TRANSMISSIVITY OF FRACTURED SHALE IN RESPONSE TO CYCLIC STRESSES

Flow via fractures in shales is of interest to a wide sections of the energy industry, both from the standpoint of production of unconventional resources, as well as shales’ potential function as a reservoir seal, in particular for carbon dioxide sequestration projects. Fractures in the subsurface are subjected to a variety of stresses, which can change as a result of fluid injection or production.

In order to evaluate how the changing stresses affect fracture morphology, transmissivity, and flow distribution, the X-ray computed tomography (CT) facility at the National Energy Technology Lab has analyzed the evolution of mechanical apertures in several shale fractures as they were subjected to cyclic changes in overburden stress. Flow data gathered at each pressure step allowed for acquisition of transmissivity measurements. Laminar, single-phase Local Cubic Law flow simulations were subsequently run on the isolated fracture morphology, and the evolution of primary flow pathways with changing confining pressure was analyzed.

Transmissivity of fractures decreased in a non-linear fashion, undergoing a rapid initial decrease with the application of overburden stress, and a more gradual change with additional increases. With successive pressurization cycles a general trend toward smaller fracture apertures and decreased transmissivity was observed, as asperities were degraded, and fine gouge material accumulated within the fracture. Flow, as modeled via simulation, appears to be largely controlled by the localization of zero aperture zones generated through these processes.