Impact of Geochemical Reactions on Performance of Enhanced Geothermal Systems Using CO2 as Working Fluid

In recent years, as part of an effort to reduce atmospheric emissions of carbon dioxide (CO2), a novel concept of operating enhanced geothermal system (EGS) using CO2 instead of water as working fluid (CO2-EGS) and achieving simultaneous geologic sequestration of CO2 has been evaluated. CO2 appears to be superior to water in extracting heat from hot fractured rock and reducing the power consumption of the fluid circulation system, because its large expansivity and lower viscosity result in substantially greater mass flow rates than those of water. However, there remain uncertainties about chemical interactions between fluids and rocks. CO2 itself would not be a strong solvent for rock minerals, but aqueous solutions of CO2 can be quite corrosive and capable of dissolving minerals and even attacking the steel liners and casings used in the well construction. Such different chemical reactions by changes of the fluid phases may significantly alter hydrodynamic properties of EGS reservoirs over time and space, affecting heat extraction rates. Therefore, it is critical to conduct comprehensive studies on chemical interactions between fluids and rocks and their impact on the operation of EGS. We have performed reactive transport modeling to study the impact of fluid-rock interactions on CO2-driven EGS, particularly focusing on mineral alteration and associated porosity changes and their impact on reservoir growth and longevity. In addition, the potential ancillary benefit of CO2 sequestration in the system is evaluated. We consider an idealized fractured reservoir system with a five-spot well configuration in a two-dimensional model. Data for mineralogical composition are taken from various EGS sites such as Soultz and Desert Peak for our numerical analyses. This modeling study could provide a guidance for identification of future CO2-EGS demonstration sites.