Methane Gas Bubbles Affecting the Formation and Distribution of Hydrates in Kaolinite Slit Pores: A Molecular Dynamics Study

Molecular dynamics simulations are applied to study the formation and distribution of hydrates in kaolinite slit pores with methane gas bubbles adsorbed on one of the pore surfaces. The results show that hydrates nucleate in the bulk of solution in pores and then grow into hydrate shells around methane gas bubbles. These hydrate shells can hinder the diffusion of methane molecules. As the result, the formed hydrates cement and distribute near the kaolinite surfaces, on which methane gas bubbles have been adsorbed before hydrate formation. Because of the adsorption of dissolved methane molecules on siloxane surfaces, methane gas bubbles are highly dynamic, with more methane molecules exchanging between bubbles and solution. Therefore, the dissolution of methane molecules is promoted, which is beneficial for hydrate formation. However, in siloxane–siloxane pores, the strong adsorption effect of siloxane surface leads to the generation of layer-like bubbles, inhibiting hydrate formation by providing a relatively low methane concentration in solution. Methane gas bubbles can be directly adsorbed on siloxane surfaces, while water molecules exist between methane gas bubbles and gibbsite surfaces. Six-water rings can be induced by methane gas bubbles on gibbsite surfaces, which may promote hydrate formation by providing partial structures of hydrate cages. The formation behaviors and distribution characteristics of hydrates in kaolinite pores with methane gas bubbles are different from those in kaolinite pores with homogeneous methane solution. The results obtained can help to understand the different formation behaviors and distribution characteristics of hydrates in sediments.