Unraveling the Role of Water Configuration in Governing Methane Occurrence and transport in Shale Nanopores: A Molecular Dynamics Study

In deep coal formations, nanopores are widely developed and commonly occupied by formation water, where water molecules can exist in various configurations, including water bridges, clusters, and films. These water configurations significantly regulate methane (CH 4 ) adsorption, desorption, and transport behaviors, yet their microscopic mechanisms remain unclear. In this study, molecular dynamics simulations are conducted to elucidate the coupled effects of pore size, water content, and driving pressure on CH 4 occurrence and migration in water-bearing nanopores. Particular attention is given to the evolution of water configurations and their role in controlling CH 4 transport mechanisms. The results demonstrate that water configuration is strongly governed by pore size and water-solid interactions. In micropores, strong confinement promotes the formation of water bridges across pore spaces, whereas water molecules preferentially aggregate near pore surfaces and form clusters or films in larger pores. The presence of water significantly reduces CH 4 adsorption capacity by occupying surface adsorption sites, with a stronger inhibition effect observed in mesopores due to the larger spreading area of water films. During pressure depletion, water bridges in micropores gradually transform into water clusters or films, facilitating CH 4 desorption and enhancing free CH 4 recovery. Furthermore, water configurations strongly regulate CH 4 transport by altering near-wall flow characteristics and weakening the boundary slip effect. The reduction in CH 4 velocity caused by water reaches 45.5% in micropores, which is higher than that in mesopores (32.0%), mainly due to the stronger blocking effect of water bridges. Increasing water content further enhances methane trapping and suppresses its migration. These findings provide molecular-level insights into the critical role of water configuration evolution in controlling methane recovery from water-bearing coal nanopores.