Direct Measurement of Changes to a Sheared Shale Fracture

Abstract:

Shales are of particular interest both for their energy production potential and their ability to provide seals for sequestered CO2. It is imperative to understand the processes that influence fracture properties in order to accurately predict subsurface behavior. In order to evaluate the coupled hydrologic and geometric changes to a fracture undergoing shear displacement, a novel apparatus was used to produce real-time shearing of fractured shale at elevated pressures. Intermittent computed tomography (CT) scans were performed during each shearing event and transmissivity measurements taken to characterize both the mechanical evolution as well as the hydrodynamic response of the fracture.   The initial fractures examined had small apertures and high resistance to flow, with most aperture values below the resolution of the CT scanner and low transmissivities. Initial shearing events, with total displacement less than two mm, minimally increased the aperture and increased the transmissivity. Further shearing resulted in fracture apertures that were large with pressure differentials across the core below the detection limit of the pressure measurement equipment. Flow models using the complex fracture geometries obtained from the CT scanning was performed to examine the micro-scale impacts of fracture evolution on the flow field that were not apparent in the bulk data. These results showed strong flow channelization within the larger aperture regions and showed that hydraulic aperture was significantly different than average mechanical apertures.   Fracture aperture evolution due to shearing was characterized with the resulting mechanical and hydrologic responses. Apertures showed increases after each shearing event. Additionally, this increased aperture resulted in increased transmissivity and subsequent increases in hydraulic aperture. Coupling the experimental work with computational analysis provided further insight into the evolution of fracture flow properties.