Subsurface Effects in a CO2 Engineered Geothermal System Thermosiphon

Abstract

Engineered geothermal systems (EGS) present an opportunity to expand the scope of geothermal implementation worldwide. Differences from traditional geothermal systems indicate that design modifications may improve their potential. A change that has been proposed is the use of CO2 as a heat extraction fluid for the reservoir. CO2 has the benefit of being a non-polar fluid (indicating no salt dissolution in a CO2-rich phase), favourable transport properties (particularly low viscosity), and a large buoyancy effect (due to operation in the supercritical phase). The disadvantage of CO2 is the larger mass flows required, leading to increased influence on the system of wellbore frictional pressure losses. Compared to a waterbased system where fluid flows are dominated by viscous flow through a porous reservoir, CO2- based systems will have significant contributing effects from turbulent wellbore friction, viscous reservoir flow, and buoyancy effects. Each of these can dominate to varying degrees depending on process design and operating parameters. Their effect on the manner in which a CO2-based system operates makes a pure thermodynamic comparison with more traditional approaches problematic. Economic comparison of different designs is necessary.