Research on differences in pore structures of coal，shale and sandstone and their effects on methane adsorption

A comprehensive understanding for the differences of pore structure among coal，shale and sandstone is of great significance for the development of coal bed methane，shale gas and tight sandstone gas. In this study，coal，shale and sandstone samples were collected，and pore structures of these samples were tested by mercury intrusion，low temperature N2 adsorption and low temperature CO2 adsorption. According to the characteristics of each test method, the joint characterization methods were used for multi-scale pore structures: micropore (<2 nm) was characterized by the low temperature CO2 adsorption, mesopore (2-50 nm) by the low temperature N2 adsorption and macropore (> 50 nm) by the mercury intrusion method. Methane isothermal adsorption experiments were performed for different samples to analyze the influence of pore structure on methane adsorption. Experimental results show that：① in the tested samples，slit pores are mainly developed in coal，and spherical pores are mainly developed in shale and sandstone. ② Pore structures of coal，shale and sandstone are quite different，and the development degree of micropore in coal is much greater than that in shale and sandstone. In coal，most pore volume and specific surface area are provided by micropore，accounting for more than 60% of the total pore volume and more than 95% of the total specific surface area. The pore volume of shale and sandstone is mainly provided by mesopores，accounting for more than 65% of the total pore volume; and the specific surface area is provided by micropores，accounting for more than 61% of the total specific surface area. ③ The order of methane adsorption capacity of different samples is coal > shale > sandstone. The adsorption of methane is mainly controlled by the specific surface area of pores. Micropores provide more space and adsorption sites for methane adsorption in coal. Therefore，the adsorption capacity of coal is far greater than that of shale and sandstone. The maximum adsorption capacity has a strong positive correlation with the specific surface area of nanopores.