Water-methane interactions in coal: Insights from molecular simulation

Water distribution can affect the storage and transport of methane in coal. There are many reports involving the distribution of water and methane in coal, but the microscopic mechanism of water-methane interaction is still unclear. In this study, realistic molecular structure models of bituminous and anthracite coals were constructed based on the chemical structures of two coal samples. And the chemical structures were analyzed by solid-state nuclear magnetic resonance, Fourier-transform infrared, and X-photoelectron spectroscopy. Then slit-pore models with different pore sizes were constructed based on the molecular structures to simulate the methane-water interaction in bituminous and anthracite coals. Results show that water molecules tend to form water bridges and water films in the pores of bituminous and anthracite coals, respectively. The significant displacement of pre-adsorbed water by methane was observed through simulation. It is found that the water molecules on the pore surface and the interfaces of small-size water bridges are more easily displaced by methane. In the larger pores, methane molecules mainly drive water molecules to agglomerate larger water bridges in bituminous coal. While methane molecules mainly drive water molecules to cover the pore surface and form a thicker water film in anthracite coal. This study provides new insights into the microscopic distribution and interaction mechanism of fluids in coal.