Geothermal exploitation by circulating supercritical CO_2 in a closed horizontal wellbore

摘要

A novel method for geothermal development via supercritical CO2 circulation through a closed horizontal wellbore is proposed. Furthermore, a comprehensive transient model is built to analyze the wellbore temperature distribution for the new method, which considers the complicated heat transfer mechanisms in the coupled wellbore/formation system and the dynamic variations of CO2 flow behavior accompanied by the heat source/sink effect. The model is validated by experimental and model results quoted in the literature. The simulated results show that the outlet temperature decreases gradually with time and ultimately approaches a constant value. Affected by the significant variations in CO2 thermophysical properties, (1) a large buoyant drive will be provided by the density difference between the CO2 in the tubing and the CO2 in the annulus, (2) the simulation error for bottomhole temperature can reach 5.23 degrees C if the effect of flow work is neglected, and (3) the outlet temperature of CO2 is significantly larger than that of water at the same injection displacement. Moreover, the outlet temperature and heat mining rate can be greatly increased by adjusting the circulation direction of working fluids, and increasing the thermal insulation of the tubing and the length of the horizontal section.