There is growing interest in the novel concept of operating Enhanced Geothermal Systems (EGS) with CO2 instead of water as heat transmission fluid (CO2-EGS). Numerical simulations of fluid dynamics and heat transfer indicate that CO2 may be superior to water in its ability to mine heat from hot fractured rock. Carbon dioxide also offers advantages with respect to wellbore hydraulics, in that its larger expansivity as compared to water would increase buoyancy forces and reduce the parasitic power consumption of the fluid circulation system. While the thermal and hydraulic aspects of a CO2-EGS system look promising, major uncertainties remain with regard to chemical interactions between fluids and rocks. We have performed reactive transport modeling to study the impact of fluid-rock interactions on CO2-EGS. Data from the quartz monzodiorite unit at the Desert Peak EGS site (Nevada) were used for the modeling analysis. A five-spot well configuration in a two-dimensional model was chosen to investigate (1) mineral alteration and associated porosity changes, and (2) impacts on reservoir growth and longevity, with ramifications for sustaining energy recovery, for estimating CO2 loss rates, and for figuring tradeoffs between power generation and CO2 mineralization (geologic storage).

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