Geology, State of Stress, and Heat in Place for a Horizontal Well Geothermal Development Project at Blue Mountain, Nevada


Steven FERCHO, Jack NORBECK, Emma MCCONVILLE, Nick HINZ, Irene WALLIS, Aleksei TITOV, Saurabh AGARWAL, Sireesh DADI, Christian GRADL, Hank BACA, Eric EDDY, Camden LANG, Katharine VOLLER, and Timothy LATIMER

Key Words:

Near-Field EGS, Horizontal Drilling, Proppant, Heat in Place, State of Stress


Stanford Geothermal Workshop




Enhanced Geothermal Systems



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3752 KB

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Fervo Energy is developing a commercial nearfield enhanced geothermal system project adjacent to the Blue Mountain geothermal field, located in northern Nevada. The goal of the project is to provide production uplift to the Blue Mountain power facility, as well as to demonstrate the efficacy of Fervo’s horizontal geothermal well design and reservoir management strategy. In this paper, we present the geologic conceptual model that formed the basis for targeting a three-well drilling campaign, which included Vertical Monitoring Well 73-22, Horizontal Injection Well 34A-22, and Horizontal Production Well 34-22, three deep wells that were drilled and completed successfully in 2022. In addition, we present key data used to constrain the state of stress and geomechanical model at the Blue Mountain site, including laboratory measurements on core samples, image logs, and petrophysical logs. Well prognoses were developed from geologic, thermal, and conceptual hydrothermal models based on available data prior to the drilling of these new wells. We then compare the geologic prognoses for all three wells against the as-drilled lithology and temperature profiles. Based on the updated geologic model following the successful drilling campaign, we estimated the heat in place and power capacity for this project. The project area intentionally targeted the hot (350 °F to 400 °F) and low-permeability, thermally conductive margins of the active hydrothermal system to demonstrate the feasibility of horizontal drilling and multistage stimulation in geothermal formations and at high-temperature conditions. The horizontal wells were strategically oriented to balance stable temperature along the laterals with optimal orientation to the local stress field. Three-dimensional temperature modeling and analysis of drilling induced fractures from offset well image logs successfully informed the well placements. The lithologies encountered at the target depths of the laterals were Mesozoic metasediments composed of interlayered phyllite and quartzite intruded by diorite and granodiorite dike swarms. Drilling results from Vertical Monitoring Well 73-22, which targeted a location approximately at the midpoint of the laterals, confirmed the predicted temperatures and depth to Mesozoic basement, which informed the drilling program of the subsequent horizontal wells. The equilibrated temperature profiles of the horizontal wells were measured by both wireline logging and fiber optic sensing equipment installed permanently behind the casing in both wells. Following the stimulation treatment in Injection Well 34A-22, Production Well 34-22 was drilled through the stimulated reservoir volume. Acoustic and resistivity image logs were obtained along the lateral of Production Well 34-22, and we observed numerous fractures aligned with the maximum horizontal stress orientation suggesting the presence of tensile fractures created during the stimulation treatment. In addition, proppant samples were detected at numerous locations while drilling the lateral section of Horizontal Production Well 34-22, suggesting that proppant injected during the stimulation treatment on Injection Well 34A-22 was transported significant distances away from the wellbore. Heat-in-place estimates confirm that the stimulated reservoir volume created from the stimulation treatment in well 34A-22 was sufficiently large to enable approximately 5 MW of electric power production over a 10-year project life, consistent with the target well performance for the horizontal doublet geothermal well system.

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