Experimental Study on Tensile Fracturing Characteristics and Crack Propagation of Deep Coal with Different Macroscopic Compositions

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.