Enhanced Geothermal Systems (EGS) require cold fluid injection as part of the process to extract heat from fractured hot rock to produce electricity. Over time, this cold fluid injection will induce pressure and temperature changes in the fractured rock which will, in-turn, have a feedback effect on fracture permeability through complex thermo-hydro-mechanical-chemical (THMC) coupling. Our study presents lab measurements that characterize shear fractures in granite from the Frontier Observatory for Research in Geothermal Energy (FORGE) site at its in-situ temperature, stress, pressure, and chemical conditions. Using the triaxial direct-shear method, we performed mechanical shearing and flow-through experiments on initially intact (i.e., unfractured) core samples. Applied reservoir conditions were 36.1 MPa total normal stress, 22.2 MPa pore pressure, and temperatures between 180 and 210 °C. Injected fluid was so-called “golf course water” to mimic the tap water chemistry that is being used at FORGE. Measurements included intact shear strength, residual shear strength, intact rock permeability, shear fracture hydraulic aperture, fracture dilation during shearing, stress-dependent fracture aperture, permeability change with temperature, and effluent chemistry changes. When sheared, fractured permeability exhibited a significant increase, often by a factor of 10^5 or more, and this enhancement was sustained over time. However, shear slip after the initial fracture event did not generate the expected rise in fracture permeability that is typical for room-temperature tests. Permeability then experienced a reduction by a factor of 10^2 due to stress-induced closure as the confining pressure was raised from 36.1 MPa to 48 MPa, and a similar magnitude reduction was again observed during cooling. Other key observations from our results include a strong tendency for silicate dissolution and magnesium salt precipitation during sustained flow. The shear-dilation was unusually low at 4° ±1° for the high-temperature conditions compared to values of 11° ±5° for room temperature tests. This low dilation coincides with smooth shear fracture surfaces that resembled slickensides and felt waxy to the touch. To the best of our knowledge, our measurements are the first-ever THMC measurements for rock fractures at EGS conditions. These experiments indicate that shear stimulation is unlikely to be effective at FORGE for geothermal reservoir flow enhancement. The experiments also indicate that mineral dissolution and precipitation will be highly active at FORGE conditions with injection water contaminants such as magnesium being key reactants for infilling precipitation in the fractures.

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