Fracture network versus single fractures: Measurement of fracture geometry with X-ray tomography

Several long-standing questions concerning fluid transport in fractured rock involve the geometrical properties of the connected void space within a fracture network. We present experimental results that compare the geometrical properties of a fracture network to those of the individual fractures in the network. To image and quantify the aperture distribution of natural fracture networks in coal, a Wood's metal injection method is combined with X-ray computerized tomography and image analysis. We find that the aperture distribution of the networks is spatially anisotropic and dependent on the number and geometry of the individual fractures. Void area in the individual fractures ranged between 45%–58%. A three-dimensional auto-correlation analysis on the fracture network and a two-dimensional analysis on the individual fractures found that the apertures were correlated over distances of 10 mm to 20 mm in the direction of flow.