Triaxial Direct-Shear In Situ Microtomography

Abstract — Shear fractures are known to facilitate fluid conductivity through rock. It is also known that a fracture’s aperture is a controlling characteristic for its fluid conductivity and that fractures tend to exhibit some form of roughness that will influence fluid flow. We evaluate mechanical strengths of en echelon shear fracture structures and find that an en echelon model predicts that fresh fractures have a tendency to be rougher, rather than more planar, and predicts a maximum amplitude for this roughness along the fracture length, all without need to call on heterogeneity. This tendency for rough fracture creation is validated by in situ x-ray images and fluid conductivity measurements from triaxial direct shear experiments on anhydrite and shale. These experiments applied various states of confining stress from 4 to 30 MPa and shear displacement magnitudes from 0 to 2 mm on initially intact rock specimens. Hydraulic, dilatational, and local fracture apertures were measured in the experiments and predicted by the model. Apertures exhibited strong anisotropy with larger flow paths forming perpendicular to the direction of shearing. Local and dilatational aperture were found to be positively correlated with increasing shear displacement but hydraulic aperture was found to vary significantly, always having values smaller than the other aperture measures at factors ranging from 0.6 to 0.0. An implication of these results is that shear fractures have a mechanism for simultaneously exhibiting very low fluid conductivity and high fluid storage volume.

Abstract — Characterization of the potential hydraulic conductivity through rock fractures at stressed conditions is needed to better predict flow behavior through actual rocks in the field away from boreholes. We have measured the hydraulic conductivity of triaxial direct-shear fractured anhydrite rock specimens in the laboratory at varied confining stress conditions and observed these fractures with X-ray imaging. Anhydrite was acquired from the Big Sky Carbon Sequestration Partnership in Montana. These experiments provide a means to evaluate the potential for hydraulically conductive fractures to form through this initially impermeable caprock in response to in-situ stress changes, as can occur due to CO2 injection. Results show that in-situ stress is a key factor for limiting fracture conductivity and provide evidence that a critical confining stress magnitude exists above which conductive fractures may no longer be possible. In addition, computed tomography sections provide information on how fracture aperture distribution can change in response to increasing stress at fracture creation. This segmented computed tomography aperture information is compared with the hydraulic aperture and specimen dilation.

Abstract — The upper Duperow formation at Kevin Dome, Montana is a low-porosity dolomite with lesser anhydrite. The unit forms a caprock for potential CO2 storage at Kevin Dome in work conducted by the Big Sky Carbon Sequestration Partnership that is part of the Department of Energy’s research portfolio. The CO2 storage reservoir is in the middle Duperow formation, which is also a dolomite but which is more porous and permeable. Although carbonates are not often considered as caprock for CO2 storage, the impermeable character of the upper Duperow indicates that it would be an effective seal for CO2. However, a good caprock should also be resilient to fracture damage as could occur during stress changes associated with injection of CO2 into the storage reservoir. In this study, we use triaxial direct-shear coreflood experiments combined with simultaneous x-ray radiography/tomography measurements to characterize the fracture behavior of massive dolomite, and the permeability of hear fractured dolomite as a function of confining stress and displacement of fractures. The experiments were conducted at confining stresses from 3.5 to 30 MPa (500 to 4300 psi). Specimens (2.5-cm diameter) were first equilibrated to the confining stress within the triaxial coreflood system, and permeability of the intact material was measured. The specimens were fractured using a directshear system in which offset, semi-circular pistons advance focus shear along the mid-plane of the specimen. Water transmissivity was continuously measured using upstream and downstream ISCO pumps during this process, and the development of fractures was identified by sudden loss of axial stress, noticeable displacement in radiography, and a marked increase in fluid flow rate. The specimen was then returned to hydrostatic stress conditions. X-ray radiography was used to characterize fracture displacement and specimen dilation. X-ray tomography was conducted at hydrostatic conditions to determine fracture aperture and geometry. The experiments were concluded with a series of fracture reactivation steps applied with the direct-shear device and net fracture displacement approached 3 mm (12% of the specimen length). Fracture reactivation resulted in a short-lived increase in permeability.

Abstract — Caprock leakage is of crucial concern for environmentally and economically sustainable development of carbon dioxide sequestration and utilization operations. One potential leakage pathway is through fractures or faults that penetrate the caprock. In this study, we investigate the permeability induced by fracturing initially intact Marcellus shale outcrop specimens at stressed conditions using a triaxial direct-shear method. Measurements of induced permeability, fracture geometry, displacement, and applied stresses were all obtained at stressed conditions to investigate the coupled processes of fracturing and fluid flow as may occur in the subsurface. Fracture geometry was directly observed at stressed conditions using X-ray radiography video. Numerical simulation was performed to evaluate the stress distribution developed in the experiments. Our experiments show that permeability induced by fracturing is strongly dependent on the stresses at which the fractures are created, the magnitude of shearing displacement, and the duration of flow. The strongest permeability contrast was observed when comparing specimens fractured at low stress to others fractured at higher stress. Measureable fracture permeability decreased by up to 7 orders of magnitude over a corresponding triaxial confining stress range of 3.5 MPa–30 MPa. These results show that increasing stress, depth, and time are all significant permeability inhibitors that may limit potential leakage through fractured caprock.

Abstract — The challenge of characterizing subsurface fluid flow has motivated extensive laboratory studies, yet fluid flow through rock specimens in which fractures are created and maintained at high-stress conditions remains underinvestigated at this time. The studies of this type that do exist do not include in situ fracture geometry measurements acquired at stressed conditions, which would be beneficial for interpreting the flow behavior. Therefore, this study investigates the apparent permeability induced by direct-shear fracture stimulation through Utica shale (a shale gas resource and potential caprock material) at high triaxial stress confinement and for the first time relates these values to simultaneously acquired in situ X-ray radiography and microtomography images. Change in fracture geometry and apparent permeability was also investigated at additional reduced stress states. Finite element and combined finite-discrete element modeling were used to evaluate the in situ observed fracturing process. Results from this study indicate that the increase in apparent permeability through fractures created at high-stress (22.2 MPa) was minimal relative to the intact rock (less than 1 order of magnitude increase), while fractures created at low stress (3.4 MPa) were significantly more permeable (2 to 4 orders of magnitude increase). This study demonstrates the benefit of in situ X-ray observation coupled with apparent permeability measurement to analyze fracture creation in the subsurface. Our results show that the permeability induced by fractures through shale at high stress can be minor and therefore favorable in application to CO2 sequestration caprock integrity but unfavorable for hydrocarbon recovery from unconventional reservoirs.