Moisture is one of the most important factors that influences coal seepage and coal-bed methane (CBM) extraction.

Adsorption isotherms of CH₄ and CO₂ on Qinshui Basin anthracite were obtained at the temperatures of 283 K, 303 K, and 323 K using the gravimetric method. The feasibility of the displacement of CH₄ by injecting CO₂ on this anthracite was verified by calculating the selectivity factor of CO₂ over CH₄ (αCO2/CH4), adsorption affinities, and thermodynamic properties of CH₄ and CO₂. Results show that the values of αCO2/CH4 are more than 4.0. Henry’s constant (KH) of CH₄ is smaller than that of CO₂, and CH₄ has a weaker affinity with coal surface. As temperature improves, KH of CO₂ and CH₄ decrease. Gibbs free energy change (ΔG) and surface potential (Ω) of CO₂ are more negative than those of CH₄, indicating that CO₂ adsorption on anthracite is more spontaneous and favorable. The absolute values of Ω and ΔG of CH₄ and CO₂ increase with pressure rises. Isosteric heat of adsorption (Qst) of CH₄ is lower than that of CO₂. With increasing loading, Qst and entropy loss (ΔS) of CH₄ decrease, while Qst and ΔS of CO₂ increase. The higher ΔS of CO₂ reveals that the adsorbed CO₂ molecules constitute a more stable rearrangement than CH₄ molecules. High temperature reduces ΔS of CH₄ and CO₂.

Coal contains a large number of fractures, whose characteristics are difficult to describe in detail, while their spatial distribution patterns may follow some macroscopic statistical laws. In this paper, several fracture geometric parameters (FGPs) were used to describe a fracture, and the coal seam was represented by a two-dimensional stochastic fracture network (SFN) which was generated and processed through a series of methods in MATLAB. Then, the processed SFN image was able to be imported into COMSOL Multiphysics and converted to a computational domain through the image function. In this way, the influences of different FGPs and their distribution patterns on the permeability of the coal seam were studied, and a finite element model to investigate gas flow properties in the coal seam was carried out. The results show that the permeability of the coal seam increased with the rising of fracture density, length, aperture, and with the decrease of the angle between the fracture orientation and the gas pressure gradient. It has also been found that large-sized fractures have a more significant contribution to coal reservoir permeability. Additionally, a numerical simulation of CBM extraction was carried out to show the potential of the proposed approach in the application of tackling practical engineering problems. According to the results, not only the connectivity of fractures but also variations of gas pressure and velocity can be displayed explicitly, which is consistent well with the actual situation.