Experimental Study on the Change of the Pore-Fracture Structure in Mining-Disturbed Coal-Series Strata: An Implication for CBM Development in Abandoned Mines

After a coal mine is abandoned, a large amount of coalbed methane (CBM) resources is still accumulated in the overlying and underlying coal strata. The structure of the coal/rock pore fracture, which is a key element for the development of CBM resources, changes significantly due to the influence of mining. In this study, the pore volume and pore size distribution (PSD) of six coals (three parallel bedding coals and three vertical bedding coals), three mudstones, and three sandstones before and after fracture are compared from the perspective of macro and micro using a triaxial compression experiment and high-pressure mercury intrusion porosimetry, and the pore-fracture evolution of the coal under different confining pressures is studied. By quantificationally characterizing the pore-fracture strain of fractured coal/rock, an equivalent model based on plastic deformation is proposed. Combined with the pore-fracture characteristics of coal/rock strata in abandoned mines, the influence of the coal/rock fracture on CBM migration is discussed. The results show that (1) the pore volumes of coal, mudstone, and sandstone are significantly reduced after the fracture. The compression of coal is more significant than that of other samples. The macropore (>50 nm) volume of coal and mudstone decreased sharply in the process of loading, while that of mesopores (2-50 nm) changed little. For sandstones, the macropore and mesopore volumes are compressed. (2) With the increase of effective stress, the compression degree of pores increases; especially, the volume of macropores decreases significantly. (3) The expansion of fracture is beneficial to the migration of CBM, while pore compression reduces the diffusion path. The ultimate performance of CBM migration in mining-disturbed strata is determined by the net effect of two competing effects. This study has a guiding role in improving the pore-fracture strain physical model of fractured coal/rock and revealing the CBM migration in abandoned mines.