Heat Transfer Performance of Medium-Deep Coaxial Geothermal Wells: A Case Study in Changsha, China

A coupled model integrating three-dimensional heat transfer in the geological formation with one-dimensional non-isothermal flow in the wellbore was developed for medium-deep coaxial geothermal wells under the geological conditions of Changsha, Central-South China. The effects of wellbore configuration, materials, and operating parameters on heat transfer performance were investigated. The results show that the outlet and bottomhole temperatures decrease rapidly initially and subsequently decline at a slower rate, while the thermally disturbed zone continues to expand. Increasing the well depth raises both the outlet temperature and heat extraction rate. A large-diameter–high-flow-rate configuration increases the heat extraction rate but reduces the outlet temperature. Reducing the thermal conductivity of the central pipe mitigates thermal short-circuiting between the inner and outer flow passages, thereby increasing both the outlet temperature and heat extraction rate. Increasing the thermal conductivity of the cementing material enhances radial heat transfer, although the incremental benefit gradually diminishes, while the segmentation depth of the cementing material has only a minor effect. Increasing the inlet flow rate lowers the outlet temperature but increases the heat extraction rate, whereas increasing the inlet temperature produces the opposite effects. These findings provide a reference for wellbore configuration design, material selection, and operating-parameter optimization in Central South China.