Characteristics of a Multi-scale Fracture Network and Its Contributions to Flow Properties in Anthracite

Fractures are the main flow channels in the coal and play a significant role in coalbed methane (CBM) production. For a comprehensive understanding of the importance of multi-scale fractures in coal, an integrated approach of laboratory experiments and numerical simulation was used to analyze the characteristics of coal fracture networks in the samples collected from the southern Qinshui Basin, China. X-ray micro-computed tomography (micro-CT) was conducted to study the geometrical and topological parameters of multi-scale fractures in samples of different sizes. The fracture parameters, including fracture equivalent diameter, angle, orientation, spacing, fracture porosity, fractal dimension, and intersection point density, were quantitatively measured from micro-CT images of the coal samples. These micro-CT data were used to generate the multi-scale discrete fracture networks (DFNs), followed by the simulation of the gas flow behavior in coal fractures using COMSOL modeling software. The results show that the fracture equivalent diameter, fracture porosity, fractal dimension, and intersection point density are higher in the bright coal lithotype, indicating the heterogeneity distribution of the fractures in different coal lithotypes. The results depict a decline in the fracture equivalent diameter, fracture angle, fracture spacing, fracture porosity, and fractal dimension with the decrease in the sample size. However, the intersection point density of fractures shows the opposite trend. According to the fracture parameters, the fracture network development degree decreases, while the relation between different fractures (connectively) increases with the decrease of the fracture scale. The DFN simulation illustrates that the permeability increases from semi-dull coal (0.02 mD) to bright coal (0.025 mD). The long fractures contribute to the fracture connectivity network by connecting to smaller fractures and might show high-flow velocity in coal seams. Overall, the results indicate that the semi-bright coal and bright coal are beneficial to the production of CBM for the development of fractures. The findings of this study can help for a better understanding of the distribution of fractures in different coal lithotypes and will be favorable for enhancing CBM production.