STRESS-PERMEABILITY EFFECTS THROUGH SHEAR FRACTURES FORMED IN THE SUBSURFACE

The potential for permeability increase through stimulated fractures and faults in the subsurface has become a significant concern, especially as associated with induced seismicity. However, the effect of stresses on the permeability of freshly formed fractures is not yet well understood. In this study, we use X-ray imaging to directly observe fracture initiation and propagation through rock at stressed conditions and we couple this information with simultaneous permeability testing. Fractures were created using a triaxial direct-shear method through initially intact specimens of fine-grained Utica shale. X-ray computed tomography images of a specimen that was fractured at 22 MPa confining stress and observed at stressed conditions, indicate that significant shear deformation can be accommodated by distributed fracturing and communition of the rock, all without creating a hydraulically connected fracture aperture pathway or significantly enhanced permeability. However, in experiments at lower confining stress (3.5 MPa), significant permeability increases of 2 to 4 orders of magnitude were observed in which discrete hydraulically open fractures were created by shear and propped open by asperities. Results from our experiments show that even moderate-stress conditions can reduce or inhibit permeability enhancement of shear-stimulated fractures. These results highlight the challenges of stimulating permeability in hydraulic fracturing operations, while illustrating the potential resilience of caprock.