Experimental investigation on the stress sensitivity of permeability in naturally fractured shale

In this paper, we present an experimental investigation regarding the stress sensitivity of permeability in naturally fractured shale. Gas permeability tests were performed on the fractured cylindrical shale samples under loading and unloading conditions. Different hydrostatic stress and gas pressure levels were chosen to investigate the dependence of permeability on stress. The permeability of the fractured shale decreases with increasing hydrostatic stress, re-increases during unloading and is irreversible during loading and unloading processes. The gas pressure exhibits a significant effect on the permeability in comparison with the hydrostatic stress. Small gas pressure changes (e.g., 2 MPa) induce a comparable change in permeability with a large hydrostatic stress change (e.g., 40 MPa). The gas pressure gradient on the permeability will be discussed. The fracture aperture was estimated by recording the volume change during loading and shows that the aperture change is consistent with the permeability evolution during loading, which is more complicated at a higher hydrostatic stress value. The roughness of the fractured surface was also analyzed and will be discussed in combination with the permeability evolution.