Influence of fracture connectivity and shape on water seepage of low-rank coal based on CT 3D reconstruction

The shape and connectivity characteristics of coal fractures are important factors that affect the permeability of coal and the migration of water in the coal during water injection. In this study, a microscopic and mesoscopic fracture model of coal is constructed by using CT 3D reconstruction technology. Based on this model, the fracture connectivity parameters are selected and analyzed, and the gray correlation method for comprehensive evaluation is adopted to determine the relationship between permeability and fracture connectivity. Then, the shape factor η is introduced, the water injection seepage flow is simulated through the Navier-Stokes equation, and the seepage in the fractures with different shape factors is analyzed. The results show that the permeability of coal have a strong correlation with the fracture connectivity parameters, and the correlation order is average coordination number > porosity > maximum Feret diameter. Furthermore, the fracture shape factor is positively correlated with the maximum Feret diameter. In addition, the steady flow velocity increases with the increase of the shape factor. The fractures with η > 2 significantly increases the permeability of coal, but fractures with large shape factors are still the most important channels for seepage.