Experimental study on the evolution of pore structure of coal samples under freeze–thaw

To investigate the effect of freeze–thaw on damage and pore structure characteristics, some coal samples were freeze-thawed by freeze–thaw test machines. The evolution law of the pore structure of coal samples under freeze–thaw cycles, the porosity, and the bound and free fluid volume of coal samples under freeze–thaw were studied by low nuclear magnetic resonance, the complexity of the pore structure of coal samples under different freeze–thaw cycles was analyzed by fractal dimension, and the influence mechanism of freeze–thaw on the pore structure of coal samples was discussed. The results indicate that: (1) freeze–thaw could damage the pore and fracture structure and reduce the wave velocity of coal samples, and the wave velocity of coal samples after 120 freeze–thaw cycles decreased 66.5% compared with that of before the freeze–thaw cycle. (2) Freeze–thaw can effectively promote the development of pore and fracture. With the increase in freeze–thaw cycles, the proportion of micropores of coal samples decreases, while the proportion of mesopores and macropores gradually increases. (3) The freeze–thaw cycle can promote the increase in the coal sample porosity, and the increment of total, residual, and effective porosity is 3.47%, 1.94%, and 1.53%, respectively, after 120 freeze–thaw. (4) The fractal dimension of total, adsorption, and seepage pore of coal samples all decrease with the increase in freeze–thaw cycles, indicating that liquid nitrogen can reduce the complexity of the pore structure and weaken the heterogeneity of the pore structure.