Molecular Insights into Methane Gas Bubbles Affecting Hydrate Formation in the Salt Solution of Kaolinite Slit-Pores

It is crucial for the exploration and exploitation of natural gas hydrate resources to determine the formation and distribution of hydrates in sediment pores. Molecular dynamics simulations are used to study the formation and distribution behaviors of methane hydrates in the salt solution in kaolinite slit-pores, investigating the combined effects of mineral surfaces, salt ions, and the state of methane. The results indicate that in the presence of methane bubbles, hydrates can primarily nucleate in the solution, with a minority nucleating on the kaolinite surfaces. Besides, surface hydrate nuclei are likely to move away with the growth of hydrates. Hydrates tend to distribute near the surfaces where methane bubbles were previously adsorbed, and salt ions can weaken the binding interaction between hydrates and the surfaces, by accumulating on the kaolinite surface or at the bubble–solution–siloxane surface three-phase contact line. Salt ions can reduce the efficiency of methane molecule exchange between bubbles and the solution, thereby inhibiting hydrate nucleation. However, salt ions do not alter the influence of the pore surface composition on the methane molecule exchange efficiency. As hydrates form, most salt ions migrate and adsorb onto kaolinite surfaces, and a small number of chloride ions can be trapped within the hydrates. Additionally, the presence of surface methane bubbles can reduce the surface area exposed to the solution, thereby decreasing the amount of salt ions adsorbed on pore surfaces. The findings obtained can help us understand the formation and distribution characteristics of hydrates in sediment pores.