Geothermal energy production utilizing a U-shaped well in combination with supercritical CO2 circulation

摘要

A new method for utilizing geothermal energy via supercritical CO2 circulation through a U-shaped well is proposed. By recycling the heat transmission fluid in a closed loop, the method can avoid continuous fluid loss and long-term formation damage to the geothermal reservoirs. Furthermore, a comprehensive model that incorporates the complicated heat transfer in the integrated wellbore and formation systems is developed to investigate the flow and thermal behavior of CO2 in the geothermal system, and is validated by the measured data. The simulated results show that the thermo-physical properties of CO2 vary dynamically in the U-shaped wellbore, providing a significant density-driven thermosiphon. For an identical injection rate, the outlet temperature and transported energy of CO2 are 108.0 degrees C and 186.81 kJ/kg, while those of water are 85.1 degrees C and 289.18 kJ/kg. Compared to the traditional double-pipe heat exchanger, the new system generates a higher outlet temperature and has a larger heat mining rate. The outlet temperature and pressure first increase and then decrease with increasing injection rate. Larger length of the horizontal section and lower thermal conductivity of the production tubing can be applied to increase the efficiency of geothermal exploitation in the proposed method.