| Title | Field Tests and Numerical Simulation of Thermal Response Test Equipment Applicable to Water Wells of Large Diameters |
|---|---|
| Authors | Satoshi TANAKA, Hikari FUJII, Hiroyuki KOSUKEGAWA, Retsu HARADA |
| Year | 2024 |
| Conference | Stanford Geothermal Workshop |
| Keywords | TRT, GHE, thermal conductivity, GSHP system, optical fiber thermometer |
| Abstract | This study aimed to validate the effectiveness of a novel TRT equipment applicable to large diameter water wells. The equipment consisted of an optical fiber thermometer, a cable heater, and cylindrical electromagnets, which heat the surrounding ground in close contact with the casing of the water well. The vertical distribution of apparent thermal conductivities of ground (λs) is estimated by applying Kelvin's line source theory to temperature changes during the heating and recovery periods. First, TRTs were conducted four times with different heating periods using the new TRT equipment in a water well (204.7 mm ID) cased with steel pipes to investigate the necessary length of the heating period. The heating periods were changed from 0.5 days to 4 days, and the heat load of the heater was set to approximately 43 W/m for all four TRTs. Thereafter, estimated λs profiles were compared with a reference profile estimated from TRTs using cable heating method in the same test field. The λs profiles estimated from the heating periods of each test differed significantly from the reference profile due to the natural convection of groundwater around the well and the difference in the positional relationship between the heater and temperature sensor at each depth. In contrast, the λs profiles estimated from the recovery period showed good agreement with the reference profile when a heating period over 2 days was applied. Subsequently, the numerical models were developed using FEFLOW ver. 7.1 to reproduce the TRTs using the new TRT equipment. The heating periods required to estimate the appropriate λs profile were evaluated for larger well diameters of 300, 400, and 500 mm. The validity of the developed models was verified through the history matching of temperature changes. The simulation results showed a proportional relationship between the inner diameter of the wells and the required heating period, indicating that the new TRT equipment can be applied to large-diameter water wells by applying appropriate heating periods. |