The influence mechanism of moisture content on lignite methane adsorption under deep high-temperature and...

The efficient development of deep coalbed CH₄ resources is strongly constrained by coal moisture, and this effect is particularly pronounced in high-moisture lignite. However, the molecular mechanism by which moisture influences CH₄ storage and migration in lignite remains insufficiently understood. To clarify this issue, this study investigates the effects of moisture on the occurrence characteristics of CH₄ in Shanxi lignite, with a particular focus on pore occupation, adsorption behavior, and diffusion performance under different moisture conditions.

Monte Carlo and molecular dynamics (MD) simulations were employed to construct pore structure models of Shanxi lignite at various moisture contents, and CH₄ adsorption isotherms, adsorption energy distributions, radial distribution functions (RDFs), and mean square displacements (MSDs) were systematically analyzed. The results show that, with increasing moisture content, pores larger than 5 Å are progressively occupied by water, substantially reducing the accessible space available for CH₄. Under different temperature-pressure-moisture conditions, moisture markedly weakens the CH₄ adsorption capacity of lignite, and this inhibitory effect becomes more pronounced at elevated temperatures. When the moisture content reaches 4 wt%, CH₄ adsorption decreases by approximately 30-40%. Water preferentially occupies high-energy adsorption sites, forcing CH₄ to migrate toward energetically weaker regions and significantly suppressing its diffusivity, with a reduction exceeding 40%. Pore size and connectivity are identified as the key factors controlling CH₄ adsorption and migration. These findings provide molecular-level insights into CH₄ occurrence in high-moisture low-rank coal and offer guidance for optimizing coalbed methane development strategies.