to a specific size range which depends on simulation conditions and remained stable in the liquid phase for the duration of the simulations (5ns).