Study on wellbore fluid flow and heat transfer of a multilateral-well CO2 enhanced geothermal system

Research efforts towards CO2 enhanced geothermal systems (CO2-EGS) are increasing due to potentially high heat extraction efficiencies. It is well known that CO2 properties are highly sensitive to the temperature and pressure and have noticeable effects on CO2-EGS heat extraction. Consequently, an understanding of wellbore CO2 fluid flow and heat transfer mechanisms is necessary when considering variable CO2 properties. In this paper, a coupled wellbore and reservoir fluid flow and heat transfer model is used to estimate multilateral-well CO2-EGS efficiency. The model is calibrated and validated by field data from the HGP-A well in Hawaii. Schematically, concentric tubulars are used allowing single well injection and production. Multiple cases are analyzed using this model. These include effects of CO2 pressure work on wellbore CO2 fluid flow and heat transfer, assessment of differences in heat extraction using varying wellbore sizes and central tubing insulation lengths, and evaluations of efficiencies under different injection-production well configurations. Results show that CO2 pressure work can induce a dramatic temperature reduction in the central tubing of the multilateral well CO2-EGS. The majority of pressure loss occurs in the formation and central tubing. The optimized design suggests that a three-layer central tubing with a central diameter of 0.19 m will maximize heat insulation and heat extraction. The effect of annulus diameter on heat extraction is negligible. Also, a lower injection well configuration results in a higher outlet temperature, thermal power output, and lower pressure losses compared to an upper injection well configuration. These results provide significant suggestions for wellbore designs that can potentially optimize multilateral-well CO2-EGS efficiency.