Geofluid circulation enhanced downhole heat exchanger for highly efficient heat extraction: Mechanism and optimization

• Downhole forced convection heat extraction superiority was experimentally proven. • Heat extraction mechanism is explained by geofluid circulation and heat transfer evolution. • Optimization of key parameters for enhanced DHE was achieved. The Downhole Heat Exchanger (DHE) is critical for the development of geothermal energy. However, conventional DHEs experience a gradual reduction in heat extraction efficiency over time. The study developed a geothermal heat extraction scheme based on forced convection of geothermal fluid (geofluid), with the heat transfer process simulated using an experimental setup. Experimental results showed that enhanced DHE achieves higher temperature rises than the conventional version. This confirms the feasibility of enhancing heat transfer efficiency through forced geofluid circulation. Building on this, the study further investigated the mechanism of the enhanced DHE through numerical simulations, aiming to reveal its heat transfer characteristics and optimize parameters including geofluid flow direction, circulation rate, production casing length, and outer casing length. The results show that the enhanced DHE outperforms the conventional version in production temperature, heat extraction power, and net power. Continuous production creates a low-temperature region near the reinjection point, affecting long-term performance. Geofluid circulation acts as the key mechanism for heat extraction, intricately linking the evolution of heat extraction with geofluid flow in the aquifer. It is recommended to circulate geofluid upstream at a rate of 400 m³/d. Extending the lengths of the middle casing and outer casing enhances heat extraction power. By balancing heat transfer efficiency and economic cost, the optimal casing lengths are 200 m for the middle casing and 300 m for the outer casing. These findings provide valuable insights into optimizing key parameters for enhanced DHEs.