Methane adsorption of nanocomposite shale in the presence of water: Insights from molecular simulations

Accurately predicting gas adsorption in shale gas is essential for estimating production capacity and optimizing extraction processes. However, the complex nature of shale reservoirs (heterogeneous structure, organic-inorganic nanopores, moisture content, etc.) presents significant challenges in understanding the mechanism of methane adsorption. To address these challenges, a nanocomposite shale model incorporating kerogen and illite with slit pore widths of 1 nm, 2 nm, and 4 nm is created. The hybrid grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations are used to investigate methane adsorption in rigid and flexible shale models (kerogen matrix and clay mineral) at three different temperatures, several pressures, and various moisture contents. The simulation results show that the methane density distributions revealed asymmetrical behavior due to the heterogeneous surfaces. Factors such as kerogen flexibility, pore width, moisture content, temperature, and pressure were found to significantly influence fluid behavior. A comparative analysis between the rigid and flexible shale models, considering varying pore widths under both dry and moist conditions, revealed noteworthy differences. Kerogen's flexibility led to increased surface roughness and decreased adsorbed density. Conversely, the presence of water reduced methane adsorption sites in illite, resulting in diverse and complex behaviors dependent on pore widths in the shale model. Finally, the results suggest that using adsorbed volume instead of classical adsorption models is recommended for converting excess adsorption into absolute adsorption. This alternative approach can provide more accurate predictions and enhance the understanding of adsorption in shale gas systems.