Evolution Characteristics and Mechanism of Microwave-Induced Fracture Structure in Loaded Coal for Enhancing Coalbed Methane Extraction

In a natural coal reservoir, the coal seam is subject to in situ stress and gas pressure constraints. Previous studies have indicated that microwave-induced fracturing significantly damages the pore and fracture structures of coal bodies under unloaded conditions. However, under the stress conditions of a geological environment, the mechanical properties of coal differ significantly from those under unloaded conditions, thereby influencing the extent of microwave-induced fracturing. To address this research gap, experiments are conducted on loaded coal samples using a real-time uniaxial-loading microwave irradiation (MI) device. A comparative analysis of the fracturing effects of microwaves on coal bodies under loaded and unloaded conditions is performed. Based on a CT scanning system, three-dimensional (3D) fracture and pore network models of coal samples before and after MI were established to reveal the evolution laws of the quantity, scale, and connectivity of the fracture structures of coal under the combined action of an external load and MI. Results indicate external load promotes mutual connection of microfractures; under the same microwave power, the growth rate of the fracture volume ratio of loaded coal samples increased on average by 34.75% compared with those of unloaded coal samples. External load during MI promotes pore throat expansion and fracture network formation, with connectivity index 45% higher in loaded coal samples. By applying the Griffith strength theory, we found that the restrictive effect of external load on the coal body thermal expansion exacerbates cumulative internal stress, enhancing thermal stress and prompting fractures in the main stress direction, which allows for better destruction of the coal body surrounding the microwave borehole in a high-stress environment. This work can advance the on-site application of microwave-induced fracturing technology in enhancing coalbed methane (CBM) extraction.