Rock Properties of FORGE Well 58-32, Milford, Utah

摘要

Well 58-32 was drilled to a depth of 7536 feet in the Milford FORGE area during the summer of 2017 to confirm the reservoir characteristics inferred from existing wells and a wide variety of both new and legacy geologic and geophysical data. Drill cuttings were collected and described at 10-foot intervals and a robust suite of geophysical and image logs were run. Thermal conductivity, density, x-ray diffraction, magnetic susceptibility, and spectral gamma ray measurements were performed on the cuttings at least every 100 feet, and these analyses show that the basement rock within the FORGE area consists of a suite of intrusive rock types that are primarily granitic. Some diorite and monzodiorite was also encountered, as was a significant volume of rock with a more intermediate composition. The laboratory analyses were used to calibrate gamma ray and density log responses, allowing meter-scale variations in rock type to be more accurately identified and interpreted throughout the well bore. Similar small-scale compositional changes can be seen in outcrop at many locations in the adjacent Mineral Mountains. The density of the granite and intermediate rock types typically range from 2.6 to 2.65 g/cm3, but the higher gamma response of the granitic rock (140-290 gAPI) can often differentiate granitic compositions from intermediate compositions (70-210 gAPI). The higher density (2.7-3.0 g/cm~3) and lower gamma values (50-80 gAPI) of the dioritic compositions is more distinctive and greatly simplifies identification. The various laboratory analyses and geophysical logs of the 58-32 well prove it was drilled into low porosity/low permeability intrusive rock with temperatures well within the U.S. Department of Energy-specified window of 175°-225°C (347°-37℉). Despite small-scale compositional changes, the geomechanical characteristics throughout the reservoir zone are reasonably consistent and are suitable for testing the techniques and technologies required to develop enhanced geothermal reservoirs.