Long-term operation performance of medium-deep borehole heat exchangers in the Guanzhong area

Technologies for heating utilizing medium-deep geothermal energy exhibit promising application prospects in the Guanzhong area of China, which boasts abundant geothermal resources. Geological parameters serve as determining factors in the performance of medium-deep borehole heating systems. This study aims to explore the performance of medium-deep borehole heating systems in the Guanzhong area, Shaanxi Province. First, this study analyzed the geological parameters of five typical areas in Guanzhong: Xingping City under the jurisdiction of Xianyang City, Weicheng District of Xianyang, and Gaoling, Huyi, and Chang’an districts of Xi’an City. Then, by combining the typical structural parameters of buried pipes, this study simulated the performance and thermal influence radius of borehole heat exchangers under long-term operation. The results show that the performance of borehole heat exchangers arranged at different locations changed significantly at the beginning but tended to stabilize after around five years of operation. As the depth of buried pipes increased from 2000 m to 2500 m, the outlet water temperatures exhibited a significant upward trend, increasing by up to 8% at the same moment. The outlet water temperatures at the end of various heating seasons varied more significantly with an increase in the burial depth. Under the long-term operation, the average annual heat transfer rates showed a downward trend, with a total decreased amplitude reaching 11%-12% within 20 years. As the burial depths of borehole heat exchangers increased, the heat transfer rates rose by up to 41.41%-53.23%. The depths of buried pipes produced insignificant effects on thermal influence radii. After 20 years of operation, pipes with different burial depths displayed maximum thermal influence radii at the bottomhole of approximately 50 m. The soil temperature was significantly influenced by the distance from borehole heat exchangers and their operation duration. Specifically, the soil temperature within a distance of 20 m from heat exchangers manifested a pronounced downward trend with fluctuations, whereas that beyond a distance of 60 m showed a minimal decrease during 20 years of operation. Owing to the considerable thicknesses of the Neogene Zhangjiapo (N₂z) and Lantian-Bahe (N₂l+b) formations, buried pipes in Xingping City and the Huyi and Gaoling districts of Xi'an yielded relatively high outlet water temperatures and heat transfer rates. This suggests that medium-deep geothermal energy in these areas is more suitable for geothermal heating. The results of this study will provide a reference for the efficient utilization of medium-deep geothermal energy in the Guanzhong area.