Experimental study on porosity and permeability characteristics of damaged coal in different shear end faces and confining pressures

Coal damage is a significant factor which affects coal permeability changes and gas flow. Damage-based seepage testing system with multi-shear end faces and nuclear magnetic resonance instrument were adopted to analyze porosity and permeability characteristics of damaged coal in different shear end faces and confining pressures. It is found that the number of macro-pores in damaged coal increases. Meanwhile, adsorption space transforms into seepage space, with better connectivity between pores. In the case of 50% shear end face, average area percentage of seepage space （relaxation time T₂>10 ms） rises from 9.89% to 14.94%. Affected by shear stress, the axial deformation of coal continuously increases, 50% shear end face causing bigger axial deformation than the 30% shear end face. The larger the confining pressure value, the smaller the axial deformation. With increasing shear stress, permeability of coal decreases first before slowly growing. When stress reaches the threshold of coal strength, permeability of damaged coal rises exponentially. The permeability value corresponding to 50% shear face is generally larger than that of 30% shear face, and the former is about 7 times that of the latter.