Laboratory fracture conductivity tests were conducted to investigate the effects of temperature, time and sliding on the conductivity of a representative fracture under in situ conditions experienced at the DOE’s FORGE (Frontier Observatory for Research in Geothermal Energy) site, near Milford, Utah. Conductivities of artificially sheared (shifted) fractures were compared to conductivities before shear displacement and associated dilation. Two types of experimental tests were performed: 1) hydro-shearing and conductivity measurement at room temperature in a triaxially loaded configuration, and 2) conductivity at elevated temperature under hydrostatic confining pressure conditions. These experiments were performed on granitic core recovered from the thermal reservoir in well 58-32 at the Utah FORGE location. The experimental results showed a significant increase in fracture conductivity after substantial shear displacement. The fracture conductivities were approximately two orders of magnitude larger in the sheared samples. Variation in conductivity depends on the mechanical and physical properties of the matrix and the fractures, as well as fracture roughness, and the size distribution of the associated asperities. While conductivity was initially substantially larger, these laboratory experiments also showed a significant decrease in fracture conductivity over even relatively modest testing times. In one test, at ambient temperature, conductivity dropped to less than 20% of its measured initial value after only 100 hours of representative effective normal stress application. In all cases, the increase of normal stress on the fracture surface, and possibly thermal stresses and loading, led to fracture conductivity reduction. There is a direct correlation between measured roughness, conductivity magnitude immediately after shear displacement and reduction in conductivity with time at load. Reservoir simulations should carefully consider this thermo-mechanical conductivity degradation. Future studies would be useful to evaluate injecting of a substantially cooler fluid and the influence of cyclic injection.

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