Micro-Nano Structure Characteristics and Gas Enrichment Mechanism of High-Rank Coals in the Southern Qinshui Basin

The exploitation and utilization of coalbed methane (CBM) are of great significance to alleviate energy supply-demand contradictions, reduce greenhouse gas emissions, and achieve the “dual carbon” strategic goals. However, the micro-nano structural characteristics of high-rank coals are the key geological factors controlling the occurrence and migration of CBM, and their mechanism is complicated. In this paper, the metamorphism, deformation, and pore-fracture structure of coal reservoirs in the southern Qinshui Basin were systematically studied by proximate analysis, low-temperature nitrogen sorption, and mercury intrusion porosimetry, and the controlling factors of its coal reservoir permeability and the mechanism of its CBM enrichment were discussed. The study shows that the coal reservoirs in the southern Qinshui Basin are dominated by high-rank anthracite, and their metamorphism decreases from south to north and from the margin to the interior; their deformation is mainly brittle deformation, and most of them are cut or broken by fracture; and their pore-fracture structure mainly consists of matrix pores and tectonic fractures. Metamorphism, deformation, and pore-fracture structure are the main controlling factors for the permeability of coal reservoirs, which are mainly reflected in the control of the formation and development of pore-fracture systems. The CBM enrichment areas experienced a dynamic enrichment, and the hydrodynamic conditions and tectonic uplift are the main influencing factors for their formation. The control of the hydrodynamic field on CBM enrichment is manifested in the influence of fluid potential energy on the phase transition of CBM and the mineralization of stratum water. The control of the tectonic uplift on CBM enrichment depends on the combined effects of the temperature and pressure on adsorption at different times. Therefore, this study not only helps to deepen the understanding of multifield coupling theory in coal reservoirs but also provides a core driving force for the efficient development of CBM resources.