Strength Characteristics and Crack Propagation Modes of Coal Samples with Different Slots under LN2 Freeze–Thaw

Abstract

In the realm of coalbed methane extraction, liquid nitrogen (LN₂) fracturing technology is proposed as a promising ecofriendly alternative for enhancing permeability. Nevertheless, integrating this technology into practical engineering applications continues to pose significant challenges. To elucidate the mechanical effects of LN₂ injection channel geometries on fracturing, this study employs uniaxial compression and acoustic emission (AE) techniques to assess changes in the mechanical properties of coal samples featuring slots and circular slots of varying angles before and after freeze–thaw. Additionally, the study examines the coupled effects of LN₂ and geo-stress on the failure mechanisms of coal samples featuring slotted geometries. The findings indicate that (1) post-LN₂ freeze–thaw, slotted coal samples exhibit decreased uniaxial compressive strength, reduced elastic modulus, and lower AE peak energy, with an increase in cumulative AE energy. (2) The angle of the slots significantly affects the mechanical properties of the coal samples, showing a positive correlation with uniaxial compressive strength, elastic modulus, and cumulative AE energy under consistent conditions. (3) Under uniaxial compression, cracks primarily localize near the slots, with a higher prevalence of tensile cracks in coal samples with angled slots of 0° and 90° and circular slots compared to those with angled slots between 30° and 60°. Following LN₂ freeze–thaw, the coal samples transition from a brittle to a ductile state, effectively facilitating the propagation of shear cracks. Based on these results, new experimental evidence supports the application of LN₂ in coal seam fracturing technology for enhanced fracturing and permeability, providing valuable insights for guiding engineering practices and optimizing LN₂ fracturing processes.