Thermo-mechanical degradation and fracture evolution in low-permeability coal subjected to cyclic heating–cryogenic cooling

This study addresses the limitations of conventional hydraulic fracturing techniques, such as high water consumption and the potential to trigger water-lock effects. It systematically investigates the thermo-mechanical degradation behavior and fracture evolution mechanisms of low-permeability coal under the influence of cyclic heating and liquid nitrogen (LN2) cooling, a promising waterless stimulation technology. Using a combination of methods, including Brazilian splitting tests, three-point bending tests, acoustic emission (AE) monitoring, and three-dimensional (3D) surface profilometry, the progressive damage characteristics of coal's mechanical integrity over 15 cycles were revealed. Key findings include: cyclic thermal shock causes an 81.98% decrease in coal's P-wave velocity, a 63.31% reduction in tensile strength, and an 85.15% attenuation in fracture toughness, indicating significant cumulative damage due to repeated thermal shock. AE analysis confirms that microcrack nucleation and propagation intensify during the cycles, with the coal's failure mode transitioning from brittle to ductile. Fractal dimension and roughness index-based fracture surface characterization shows a significant increase in fracture network complexity with the number of cycles.