Relationship between the Geological Origins of Pore-Fracture and Methane Adsorption Behaviors in High-Rank Coal

Coal is characterized by a complex pore-fracture network and functional groups, which are derived from various geological origins and which further affect methane adsorption. To explore the relationship between the geological origins of pore-fractures and methane adsorption behaviors, we conducted pore structure tests and adsorption isotherms on six Qinshui high-rank coals. The pores and fractures were observed using an optical microscope (OM), a field emission scanning electron microscope (FESEM), and a high-resolution transmission electron microscope (HRTEM), and the pore structure parameters were determined using mercury intrusion and low-pressure N₂ and CO₂ adsorption. High-pressure CH₄ adsorption isotherms were obtained at 30 °C using the manometric method. Results show that the Qinshui high-rank coals develop five stages of pore size distribution, consisting of the smaller micropore stage (0.3-1 nm), the larger micropore and smaller mesopore stage (1-10 nm), the mesopore and smaller macropore stage (10-110 nm), the microfracture stage (0.11-40 μm), and the larger macropore stage (>40 μm). The micropores dominate the total pore volume (PV) and specific surface area (SSA). Pores and fractures of various morphologies and sizes have different geological origins, which are related to coalification and stress field evolution. Methane adsorption on coals mainly occurs in the micropores as a form of volume filling. The maximum pore size for complete gas filling (MPSCGF) ranges from 0.60 to 0.88 nm in Qinshui high-rank coals. The coal-forming geological processes, such as coalification and stress field evolution, contribute to various pores and fractures, which show different pore sizes and functional groups. The geological origins of pores and fractures control the methane adsorption behaviors in coals by way of the pore size and functional groups. Surface coverage-related methane adsorption behavior occurs in fractures, primary pores, and large-scale secondary pores, while micropore filling is the methane adsorption behavior in macromolecular pores and small-scale secondary pores. The aim of this study is to provide a new insight into the methane adsorption on coals from the geological process of the formation and modification of pores and fractures.