Permeability measurements during triaxial and direct shear loading using a novel X-ray transparent apparatus: Fractured shale examples from Beetaloo basin, Australia

Abstract

Coupled multiphysics constitutive models are generally developed based on macro-(core) scale measurements often without an in-depth knowledge of involved micro-scale phenomena. Recent addition of micro-scale imaging capabilities to triaxial and direct shear systems has enabled the characterisation of micro-scale phenomena during failure process using techniques such as 3D X-ray computed tomography. These X-ray transparent deformation-flow cells, however, are often force controlled, and incapable of measuring the displacement independently. They also have difficulties in controlling the temperature, and often do not allow measurement of permeability or cannot combine both direct shear and triaxial capabilities in one system. We therefore present the development and the first trial of an X-ray transparent apparatus capable of controlling the sample temperature without influencing X-ray radiation. The designed system allows for measurement of axial deformation both in load and displacement control-mode where permeability measurements can be conducted during the experiment. In order to assess the robustness of the designed system, the conductivity of fractured shale samples from the Beetaloo basin, Australia were measured under triaxial and direct shear loading. The triaxial experiment was conducted on a sample drilled parallel to bedding planes at 10 MPa confinement and up to 30 MPa axial stress. The gas permeability was continually measured during the loading and micro CT-images were acquired before-loading, at peak-load and after-unloading to investigate any induced damage and its potential effect on the permeability. In addition, a direct shear test was conducted on another Beetaloo shale sample with 0.5 MPa normal stress and 0.5 mm/s shear displacement. The fracture conductivity was measured throughout the test and sample was scanned before-shearing and right after-peak to investigate the effect of fracture shearing on overall fracture conductivity and aperture variations.