The Utah Frontier Observatory for Geothermal Research (FORGE): Results of Recent Drilling and Geoscientific Surveys

摘要

The U.S. Department of Energy's FORGE (Frontier Observatory for Research in Geothermal Energy) initiative will bring Enhanced Geothermal System (EGS) development to commercial viability and visibility by creating a controlled environment where EGS technologies can be developed and tested. The Utah FORGE site is located in central Utah, 350 km south of Salt Lake City. The region has been extensively explored in support of geothermal development at Roosevelt Hot Springs. Since the 1970s, more than 100 wells, the deepest to 3.8 km, have been drilled in the vicinity of the site. Well 58-32 was drilled to a depth of 2296 m (7536 ft) on the FORGE site in 2017 to characterize the underlying reservoir rocks. Additional supporting data was provided by new geologic mapping and 3-D seismic reflection, gravity, hydrologic and soil gas surveys. The well encountered a bottom hole temperature of 197°C in Tertiary granitic rocks. Although the well encountered more than 2000 natural fractures, measured permeabilities are low, less than 30 microdarcies. Induced fractures indicate the maximum horizontal stress trends NNE-SSW. Stress gradients based on analysis of microhydraulic and Diagnostic Fracture Injection Tests range from 0.58 - 0.63 psi/ft for σ_(Hmin) and 0.68 - 0.82 psi/ft for σ_(Hmax). A gradient of 1.13 psi/ft was calculated for σ_V. Enhancement and growth of the induced fractures are clearly apparent when pre- and post-injection Formation Microscanner Image logs are compared. These changes demonstrate the suitability of the FORGE reservoir rocks for Enhanced Geothermal System development.