Physicochemical Controls on Methane Adsorption Capacity of Coal Seams Beneath Red Beds: Roles of Pore Structure and Surface Chemistry

Abstract The abnormal occurrence of coalbed methane (CBM) beneath thick red beds poses a major challenge to gas disaster prevention and safe coal mining. Previous studies have mainly focused on the sealing capacity of red beds and their influence on CBM migration and accumulation, whereas the response of the underlying coal matrix, particularly its methane adsorption behavior, pore structure, and surface chemistry, remains insufficiently understood. Consequently, the mechanisms linking red beds occurrence conditions to variations in the methane storage capacity of underlying coal seams have not yet been fully quantified. Taking the Xutuan Coal Mine in the Huaibei Coalfield as a case study, this work integrates high-pressure methane adsorption–desorption tests, low-pressure N2/CO2 adsorption experiments, and Fourier-transform infrared spectroscopy to reveal the effects of red beds on methane adsorption behavior, pore structure, and chemical composition of coal. The results show that, with increasing red beds thickness and decreasing distance between the coal seam and the red beds, micropore volume generally decreases, whereas the relative abundance of oxygen-containing functional groups increases. Correlation analysis further shows that methane adsorption capacity is positively correlated with micropore volume and negatively correlated with the relative abundance of oxygen-containing functional groups, with correlation coefficients of 1.0 and −1.0, respectively. These results indicate that both micropore structure and surface chemical characteristics are closely related to the variation in methane adsorption capacity. Combined with the geological characteristics of the red beds, the observed micropore variation may be related to stress-related pore compression, whereas the increase in oxygen-containing functional groups reflects changes in the surface chemical structure of the coal matrix. Fe-related oxidation is proposed as a geochemical hypothesis for the observed variation in oxygen-containing functional groups, based on the regional geological background of the red beds. Furthermore, a normalized red beds geometric influence index, k, was proposed to integrate red beds thickness and its spatial relationship with the coal seam. Field gas content exhibits a strong negative correlation with k, indicating that the proposed index can provide a quantitative reference for the relative evaluation of methane occurrence under comparable red beds geological conditions.