Relations between tomographic and geomechanical observations Fracture growth and sealing in the subsurface are often complex, yielding tortuous pathways showing significant horizontal and vertical extent (Xxxxxx and Xxxxxxxxx, 2012; Thiercelin et al., 1987). Hydraulic fracturing experiments, at either constant strain rate or constant stress condition, were performed to understand the geomechanical properties of shales. Synchrotron-based time-resolved X-ray tomography (4D XCT) allows for the understanding of the local conditions around a fracture as it develops and seals (Xxxxxxxx Xxxx et al., 2017). The relations between tomographic and geomechanical observations therefore improves the understanding of shale mechanical properties significantly and have strong implications for reservoir conditions and sweet spot identification. The in-situ imaging experiments were conducted on samples manufactured in two orientations with respect to the bedding planes. The confining pressure was applied gradually, with regular pauses to record tomographs of the sample material under increasing confinement. Once the desired confining pressure was reached, the pressurisation system was reconnected and used to increase the borehole pressure, simulating a rise in fluid injection pressure. Injection pressure was raised gradually, with pauses for tomographic imaging. Once the breakdown pressure was reached, the fracture growth (and resultant pressure drop) was found to be rapid and uncontrolled. The injection pressure was a function of the volume of fluid injected. After failure, the injection pressure was lowered to zero, and the confining pressure was raised, in order to image the pressure- driven closure of fractures (Figure 4). Fractures occur in the borehole-parallel orientation in bedding-parallel samples of all materials. In bedding-perpendicular samples, fractures only develop in the borehole-parallel orientation in the unlaminated materials. In materials where layering is present, the fractures were commonly seen to grow in a borehole- perpendicular orientation, so as to follow the layering in the material. Breakdown pressure during these experiments was found to increase with confining pressure, but was seen to be isotropic between the two sample orientations, and not strongly sensitive to the different material. At ambient conditions these microcracks have a strong effect on the propagation of fractures, but this is suppressed by the microfracture closure at elevated. Existing fractures were observ...
