Experimental study on the fissure structure and permeability evolution characteristics of coal under liquid nitrogen freezing and freeze–thaw

Liquid nitrogen (LN2) fracturing technology as a kind of waterless fracturing technology has received extensive attention in recent years. In order to investigate the evolution law of fissure structure and seepage characteristics of coal samples under LN2 freezing and freeze–thaw, the evolution of fissure of coal samples before and after LN2 freeze–thaw was monitored by micro-computed tomography, the change of permeability of coal samples under different LN2 freezing time and freeze–thaw cycles was measured, and the damage mechanism of LN2 freezing and freeze–thaw to coal was discussed. The experimental results show that (1) LN2 freeze–thaw can cause the shrinkage of the coal matrix, resulting in damage to the fissure structure of the coal sample, which promotes the initiation, expansion, and extension of fissure of coal sample to form new fissure and, finally, forms a fracture network. (2) The permeability and the increment of permeability of coal samples increase with increase of LN2 freezing time and the number of freeze–thaw cycles under different gas pressure and confining pressure condition. (3) Under the same freezing time, the permeability growth rate of coal samples under LN2 freeze–thaw condition is significantly greater than that of coal samples under LN2 freezing condition. (4) The frost-heave force and thermal stress are the main factors leading to the damage, promoting fissure formation and increasing permeability of coal samples LN2 freezing and freeze–thaw. This study provides a theoretical basis for the understanding of fracturing technology with LN2.