Volume 64

Numerical Simulation of Heat Extraction and Heat Injection Processes Integrated with a Geothermal-Solar Heating System Tao Liu , Zheng Wang , Xinli Lu, Wei Zhang, Jiali Liu , Guangtong Zhao 2, Ping Zhao

Abstract

Long-term heat extraction from a closed-loop geothermal well results in a temperature decrease of formation around the well, which in turn lowers the outlet temperature of the heat extraction fluid (water). To improve this unfavorable situation, an integrated heat extraction–heat injection model has been developed in this study. The model is used to analyze formation temperature decline and its distribution after the first heating season, fluid temperature distribution in the well during heat injection, and fluid outlet temperature during the second heating season. The results indicate that during the first heat-extraction period, the formation temperature decreases under different flow rates reached a minimum value at depth of about 2860m. After 180 days of heat extraction, a plume-shaped low-temperature zone is developed around the borehole, with the maximum influence radius reaching about 12.6m at a depth of 2860m. During heat injection, the injected fluid in the inner pipe with a lower flow rate shows a U-shaped temperature profile. The upward flow in the outer pipe (annulus) transfers heat to the formation all the way during ascent when the flowrates are higher than 0.5kg/s, showing a monotonic decrease in temperature. A comparison of heat re-extraction shows that the heat-injection reduces the average temperature decline by about 50.8%. This study demonstrates that appropriate heat-injection, along with flow rate control, can significantly improve the temperature stability of formation around the geothermal well.

Keywords closed-loop geothermal well, deep borehole heat exchanger, formation temperature decline, improvement due to heat injection, influence radius