Stress-dependent fracture porosity and permeability of fractured coal: an in-situ X-ray tomography study

Coal porosity and permeability are highly stress-sensitive. Although the stress-dependent permeability behaviour of coal has been well documented in the literature, few studies are available which directly observe the dependency of fracture aperture, porosity and connectivity on effective stress. In this study, in-situ X-ray tomography was conducted using a novel X-ray transparent tri-axial core holder to provide a detailed characterization of coal fracture network evolution due to increased effective stress. Both conventional and synchrotron X-ray sources were used to image two fractured anthracite coals (named sample A and sample B) at resolutions of 38.7 and 18.1 μm, respectively. Permeability was measured at effective stresses from 0.5 to 6 MPa for sample A and from 0.5 to 11.0 MPa for sample B. Results show that the permeability for both samples decreased by one order of magnitude following an exponential function. X-ray CT images indicate that large fractures were quickly compressed and isolated to small fractures, which accordingly caused dramatic porosity reduction by a power law. The fracture contact area increases in response to increased stresses, resulting in a decrease of fracture compressibility. Both an exponential decreasing trend and a linear decreasing trend were observed. When the fracture contact area increases, the tortuosity and spatial connectivity of the fractures become important, because these attributes significantly affect permeability. Preliminary studies on the correlation between permeability and CT-resolved porosity and that between permeability and CT-resolved aperture are not consistent with the cubic law. This may indicate that the fractures cannot be adequately represented as smooth parallel plates. A mixture of tube and plate model is proposed to better describe fracture geometry considering its rough nature.