Characterization of Drill Core from Stimulated Crystalline Rock Following Hydraulic Fracturing at the Utah FORGE Geothermal Test Site

Abstract

In April 2022, three hydraulic stimulation stages were completed near the toe of injection well 16A(78)-32. In April-June 2023 the production well, 16B(78)-32, was drilled ~300 ft above and roughly parallel to the highly-deviated injection well. Three intervals in the production well cored through stimulated crystalline reservoir rock, totaling 135.8 ft. Core from stimulation 1 (10,430 to 10,493 ft) and 3 (9,800 to 9,853 ft) consists of dark-colored, banded gneiss. Core from stimulation 2 (10,250 to 10,304 ft) consists of light-colored granitoid. Fractures in the core were assigned the following classifications: 1) planar; 2) semi-planar; 3) unbroken mineralized; 3) rough; 4) concave-convex; and 5) curviplanar. Within individual core runs coherent fracture measurements of planar features (planar, semi-planar and unbroken mineralized fractures) were obtained. Measured fracture orientations were rotated to match the trajectory of the deviated well. A depth correction and an axial rotation was then applied to best fit microresistivity image logs. Planar features dominantly strike NNW-SSE in alignment with the maximum horizontal stress. Planar features were most common in gneiss from the stimulation 1 and 3 cores, with the stimulation 3 core containing most planar features. The stimulation 3 core may have been adjacent to a reactivated natural fracture zone. Fractures containing secondary vein minerals were open and filled with fluid in the reservoir prior to the drilling of 16B(78)-32. The majority of the planar features are mineralized, with calcite being the most common phase. Calcite ± anhydrite may have precipitated from the injected fluids at reservoir temperatures. Cross-cutting relationships suggest closely spaced, curviplanar fractures in the bottom of the stimulation 1 core are impermeable features in the reservoir.