Molecular simulation of methane adsorption/desorption characteristics of deformed kerogen in shales

Current studies on the methane adsorption/desorption behavior in kerogen are primarily conducted based on fixed pore structures. Consequently, the influence patterns and mechanisms of the pore deformation of kerogen on methane adsorption/desorption remain poorly understood. Using molecular simulation, we explore both the methane adsorption/desorption behavior in shale kerogen and the coupled pore deformation characteristics. By constructing and characterizing a nanopore model of kerogen, we investigate the influence of pressure on kerogen deformation. Using a coupling method combining Grand Canonical Monte Carlo (GCMC) and molecular dynamics (also referred to as the GCMC-MD method), we analyze the coupling characteristics of methane adsorption and kerogen deformation.Additionally, through methane desorption simulation using a kerogen model under high-pressure adsorption equilibrium, we investigate the characteristics and causes of methane desorption hysteresis. The results indicate that ultramicropores in Type ⅡD kerogen serve as the primary migration pathways, while isolated ultramicropores also have the potential to act as migration pathways due to the influence of pore deformation. The pore structures of kerogen generally are reversible during the pressure increase and decrease cycles. The methane adsorption-induced kerogen deformation can be divided into two stages: volume expansion and pore reorganization. The structural deformation of kerogen enhances the methane adsorption capacity, while the selective adsorption of methane onto high-affinity heterogeneous sites in kerogen leads to pronounced changes in pore space. Small desorption hysteresis loops are observed within the deformed structures of kerogen. The methane desorption hysteresis is primarily caused by pore deformation rather than the thermodynamic differences between adsorption and desorption and the capillary condensation effect.