Mechanism of Liquid Nitrogen Freezing–Blasting Synergistic Fracturing for Coal Seam Permeability Enhancement

Abstract To enhance gas extraction from low-permeability coal seams, this study introduces a synergistic liquid nitrogen (LN2) freezing–blasting method designed to mechanically precondition coal and promote fracture propagation during blasting. Coal specimens with varying moisture contents (MC) were subjected to controlled LN2 freezing regimes, followed by uniaxial, triaxial, and tensile testing using a coal–rock triaxial creep apparatus. Subsequent blasting experiments under biaxial lateral loading elucidated fracture propagation and coalescence behaviors in frozen coal. The results show that LN2 freezing significantly alters the mechanical behavior of coal and promotes fracture initiation. Under uniaxial loading, specimens frozen for 60 minutes at 12% MC exhibited the strongest enhancement, with both compressive strength and elastic modulus more than doubling compared with unfrozen dry coal. This strengthening effect became more pronounced under triaxial confinement. In contrast, tensile strength associated with fracture initiation was degraded due to LN2 vaporization-induced thermal shock and frost-heave effects, reaching only 0.65 MPa at 60 minutes of freezing and 12% MC, a 126% reduction compared with unfrozen dry coal. Subsequent blasting experiments demonstrated that LN2 pretreatment significantly improved fracture development and connectivity, promoting the transfer of blasting energy from the near-field crushed zone to far-field interconnected fractures. Under optimal conditions (60 minutes of freezing and 12% MC), the brittleness index reached 24.66, surface-penetrating fractures extended up to 108.8 mm, and the crushed zone length was reduced to only 14.7% of that in unfrozen dry coal.