Factors Influencing the Methane Adsorption Capacity of Coal and Adsorption Heat Variations

Adsorption capacity is a key parameter to evaluate the underground storage capacity of natural gas and the reliability of CO2 sequestration. To explore the effects of temperature and pore structure and properties on adsorption capacity, the quantitative characterization of the effect of temperature on the adsorption capacity of coal and on changes in the heat of adsorption of coal samples during the adsorption process, nine coal samples with thermal maturities ranging from 0.47% to 5.16% vitrinite reflectance (Ro) were analyzed by isothermal adsorption experiments at from 20 to 100 °C in 20 °C steps, and the pore structure was analyzed by low-temperature N2 and CO2 adsorption. Moreover, the isosteric heat based on a fixed amount of adsorption and the ultimate heat of CH4 adsorption in different rank coals were also analyzed. The results show that the adsorption capacity of all coals decreases with increasing temperatures, with the maximum volume adsorbed being 11.77–33.34 cm3/g at 20 °C and 9.87–28.99 cm3/g at 100 °C. The change in adsorption ranges from 1.90 to 14.18 cm3/g per 20 °C change, with the coals from 0.71% Ro to 3.65% Ro showing the least variability: 2.49 cm3/g for every 20 °C change. Coals are dominated by 0.3–0.8 nm micropores, and the isosteric heat of adsorption variation is influenced by the coal surface roughness and intermolecular forces during adsorption. With increasing coal rank, the ultimate heat of adsorption for coal follows the same trend as the adsorption capacity and is positively correlated with the volume and surface area of micropores. The adsorption capacity can be well predicted by the isosteric heat of adsorption only at temperatures ≥ 60 °C. The results can be beneficial for understanding the gas storage in deep reservoirs and promoting the efficient production of deep coalbed methane.