Experimental study on coal pore structure deterioration under freeze–thaw cycles

This study investigated the impact of consecutive freeze–thaw cycles on the coal pore structure deterioration. Eight coal samples (cores and thin sections) underwent different numbers of freeze–thaw cycles. The effect of freeze–thaw cycles on microscopic characteristics of the coal samples was evaluated by nuclear magnetic resonance and scanning electron microscope. The experimental results showed that the coal samples were significantly deteriorated by freeze–thaw cycles, inducing numerous cracks generated on the sample surfaces. The total porosity of coal continuously increased with consecutive freeze–thaw cycles. The number of pores in the coal samples increased under the effect of freeze–thaw stress. Moreover, the proportion of pores in the coal after freeze–thaw cycles altered. The proportion of micropores and mini-pores gradually decreased, while that of meso-pores and macro-pores (cracks) increased. Therefore, freeze–thaw stress was conducive to gas desorption, diffusion, and transportation, which ultimately improved the efficiency of coal bed methane extraction from coal seams.