Numerical study on heat extraction performance and parameter optimization of a U-shaped geothermal well

The U-shaped geothermal well, as a method for extracting geothermal energy, is of critical importance for utilizing energy in geothermal reservoirs. To investigate factors that influence heat extraction efficiency of U-shaped geothermal wells, a numerical model with a borehole heat exchanger was first verified. A 3200-m deep U-shaped geothermal well was then modeled based on the geological and reservoir data of the Huangling area in Shaanxi Province, China. Sensitivity analysis was conducted focusing on key factors such as geotechnical thermal properties, injection flow rate, and injection temperature. The Grey Relational Analysis method was applied to rank the influence of these parameters on outlet temperature and heat extraction efficiency, followed by optimization to identify the optimal operating parameters. The relative importance of factors influencing outlet temperature is as follows: injection flow rate > injection temperature > thermal conductivity of geological formation, while for heat extraction efficiency, the order is injection temperature > injection flow rate > thermal conductivity of geological formation. The optimal injection flow rate and temperature for U-shaped geothermal wells in this area are 100 m3/h and 10 °C, respectively. This study could serve as an example for optimizing heat extraction from medium- to deep-depth geothermal reservoirs.