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Abstract The Utah FORGE (Frontier Observatory for Research in Geothermal Energy) is a U.S. Department of Energy-sponsored field laboratory for demonstrating and promoting the feasibility of hydraulically fracturing thermally conductive granite (or similar) formations to create an Enhanced Geothermal System (EGS). One embodiment of an EGS, as adopted at FORGE, is to drill injection and production wells and interconnect them with hydraulic fracturing to form a subsurface heat exchange system. Three small fracturing treatments in the injection well were carried out in April 2022. In April 2024, eight fracturing stages were pumped in the injection well, and four fracturing stages were pumped in a vertically offset but parallel, sub-horizontal production well. The static bottomhole temperature is 430°F. The stages in the production well were perforated based on fracture intersections or proximity detected by fiber optics that had been previously cemented on the outside of the casing string in that well. To establish optimal procedures for treating granitic or similar reservoirs, the eight stages incorporated several variables to test, at least on a preliminary basis. These variables included stage volume, proppant placement, testing an ultralightweight proppant (ULWP), fluids – slickwater versus crosslinked carboxymethyl hydroxypropyl guar (XL CMHPG), and the number of perforation clusters. Most stages treated well, some verging on screening out. In two stages, proppant was not pumped as the treating pressure reached the maximum allowable surface pressure before achieving a pump rate sufficient to successfully carry proppant. This suggested unexpected geologic differences in this granitoid batholith. Regardless, all other stages treated well and are described. Silica sand was chosen for the proppant since the FORGE wells are not going to be used for commercial purposes. This is one of the few times that wells have been intentionally interconnected successfully. There was some success at the Fenton Hill project in the 1980s and Fervo Energy successfully interconnected wells in a geothermal play in Nevada. Fervo Energy has also started development of a new EGS project offsetting the Utah FORGE acreage. The importance of fiber optics is documented, along with the role of stimulating both wells and characteristic treating records for the production well. A nine-hour circulation test performed immediately after stimulation treatments confirmed significant hydraulic connectivity between the two wells. A successful longer-term circulation test conducted three months after the stimulation program is also described. Efficiency built to about 90% before a planned shutdown and flowing temperature tracked increasing flow rate from the production well – reaching a maximum of 385°F.
England et al. (Tue,) studied this question.