Experimental Study of Shear Fracture Reactivation in Marcellus Shale

The source of hydrocarbon during production of unconventional shale reservoirs, such as the Marcellus shale, remains a mystery. The three primary sources are the induced hydraulic fractures, the existing natural fracture network, and the matrix. A model of hydrocarbon production requires knowledge of the amount of hydrocarbon and the permeability of each of these features among other parameters. In this study, we focus on the experimental characterization of shear fractures in order to understand the extent to which natural fracture reactivation during hydraulic fracturing can produce sufficient permeability and hydrocarbon volumes to impact overall hydrocarbon production.

Experiments were conducted on Marcellus shale core using a triaxial direct-shear coreflood system coupled with simultaneous x-ray radiography and tomography. The system allows us to collect data on the shear strength of the shale, permeability of the shale pre- and post-fracture, and fracture aperture measured without disturbing the shale from subsurface conditions. These data are collected as a function of confining stress, shear displacement, and pore-pressure fracture reactivation. We observe that elevated pore pressure can reactivate fractures (hydroshear) under stress conditions that are consistent with our stress-activated shear displacement measurements. Fracture permeability is significantly impacted by the initial confining stress (differences of orders of magnitude) and is reduced by increasing effective stress as would occur during reservoir depletion (order of magnitude impact). We find strong anisotropy in fracture apertures that implies an anisotropic permeability field with flow favored perpendicular to the shear-displacement direction. Finally, we observe non-planar shear structures that create roughness and flow constrictions arising from connection of en échelon shear segments.