Adsorption/desorption characteristics of methane on moist shale under high temperature and pressure: an experimental and molecular simulation study

The adsorption/desorption characteristics of methane (CH₄) on moist shale are of great significance for shale gas exploration and production. However, the influence of moisture on CH₄ adsorption/desorption under high temperature and pressure conditions, which is consistent to shale reservoirs (burial depths about 3500-4500 m) in China, remained unclear. In this study, quantitative analysis toward moisture dependence of CH₄ adsorption/desorption capability on shales was investigated through experimentation and molecular dynamics simulation under moisture contents of 0%, 0.204%, 0.445%, 0.677%, and 0.965%. Results show that with increasing moisture content, the isothermal adsorption capacity of CH₄ decrease, and it reaches 36.80% and 10.00% at moisture content of 0.965% in experimentation and simulation, respectively. Simultaneously, the hysteresis index of CH₄ desorption increase by 19.64% and 4.52%. The role of water molecules hindering CH₄ desorption under low and high moisture content was clarified. At low moisture content, water molecules are mainly adsorbed on the pore walls, thereby reducing the size of the pore throat and hindering CH₄ transport pathways. At high moisture content, many water molecules escape from the original adsorption sites and form clusters in the middle of the pores, blocking the pore throats. Meanwhile, CH₄ is re-adsorbed onto the exposed adsorption sites of water, which leads to CH₄ desorption hysteresis. The results provide valuable insights for shale gas exploration and production under practical water-bearing shale reservoir conditions.