Numerical investigation of fluid flow and heat transfer in a doublet enhanced geothermal system with CO2 as the working fluid (CO2–EGS)

Enhanced geothermal system with CO2 instead of water as the working fluid (CO2–EGS ) has attracted much interest due to the additional benefit of CO2 geological storage during the power generation process. This paper describes numerical analyses of a doublet CO2–EGS system, focusing on the influence of the CO2 injection rate, the permeability of induced fractures near the wellbores, the injection/production well perforation placement, the working fluid, and the heat transfer between the wellbores and the surrounding reservoir. The larger permeability in the induced fractures around the wellbores allows the fluid to more easily flow through the reservoir up to a critical fracture permeability, with further increases of the fracture permeability further reducing the pressure loss a little but the cost is not worth the added benefit. Induced fractures around the wellbores result in little difference among different wellbores perforation locations in the reservoir. Increased CO2 injection rates reduce the heat transfer between the wellbores and the surrounding reservoir so that this heat transfer can be neglected at large mass flow rates. With CO2 as the working fluid, the CO2 temperature decreases significantly going up the production well. This paper presents some important implications for CO2–EGS system for further numerical studies as well as for practical projects.