Experimental investigation on the gas flow characteristics of coal samples with different fracture network complexities

As for deep fuel exploitation of coal and gas,it is practical to study the gas flow characteristics of coal by considering a fractured medium. Coal samples containing different fracture networks were obtained in laboratory. The fracture network was quantitatively classified by an industrial computed tomography test. Several gas flow tests were conducted and classic permeability models were investigated in terms of their applicability to the prefractured coal samples. The main results are as follows: an index was defined to describe the complexity of the fracture network, and the complexity index of the fractured coal samples changes from 0.4 to 0.7. With an increase in the complexity index, the fractured coal permeability presents power-function growth. The maximum permeability ratios of prefractured coal samples to those without prefracturing reached to over 300 times, respectively. With the effect of gas pressure, the fractured coal presents 5 typical permeability evolution patterns. The increase in confining pressure weakens the compression effect of the effective stress on the fracture network and enhances the possibility that the gas adsorption shrinkage effect on the coal matrix. Considering the effect the prefractured fracture network, the new empirical permeability model can give a good description for the permeability evolution of fractured samples, which is in accord with the experimental phenomena of permeability rebound and recovery.