Experimental Study on Permeability Evolution of Multi-scale Fracture in Coal Under Varying Pore Pressures

Due to the application of hydraulic fracturing, a proper understanding of the permeability evolution of multi-scale fractures in coal is essential for predicting CBM production and formulating development strategies. In this paper, a combination of laboratory experiments and analytical model was adopted to investigate the permeability evolution patterns of multi-scale fractures in coal under varying pore pressures. Experimental results reveal that with the increase of pore pressure, the permeability of intact coal presents a trend of first decreasing and then increasing, whereas the permeability of self-propped coal samples exhibits a monotonic increasing trend. Additionally, the permeability of propped fracture is significantly higher than that of intact coal and self-propped samples, showing an approximately linear increase with increasing pore pressure. Notably, under the same pore pressure conditions, the higher proppant concentration and larger proppant size corresponds to greater propped fracture permeability. The experiment al results also indicate that the permeability of fracture propped with larger-sized proppants are more sensitive to pore pressure changes. At low proppant concentrations, propped fracture permeability increases significantly with increasing concentration, however, this enhancement diminishes as the concentration further increases.