Coal permeability, a key parameter influencing coalbed methane production and geological storage, is strongly governed by the dual-porosity nature of coal and the stress-dependent evolution of its fracture network. This study investigates the development characteristics of filled and unfilled fractures, and the resulting permeability anisotropy, in typical bright and dull coals from the deep 8# coal seam of the Ordos Basin. Utilizing CT scanning and permeability anisotropy testing, we analyze how fracture development impacts coal permeability and its evolution under stress. Bright coal exhibits a grid-like distribution of mineral-filled fractures with good vertical connectivity, and a complex network of unfilled fractures. In contrast, dull coal displays a scattered distribution of mineral-filled fractures with poor vertical connectivity and a limited number of unfilled fractures. Results indicate an exponential decay trend in permeability with increasing confining pressure, strongly correlated with fracture system development. Permeability also demonstrates significant heterogeneity (face cleat > butt cleat > vertical). Bright coal exhibits a greater permeability decay rate than dull coal, indicating heightened stress sensitivity, while its permeability anisotropy is weaker, aligning with the observed fracture development patterns.

Deep coalbed methane (CBM) demonstrates significant production potential, and a fervent exploration and development boom is currently underway in China. The permeability of coal reservoirs is heavily influenced by pore–fracture structure heterogeneity. Some researches have been conducted on deep coals’ pore–fracture structure; however, these studies mostly consider coal as a homogeneous material, neglecting the heterogeneity of the macrolithotypes within the coal. In this study, 33 deep coals with burial depths of more than 2000 m were obtained from the Daning-Jixian block of the Ordos Basin, covering all macrolithotypes: bright coal (BC), semi-bright coal (SBC), semi-dull coal (SDC), and dull coal (DC). These samples were subjected to three sets of NMR tests in dry, fully saturated, and irreducible water conditions, with the pore–fracture structure characteristics being analyzed. The results demonstrate that the sampled deep coals’ pore–fracture structure is highly heterogeneous, with transitional pores being dominant, followed by mesopores, “macropores and fractures”, and micropores. The NMR T_2C ranges from 0.61 to 2.44 ms, with an average of 1.19 ms; a higher T_2C value indicates more developed micropores. The ranges for producible water porosity (φ_pr) and producible water saturation (S_pr) are 0.31–7.24% (avg. 2.42%) and 6.97–71.47% (avg. 31.06%), respectively. Both of them exhibit a high positive correlation with the total volumes of “macropores and fractures” and mesopores. Compared to SDC and DC, the BC and SBC, especially the former, overall contain more “macropores and fractures” and mesopores, fewer transitional pores and micropores, and higher φ_pr and S_pr. These findings suggest that regions with abundant BC and SBC should be prioritized during deep CBM exploration and production due to the inherently superior permeability and gas extraction potential of BC and SBC, and these coals are likely to require less intensive stimulation to achieve higher recovery rates and could provide more sustainable gas production over time.

The development characteristics of primary fractures in coal serve as a prerequisite, influencing reservoir stimulation outcomes. The propagation of experimentally induced tensile cracks in the reservoir is closely related to the coal’s tensile fracture properties and the distribution of natural fractures. The effectiveness of reservoir stimulation directly determines the productivity of coalbed methane (CBM) wells, with clear variations in natural fracture development observed across different macroscopic coal components. Therefore, accurately evaluating the tensile fracture characteristics of different macroscopic coal components and the interaction patterns between experimentally induced and natural fractures is of great importance for deep CBM resources. This study focuses on vitrain, clarain, and durain from the deep #8 coal seam in the Daning-Jixian Block, located in the southern part of the Jinxi Flexural Fold Belt on the eastern margin of the Ordos Basin, which exhibit varying degrees of natural fracture development. Using the Brazilian splitting test and the centrally grooved three-point bending test, the tensile strength and Mode I fracture toughness of different macroscopic coal components were investigated. The study reveals the propagation behavior of pure tensile cracks and the resulting fracture network morphology in coal specimens with developed natural fractures. The presence of natural fractures reduces the tensile strength and fracture toughness of coal while increasing its brittleness. Compared to durain specimens (with average tensile strength of 2.09 MPa and fracture toughness of 0.338 MPa·m⁰·⁵), vitrain (avg. 0.92 MPa, 0.234 MPa·m⁰·⁵) and clarain (avg. 1.40 MPa, 0.185 MPa·m⁰·⁵) exhibit clearly lower tensile strength and fracture toughness, along with more pronounced brittle characteristics. Differences in geomechanical parameters among macroscopic coal components lead to distinct fracture behaviors: high-strength, high-fracture-toughness durain specimens require higher pressure to initiate fractures, which then propagate in a relatively regular manner; in contrast, low-strength, low-fracture-toughness vitrain and clarain fracture more easily, but their fracture paths are clearly influenced by natural fractures, promoting the formation of complex fracture networks. This study quantitatively characterizes the deflection and arrest behavior of experimentally induced tensile cracks interacting with dense natural fractures, providing a mechanistic basis for understanding fracture network complexity in deep coal.