Hydrogen occurrence in nanopores of coalbed methane formations: A molecular simulation study

Owing to their intrinsic nanoporous structure and high gas-holding capacity, coalbed methane (CBM) formations represent promising reservoirs for underground hydrogen storage. However, the coupled effects of temperature, pressure, pore size, indigenous gases, and water content on H2 behavior remain insufficiently understood. This study employs classical molecular simulations, integrating grand canonical Monte Carlo and molecular dynamics approaches, to investigate H2 storage and diffusion in representative CBM nanopores under pressures of 10–40 MPa, temperatures of 313–343 K, pore sizes of 0.5–5.0 nm, various cushion gases (CH4, CO2, and their mixtures), and water contents ranging from 7.17% to 29.59%. The results indicate that H2–coal interactions are predominantly governed by weak van der Waals forces, with H2 primarily existing in free and compressed states, and adsorption contributing only marginally. Elevated pressure enhances H2 storage through volumetric filling and compression, whereas increased temperature attenuates adsorption while promoting diffusion. Larger pores facilitate a more uniform distribution of H2 and higher molecular mobility. CH4 and CO2 reduce the accessible volume available to H2 through competitive adsorption, with CO2 exhibiting a stronger affinity toward coal surfaces. Increased water content attenuates H2–coal interactions and constrains molecular migration, without substantially altering the overall spatial distribution of H2. Wettability analyses reveal that H2 injection exerts minimal influence on coal–water interfaces, with no significant wettability inversion or water-film reorganization. Collectively, these findings elucidate the molecular mechanisms governing H2 occurrence and diffusion in CBM formations, providing critical insights for assessing reservoir suitability and optimizing strategies for underground hydrogen storage.