Experimental Investigation of Shear Fracture Development and Fluid Flow in Dolomite

摘要

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 shear 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.