Integration of Borehole Data for Hydrothermal Alteration, Natural Fracture, and Permeability Characterization at the Fallon FORGE EGS Site, Nevada, USA

The Fallon FORGE EGS (Engineered or Enhanced Geothermal System) site in Churchill County, Nevada, USA, underwent detailed geoscientific characterization between 2015 and 2018 to evaluate its suitability to operate as a field demonstration site to test and develop EGS technologies. Site activities culminated in the drilling of a 2481 m EGS observation well (21-31) in February-March 2018 to further verify the geology and rock properties of the proposed EGS reservoir formations. Here we integrate and evaluate borehole data from well 21-31, including drill cuttings and sidewall cores (petrographic, XRD, hyperspectral), wireline geophysical logs (e.g., gamma, neutron porosity, resistivity, sonic scanner), borehole imagers (FMI, BHTV), and pressure and temperature logs. The primary lithologies intersected in the lower 500 m of well 21-31 (the target EGS reservoir interval; pre-Miocene section) include volcanic (altered-basalt, altered-rhyolite tuffs, andesite), plutonic (porphyritic and equigranular felsic intrusions), and metamorphic (quartzite) units. Hydrothermal alteration and veining is common in the pre-Miocene interval of well 21-31, and includes propylitic alteration assemblages that suggest temperatures >250°C. Natural fractures observed in FMI logs are abundant throughout the pre-Miocene section, with dominant orientations of NNE-SSW. This is subparallel to the interpreted SHmax orientation of 009° ± 14 that is indicated by induced structures in the wellbore, and therefore the natural fractures are optimally oriented for hydraulic stimulation. Mud loss locations and a non-equilibrated temperature log suggest that there are some zones with relatively-higher permeability and/or open fractures. However, injection test results indicate an injectivity index of 0.025 liters/min/kPa, which is an order of magnitude lower than a typical threshold for commercial wells.