Comprehensive analysis of supercritical CO2 heat transfer in U-shaped geothermal well: Operational modes and parameter sensitivity analysis

Abstract

Closed-Loop Geothermal System (CLGS) is a promising approach for efficient geothermal energy utilization. This study develops a numerical model to simulate supercritical CO₂ (SCO₂) heat extraction in a U-shaped well. The governing equations are solved using the finite difference method, and the model is validated with field data. The effects of working fluid (SCO₂ and water), injection temperature, flow rate, pressure, horizontal well length, and insulation layer length on thermal performance are systematically analyzed. Moreover, four heat extraction cases, including continuous heating and intermittent heating, are compared. When the temperature difference between injection and production wells reaches 160 °C, a density difference of 500 kg/m³ induces a thermal siphon effect, reducing pumping power and enhancing heat extraction efficiency. At 20 MPa and 90 °C, the density-to-viscosity ratio of SCO₂ is 13.1—about 4.4 times that of water—indicating superior flowability. Under equal pressure difference, SCO₂ increases outlet temperature and thermal power by 19.8% and 28.8%, respectively. Compared with continuous operation, intermittent operation exhibits better heat transfer and thermal sustainability. While increasing the flow rate enhances thermal power, it lowers the outlet temperature. There is an optimal injection pressure (16 MPa) in the heat extraction system, at which both the outlet temperature and the heat extraction power reach their maximum values. The Plackett–Burman(PB) experimental design method is used to quantify the relative influence of these parameters. These findings provide theoretical support for optimizing SCO₂-based closed-loop geothermal systems.