Characteristics of Pores and Fractures in Primary-Fragmented Structured Coal and the Associated Influence Mechanism on Permeability

Coal-bed methane development is closely related to the pore-fracture characteristics and permeability of the coal reservoirs. After the primary structured coal is damaged and deformed, its pore-fracture system changes, which affects the permeability of the coal reservoir. To investigate the pore-fracture characteristics of deep coal with different coal-body structures and its impact on permeability, the coal seams of the Longtan formation in the Changning block of southern Sichuan were taken as the research object. A systematic study was conducted based on high-pressure mercury intrusion porosimetry, scanning electron microscopy, and full stress-strain permeability experiments. The results indicate that the pore connectivity of primary structured coal is superior to that of fragmented structured coal. Tectonic stress can modify the structure of the seepage pores and fractures in coal samples. The permeability of coal seams containing fractures is higher than that of intact coal seams after stress-induced transformation, suggesting that brittle tectonic deformation facilitates the modification of seepage pores and microfractures, which is more conducive to the development of coalbed methane. Modification of pores and fractures by strain-induced damage in coal seams under stress is the underlying cause of permeability changes in coal seams.