Methane Gas Transport in Ca-MMT Shale Nanoslits Considering Water Content Effects: Insights from Molecular Dynamics Simulations

Two-phase flow remains a significant challenge in the development of water-bearing shale gas, particularly regarding the flow of gases through clay minerals in such formations. Nonequilibrium molecular dynamics simulation is investigated to research the two-phase flow of water (H₂O) and methane (CH₄) through Ca-montmorillonite (MMT) shale nanoslits. The results indicate that water molecules preferentially adsorb onto the surfaces of the Ca-MMT shale nanoslits, leading to the formation of water bridges within the nanoslits as water content increases. Notably, CH₄ molecules exhibit complete solubility in the water phase, and their maximum velocity gradually decreases, resulting in a flattened parabolic flow profile with a higher gas-water percent (GWP). In the two-phase flow, CH₄ continues to flow without slip in the inorganic nanopores due to water molecules occupying the surfaces and creating a water film. However, when the water content (GWP) exceeds 80.87%, CH₄ gas molecules become dissolved in the water, filling the inorganic nanoslits and leading to a zero-gas flux. This condition causes CH₄ retention in adjacent nanoslits, creating a "water lock gas" effect. This research provides a theoretical foundation for understanding multiphase flow in water-bearing shale gas and offers a refined description of the shale gas extraction process for future application.