Investigation into the pore structures and CH4 adsorption capacities of clay minerals in coal reservoirs in the Yangquan Mining District, North China

Abstract

Abstract The rising energy demands worldwide and difficulty in developing novel clean energy sources have greatly stimulated the exploitation of coalbed methane. Clay minerals are common fractions of coal; thus, understanding their CH4 adsorption capacities and pore structures is vital. In this study, coal, parting, roof, and floor samples were collected from the Yangquan Mining District. The mineral compositions, CH4 adsorption capacities, and pore structures of the samples were analyzed using X-ray diffraction, the CH4 isothermal adsorption method, and the low-temperature N2 adsorption method, respectively. The results indicated that organic matter had a much higher CH4 adsorption capacity (33.80 cm3/g, 35°C) than that of clay minerals. The CH4 adsorption capacities of various clay minerals are significantly different, with smectite (18.01 cm3/g), kaolinite (5.81 cm3/g), mixed-layer illite-smectite (4.47 cm3/g), and illite (2.08 cm3/g) present in decreasing order. The pore sizes of the samples consisted of sizes <6 nm, and six pore size groups (Groups 1–6) were identified in the PSD patterns. These pore size groups were associated with different clay minerals. We propose that the CH4 adsorption capacities of clay minerals are mainly influenced by their pore structures, which are in turn associated with their species and formation processes. Furthermore, the conversion of kaolinite to illite, and the illitization of mixed-layer smectite-illite, exerted a negative effect on their CH4 adsorption capacities.