Numerical investigation on heat extraction performance of a CO2 enhanced geothermal system with multilateral wells

A CO2 enhanced geothermal system (EGS) with multilateral wells is proposed to exploit hot dry rock (HDR) in this paper. For this EGS, several upper injection and lower production multilateral wells are sidetracked from one main wellbore in HDR. CO2 is injected from the injection multilateral wells into the reservoir and then extracts heat. Subsequently, high temperature CO2 is produced from production multilateral wells and returns to the surface through the insulated tubing in the main wellbore. In this study, a 3D fluid flow and heat transfer model for this CO2-EGS is established. The effects of key factors on the heat extraction performance of this CO2-EGS are studied. The performances of double-well and multilateral-well CO2-EGS are compared. The results indicate that multilateral-well CO2-EGS has a greater heat extraction performance than conventional double-well CO2-EGS. The lower production pressure, more multilateral wells and longer multilateral wells can improve CO2-EGS performance, while the lateral-well diameter has negligible effects on CO2-EGS performance. The injection mass flow rate and well spacing should be optimized to obtain a stable output thermal power and long enough service-time. This work offers good guidance for the optimization design of CO2-EGS with multilateral wells.