Effect of Shear Slip on Fault Permeability in Shale Reservoir Rocks

Abstract:

Understanding flow along faults and fractures in shales is important for better understanding of hydraulic stimulation in unconventional reservoirs. For example, the re-activation of faults and fractures during hydraulic stimulation appears to be an important process contributing to reservoir permeability. In this study, we examine the effect of shear slip on fault permeability in shale reservoir rocks. We perform shear experiments in a triaxial apparatus on two types of samples: 1) a sample sawcut at 30° to the cylindrical axis and 2) a naturally broken sample. Both samples are from 3481 m (11422 ft) depth within the Haynesville reservoir containing 22 wt.% clay. First, we hydrostatically load the samples to a confining pressure of 15 MPa (2176 psi), followed by triaxial loading in which a constant axial displacement rate of 1 μm/s is applied for increments in axial displacement of initially 0.25 mm and later 1 mm. After each shear increment, we perform measurements of fault permeability at a constant mean pore pressure of 2.1 MPa (300 psi) using the steady state Darcy flow method. Boreholes drilled parallel to the cylindrical axis on either side of the shale sample allow pore fluid to access the fault plane. The coefficient of friction increases with shearing from 0.53 to 0.61 for the sawcut sample and from about 0.60 to 0.74 for the naturally broken sample. The sawcut sample indicates stable sliding behavior whereas small stick-slip events occur in the naturally broken sample. Upon shearing, fault permeability decreases by about 2.5 and 1.5 orders of magnitude within the first mm of shear displacement for the sawcut and naturally broken sample, respectively. Fault permeability of both samples continues to slowly decrease up to a maximum axial displacement of 4 mm and 2 mm, respectively. Laser scanning images before and after shearing show the formation of small striations in the direction of slip for the sawcut sample and the break-off of several grain fragments and formation of gouge for the naturally broken sample. New flow pathways appear to develop during shearing of the naturally broken shale sample due to grains being plucked off of the sliding surfaces and crushed, resulting in a smaller fault permeability reduction than for the sawcut sample with smoother fault surfaces.