Strength Characteristics and Fracture Propagation Patterns of Coal Samples With Different Slots Subjected to Liquid Nitrogen Freeze-Thaw Treatment

In the field of coalbed methane (CBM) extraction, liquid nitrogen (LN2) fracturing technology has been proposed as a potential environmentally-friendly alternative for enhancement of permeability. However, its application in engineering practices remains a significant challenge. This study aims to explore the mechanical mechanisms of LN2 fracturing through directional drilling in coal seams, revealing the characteristics of mechanical behavior of coal bodies with different slot morphologies under the influence of LN2. Utilizing uniaxial compression and acoustic emission (AE) techniques, this paper examines the changes in mechanical properties of coal samples with circular slots and different angled slots (0°, 30°, 45°, 60°, 90°) before and after LN2 freezing-thawing, and discusses the coupling effect of LN2 and in-situ stress on the fracturing mechanism of slot coal bodies. The results indicate that: (1) LN2 freezing-thawing reduces the uniaxial compressive strength and elastic modulus of slot coal samples, while increasing peak strain, maximum AE counts, and cumulative counts. (2) The angle of the slot has a certain impact on the mechanical properties of the coal samples; under the same conditions, the slot angle correlates positively with uniaxial compressive strength, elastic modulus, maximum AE counts, and cumulative counts. (3) Under uniaxial compression, cracks are concentrated near the slot, with 0°, 90° and circular slot coal samples showing a higher proportion of tensile cracks. Post-LN2 treatment, slot coal samples transition from brittleness to ductility-plasticity, effectively promoting the expansion of shear cracks. Based on these findings, new experimental evidence is provided for the application of LN2 in enhancing permeability and fracturing technology in coal seams, offering theoretical value and practical significance for guiding engineering practices and optimizing LN2 fracturing processes.