Heat Extraction Performance of a Deep Downhole Heat Exchanger

The experimental test for evaluating the heat extraction performance for a deep Downhole Heat Exchanger (DHE) was carefully designed and performed in the last winter in Tanggu area, Tianjin, China. The test well in which DHE was installed had been used as a reinjection well for two years. The reservoir temperature at the bottom permeable layer of the well decreased to a maximum temperature of 51 degrees C from its original of 64 degrees C due to the reinjection of cold water. Up the reinjected cold water layer, there is an bedrock layer that has a geothermal gradient approximately of 2 degrees C per 100 meters deep. A novel DHE configuration was designed and constructed in the test. The results show that the new DHE configuration works very well for extracting heat from a deep geothermal reservoir. The stable outlet temperature and the thermal power from the deep DHE can be reached after continuously heat extracting in about 2 weeks. The maximum heating power according to the test data is about 270 kW, which is much larger than some theoretically calculated values of deep Borehole Heat Exchanger (DBHE) using the existing conduction model. (C) 2019 The Authors. Published by Elsevier Ltd.