Molecular Insights of Deep Coalbed Methane Adsorption Characteristics and Production Mechanisms in a Slit-Pore Model

More than 50% of global coalbed methane (CBM) resources are estimated to reside in deep coal seams (depth >1500 m). The CBM retained in deep coal seams has higher free gas content with less formation water compared to the CBM in shallow coal seams, leading to different production methods. Current research has primarily addressed the shallow CBM extraction, with limited transferability to deep CBM (DCBM) reservoirs due to the distinct reservoir characteristics and occurrence state. In this work, we developed a slit-pore model to investigate the adsorption behaviors and production mechanisms of DCBM using molecular dynamics simulations. The slit-pore model includes a micropore, a macropore, and a fracture/cleat to simulate the flow unit of anthracite. We analyzed the adsorption patterns at different production stages and quantitatively evaluated the production performance of pressure-relief production and pressure-control production. DCBM components (CH₄, C₂H₆, N₂, and CO₂) are heterogeneously distributed in the multiscale model, with higher proportions of CH₄, C₂H₆, and CO₂ in the micropore due to the high adsorption affinity and small molecular diameters. CH₄, C₂H₆, and CO₂ exhibit monolayer adsorption in the micropore, and the production methods do not affect the adsorption pattern. Compared to the pressure-relief production, the pressure-control production yields higher DCBM production. The mass transfer from the macropore and the water-blocking effect in the micropore collectively constrain the fluid release from the micropore during pressure-relief production. This work provides a comprehensive understanding of the DCBM adsorption behaviors and production mechanisms.