Triaxial direct shear fractured marcellus (MSEEL) shale - Peak and residual shear strength, permeability, and hydroshear potential

Pre-existing fractures in the subsurface can serve as preferential fluid flow pathways, and have the potential to be shear activated during hydraulic stimulation operations. This shearing could significantly increase natural fracture permeability, improve access to additional reservoir volume for production, and contribute to induced seismicity if not adequately managed. We perform triaxial direct shear experiments to evaluate the permeability of freshly created fractures as a function of stress (e.g. depth) using specimens of carbonate-rich Marcellus Shale. The strength required to form and reactivate fractures was measured over a range of effective confining stresses from 2 to 30 MPa. Initially intact, specimens of 25 mm diameter and 25 mm length were stressed to reservoir conditions, fractured by direct-shear, and then subjected to shearing displacements of up to 3 mm. Continuous permeability measurements were acquired through the course of experiments. Simultaneous X-ray video and computed tomography were used to directly measure fracture displacement and apertures at stressed conditions. The creation of fractures at higher effective stresses resulted in an overall lower permeability compared to the compression of fractures created at lower effective pressures. Results include the analysis of transient fracture permeability following renewed shear displacement and direct evidence of hydroshearing from fracture reactivation caused by increases in fracture pore pressure.