The concept whereby heat is recovered from a geothermal reservoir by replacing the native aqueous phase with essentially pure high-pressure carbon dioxide as the working fluid is referred to as an Enhanced Geothermal System with CO2 (EGSCO2). The concept has yet to be tested in the field, and the chemical consequences, both within the reservoir itself, and at its periphery, are not fully understood. Modeling the chemical evolution of EGSCO2 systems over a range of operating parameters, spatial dimensions, host rock compositions and geohydrologic properties is needed, but requires substantial development beyond the current state of the art. At issue is how and to what extent this development might be required in order to answer questions that could critically affect the feasibility and successful implementation of the technology.

We review the literature on natural analogues, including wall rock alteration in gold deposits and gas-rich geothermal systems, as well as experimental studies, thermodynamic models for secondary minerals, equation-of-state formulations in the system NaCl-CO2-H2O and state-of-the-art electrolyte models, to gain insight into CO2-induced fluid-rock interactions for temperatures in the range 10 - 350ºC, pressures from 0.1 - 60 MPa, and salinities from 0 - 6 molal NaCl. We then propose a multi-step process through which developments in economic geology, igneous and metamorphic petrology, geothermal chemistry and the physical chemistry of electrolytes might be progressively integrated by taking advantage of existing computer codes, while simultaneously resolving critical uncertainties affecting the implementation of EGSCO2.

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