Comparison of production characteristics of various coaxial closed-loop geothermal systems

The effective exploitation of deep geothermal resources has been a research focus in the geothermal industry. The coaxial closed-loop geothermal system (CCGS) is expected to be the most feasible technique for developing deep geothermal resources in the form of closed-loop circulation. It avoids high-cost fracturing and environmental problems, making it preferable when compared with the open-loop geothermal system. Current studies mainly focus on the thermal performance of the CCGS with one well type. However, the comprehensive performances of various CCGSs have a noticeable difference in geothermal energy exploitation. It is essential to implement a detailed comparative analysis of various CCGSs to determine the production characteristics and enable their application. In this study, a 3D transient thermal-hydraulic model considering the effect of fluid gravity and viscous friction in the wellbore is proposed to realise a CCGS comprehensive investigation. The temperature field, energy, flow, and economic characteristics of the CCGSs with vertical-well, horizontal-well, and multilateral-well types are compared. To confirm the efficacy of the CCGS, the energy, flow and economic efficiencies are introduced. The results indicate that fluid gravity is beneficial for decreasing the injection pressure and pump consumption. The fluid viscous friction is important for the CCGS energy, flow, and economic performances. The precedence order obtained in terms of the long-run efficiency for the application is multilateral-well CCGS > vertical-well CCGS > horizontal-well CCGS. The key findings offer a significant reference for the CCGS scientific study.