Experimental study on infiltration and freeze–thaw damage of water-bearing coal samples with cryogenic liquid nitrogen

Liquid nitrogen, an ultra-low-temperature fluid, can cause fissures in coal, especially water-bearing coal. Several coal samples with different water saturation levels were used in this study and compared with a dry sample. The surface changes of each sample were photographed using metallographic microscopy to investigate the effects of liquid nitrogen soaking. Our results show that: 1) For multiple liquid nitrogen freeze–thaw cycles, the porosity of the water-bearing coal samples increased with an increase in the saturation level. Due to the effects of liquid nitrogen circulation freezing and thawing, the porosity can be increased effectively. 2) After being dipped into liquid nitrogen, the length and the width of fissures in the coal samples increased, and some new fissures were simultaneously produced. Liquid nitrogen soaking caused the fissures to expand, which led to an increase in the fissure proportion. The combined effects of (1) and (2) lead us to conclude that the desired cracking effect was achieved. 3) From percolation theory, the connectivity of pore fissures effectively increased due to the effects of the liquid nitrogen, which is directly related to the water-bearing percentage. The more saturated the coal sample, the more connected the pore fissures become. It can be seen that liquid nitrogen is beneficial for improving coal seam permeability and modifying coal properties.