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.