| Abstract |
Self-propping asperities in fractures are key to providing permeability in enhanced geothermal systems (EGS). The apertures resulting from shear offsets provide the permeability, however the lifespan of the asperities is limited by dissolution, precipitation, and mechanical deformation. These in turn are functions of the temperature, pressure, fluid chemistry, mineralogy, and stress state of the system. Quantification of the changes in permeability under the conditions that could be expected in an enhanced geothermal system has not been previously performed and is currently ongoing. In performing this work, our goal is to better understand how different rock types, mineral compositions, and fracture surface textures affect the life span of fractures. Such knowledge would enable selection of reservoir rock to optimize long-term fracturing effectiveness. To achieve our goal, we developed and built a specialized laboratory system allowing the application of a normal stress on a fractured disk-shaped sample while flowing water radially through the aperture. In our system, we are testing geothermally relevant rock types to assess changes in permeability of induced (radial) slip-shear fractures under conditions relevant to EGS (our effective fracture normal stress is up to 20 MPa, temperature up to 300°C) and monitor the geometric aperture, permeability, and effluent chemistry. Prior to the tests, we characterize our rock mineralogically and measure the surface profile of both surfaces. We assemble the fracture with a slight radial offset to provide an initial aperture. After making a series of system measurements, we apply our desired normal stress, raise the temperature, and flow water. We monitor the effluent chemistry and the pressure differential between the inlet and outlet, which allows our calculation of the hydraulic aperture. Post-test characterization is similar to the pretest characterization, allowing computation of the pre and post-test aperture. Opening the fracture allows characterization changes and their spatial distribution with respect to initial aperture. In this paper, we will present the results of two tests: Stripa granite at 150°C, and a meta-sedimentary rock at 250°C retrieved from a depth of 1485.3 m from well BCH-03 at Brady. |