Mechanism of the influence of high-temperature and high-pressure storage conditions on coal pore structure and methane adsorption kinetics

At present, the exploitation of shallow coal seams in China is nearly exhausted, while the exploitation of deep coal seams is confronted with a high-temperature and high-pressure reservoir environment, which directly affects the adsorption performance of coal seams. The adsorption characteristics of coal seams are a crucial factor influencing the effectiveness of coalbed methane control. To obtain the gas adsorption characteristics of coal under high-temperature and high-pressure conditions, this study combines pore structure experiments, isothermal adsorption experiments, and molecular dynamics isothermal adsorption simulations to investigate the gas adsorption performance of coal under different temperature and pressure conditions from both macroscopic and microscopic perspectives. The research results show that: the effect of pressure on pores is greater than that of temperature. When coal is stored in a high-temperature and high-pressure environment, the pore volume decreases, the pore area increases, and the connectivity of coal deteriorates. When the gas temperature rises from 303.15 K to 363.15 K, the average maximum adsorption capacity of coal decreases by 2 cm³/g for every 20 K increase; when the gas pressure rises from 0 MPa to 10 MPa, the average maximum adsorption capacity of coal increases by 1 cm³/g for every 1 MPa increase. The Langmuir volume (V) exhibits a significant linear relationship with the pore volume and specific surface area of micropores; the Langmuir pressure (P) also has a certain correlation with the pore volume and specific surface area of mesopores. This indicates that the micropore structure in coal is the key factor controlling its methane ultimate adsorption capacity, while the mesopore structure, although not directly determining the adsorption capacity, affects the morphological characteristics of the methane adsorption isotherm to a certain extent. The affinity distribution function curves under different pressures and temperatures show “multiple peaks”, indicating the surface inhomogeneity of deep coal macromolecules and the presence of multiple adsorption sites. During the adsorption process of coal macromolecules, gas molecules first occupy the strong adsorption regions, and then gradually fill the weak adsorption regions. The adsorption of deep coal seams is no longer a single monolayer adsorption, but also involves micropore filling adsorption. When the temperature increases from 303.15 K to 363.15 K and the pressure increases from 0 MPa to 10 MPa, the adsorption heat is always less than 42 kJ/mol, indicating that the adsorption on the coal molecular surface belongs to physical adsorption. These findings clarify the adsorption characteristics of deep coal seams and provide a theoretical basis for the exploitation of deep coalbed methane.