Investigating the mechanisms of water films and water bridges on methane storage and transport behavior in deep coal seams: A molecular simulation study

The occurrence states of water in coal, particularly as water films and water bridges, exert a significant influence on coalbed methane production. However, its role in gas storage and production in deep coal reservoirs remains unclear. In this study, scanning electron microscopy and low-pressure CO2/N2 adsorption experiments were conducted to obtain pore structure characteristics, followed by methane adsorption experiments with different relative humidity levels to investigate the water-induced variation in methane adsorption capacity. Using 13C nuclear magnetic resonance and Fourier transform infrared spectroscopy experiments, the three-dimensional molecular structure model of the Suide coal sample was constructed. Subsequently, molecular dynamics simulations were employed to build methane–water interaction models and calculate effective diffusion coefficients and production efficiencies. The results reveal that the spatial distribution and formation of clusters of water molecules on pore surfaces are affected by water content. In the first stage (Sw = 2.52%–6.79%, 2.58%–7.14%, and 2.53%–6.75% in the 1, 2, and 4 nm pores, respectively), the water molecules adhere to the surface of the pore walls and begin to gather to form water clusters. In the second stage (Sw = 6.79%–22.59%, 7.14%–23.81%, and 6.75%–20.81% in the 1, 2, and 4 nm pores, respectively), water clusters form continuous films along the pore surfaces by coming together. In the third stage (Sw = 22.59%–37.75%, 23.81%–36.34%, and 20.81%–37.59% in the 1, 2, and 4 nm pores, respectively), the water molecules form two states, known as the water films and the water bridges, in the 1 nm pores. The water bridge gradually disappears in the 2 and 4 nm pores. These water structures restrict methane transport. First, a few water molecules form clusters, resulting in a gradual decrease in the efficiency of methane production from 37.48% to 36.38% at Sw = 0–2.58%. Conversely, many water molecules combine with each other to form a film, and the efficiency of methane production increases from 37.48% to 46.10% at Sw = 7.14%–23.81%. The formation of water bridges limits the migration of methane molecules by blocking the pore tunnels, thereby decreasing the efficiency of methane production from 46.10% to 5.70% at Sw = 23.81%–43.01%. This study provides insight into how water films and water bridges form in coal and reveals their effect on methane storage and production.