Production Chemistry Evidence for an EGS Type Reservoir in Roosevelt Hot Springs and Implications for Utah FORGE

Abstract

The Roosevelt Hot Springs hydrothermal system is a Basin and Range-type geothermal resource located at the base of the Mineral Mountains in southwestern Utah, 4 km east of the Utah FORGE EGS site. Exploration drilling down to 2000 m depth in the 1970s proved the existence of a hot water hydrothermal plume that covers ~3 km2. The conventional reservoir is made of fractured, Oligocene-Miocene granitoid rock that in terms of rock type and properties is analogous to the Utah FORGE EGS reservoir. Since 1984, the Blundell power plant has been in continuous production, obtaining fluid supply (240-290 kg/s) from four wells with feed point temperatures of 240° to 265° C. A nearby deep well (14-2) has been used for long term but variable injection that represents between 25 and 70% of the total injectate. Time series analysis of production fluid chemistry including pre-production, 1991-1992, 2015 and 2016, provide clear evidence of injection chemical breakthrough as represented by increases in Cl and modest drops in enthalpy, relatively early in the production history. Taking into account continuous steam loss and inmixing effects, simple models show that EGS type heat extraction has been significant and helped to sustain reservoir productivity for at least 25 years. We infer from the surface geology that sub-vertical fracture-related permeability in the reservoir creates baffles that facilitate percolating fluid flow and heat exchange as the injectate migrates into the production zone. These results show the promise of developing an EGS reservoir at the Utah FORGE site.