Hot dry rock resources are the most studied alternative to hydrothermal geologies, with an estimated potential capacity of approximately 5,000 GW e in the U.S. alone. Hydrothermal resources provide access to hot brine or steam through naturally permeable geologic structures, such as pores or faults in the subsurface. In contrast, accessing the heat within hot dry rock resources requires artificially increasing permeability by hydraulic stimulation and then pumping an external fluid through the enhanced-permeability system. Water is typically considered as the external fluid, but CO 2 has been shown to extract heat more efficiently due to its advantageous thermophysical properties. Because the fluid and fracture network are unnatural, hot dry rock is typically referred to as an "enhanced" geothermal system (EGS) resource.