Asperities resulting from shear offsets in fractures prop the fractures open and are key to providing permeability in enhanced geothermal systems (EGS). These asperities and the resulting permeability are controlled by rock dissolution, precipitation, and mechanical deformation. These in turn are functions of the temperature, pressure, fluid chemistry, mineralogy, and stress state of the system. Fracture permeability changes under the conditions that could be expected in EGS reservoirs have been quantified in our laboratory experiments. Through experiments our objective has been to gain a better understanding of how different rock types, mineral compositions, and fracture surface topography affect the life span of fractures, enabling selection of reservoir rock to optimize long-term fracturing effectiveness. Through modeling, we aim to extend these results. Our experimental tests were performed in a custom laboratory apparatus which applies normal stress on a disk-shaped rock sample containing a fracture that is perpendicular to the cylinder axis. We apply a shear offset to establish an initial aperture. We introduce water such that it may flow radially through the fracture aperture, and we measure the aperture change, fracture permeability, and effluent chemistry. In this system, we perform extended duration tests on geothermally relevant rock types to assess changes in permeability of sheared induced extensile fractures under conditions relevant to EGS (effective fracture normal stress up to several tens of MPa, temperature up to 250°C). Our samples are from Stripa, (Sweden), Brady Hot Springs (NV), and Desert Peak (NV) and include a granite, a meta-mudstone-volcanic from beneath 4800 feet, a rhyolite sample from beneath 3590 feet, and silicified rhyolite tuff from beneath 2476 feet. Before and after the tests we characterize our rock mineralogy and mechanical properties (from P and S-wave velocities), and measure the surface profile of both surfaces. We assemble the fracture with a slight angular offset about the core axis, to provide an initial aperture. After making a series of system measurements, we apply our desired normal stress, raise the temperature, and flow water. Opening the fracture allows characterization of changes and the spatial distribution with respect to initial aperture. A rich suite of data has been collected. Geomechanical modeling using TREACTMECH, and THMC modeling using TOUGHREACT are being performed to improve our overall understanding of the coupled chemical-mechanical-hydrological processes. Our modeling efforts include modeling artificial aperture fields and those derived from the experiments.

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