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Estimating Reserves for Enhanced Geothermal Systems Using Field Data and Analytical Models Richard HOLT [RESPEC, USA] |
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The U.S. Department of Energy (DOE) Geothermal Technologies Office (GTO) Frontier Observatory for Research in Geothermal Energy (FORGE) initiative is a dedicated field site in Milford, Utah, FORGE achieved high levels of technical success in drilling a highly slanted well doublet, fracturing the two wells, and ultimately demonstrating commercial levels of injection, production, and temperature though a long-term flow test. FORGE provides a public repository of data, however, there are private ventures also reporting technical successes in EGS, although these data are proprietary and not publicly available. While FORGE is continuing its research-based mission, it has provided enough data for private industry to begin making non-research, for-profit commercial applications of EGS in the field generally. Before proceeding with investment and development, private industry needs to make informed decisions and reduce risk via techno-economic assessment. Because EGS is transitioning from research to commercial applications, there are few analysis tools or frameworks developed and implemented by independent third parties. In Holt (2025), the FORGE flow test was analyzed using a simplified approach adapted from conventional geothermal reservoir engineering. In Holt (2025), the fractured volume of rock was estimated as a uniform homogeneous cylindrical “reservoir” and the Monte Carlo method of reserves from Muffler (1979) was applied with a bulk heat recovery factor to calculate P90 reserves. Authors in EGS have used the analytical solution of Gringarten (1975) to make heat recovery and reservoir performance calculations for idealized EGS systems, Doe (2014) and Fercho (2025). In this paper, the techniques of Muffler (1979) and Gringarten (1975) are combined to create a framework that allows EGS reserves to be calculated considering a non-ideal, measured distribution of fluid flow among fractures in a well doublet, and by extension, a full-field development. The result is a reasonably straightforward approach to calculate EGS reserves, power generation versus time, and timing of needed makeup wells.
Topic: Reservoir Engineering