Pore structure characteristics of deep coal seams under the synergistic effect of high temperature and high pressure and their impact on coalbed methane adsorption

Abstract The content of coalbed methane is an important parameter for preventing coal and gas outbursts and preventing coal mine accidents. In order to explore the adsorption law of deep coalbed methane, this paper selects coal samples from two high-quality mining areas, and uses low-temperature nitrogen adsorption, mercury intrusion porosimetry, and high-pressure volume method to study the pore structure characteristics and adsorption properties of deep coalbeds from microscopic and macroscopic perspectives. It reveals the influence of temperature, pressure, and pore structure on the content of coalbed methane, and constructs a deep coalbed methane content adsorption model considering the synergistic effect of these factors. The research results show that the hysteresis degree of the intrusion and extrusion mercury curves of deep coalbeds decreases, the number of semi-closed pores increases, and the connectivity of coal deteriorates. The low-temperature nitrogen adsorption-desorption curves all conform to Type II isotherms. When the gas pressure reaches 0.9 MPa, the adsorption and desorption curves completely overlap, indicating the end of monolayer adsorption. More than 80% of the deep coalbed methane is stored in pores ranging from 2 to 10 nm in the form of micropore filling, while less than 10% is adsorbed as monolayers in pores larger than 10 nm. There is a significant linear relationship between the Langmuir volume (V) and the micropore volume and BET. The Langmuir pressure (P0 is related to the volume and specific surface area of mesopores, indicating that micropore structure controls the ultimate adsorption capacity of coal for methane, while mesopore structure affects the morphological characteristics of methane adsorption isotherms. The adsorption zones of deep coalbeds are divided into micropore weak adsorption zone, micropore strong adsorption zone, and monolayer adsorption, and a temperature-pressure-pore synergistic adsorption model for deep coalbed methane is constructed. The coefficient of determination (R2) of this model reaches 0.999, which can well characterize the adsorption characteristics of deep coalbed methane. By applying the constructed adsorption model, it is found that the content of coalbed methane tends to decrease with increasing depth, laying a theoretical foundation for revealing the law of changes in deep coalbed methane content.