Actively heated fiber optics based thermal response test: A field demonstration

Accurate estimation of thermal ground properties is needed to optimally apply shallow geothermal energy technologies, which are of growing importance for the heating and cooling sector. A special challenge is posed by the often significant heterogeneity and variability of the geological media at a site. As an innovative investigation method, here the focus is on the actively heated fiber optics based thermal response test (ATRT). A type of copper mesh heated optical cable (CMHC), which both serves as a heating source and a temperature sensing cable, was applied in the field in a borehole. By inducing the electric current to the cable at a relatively low power of 26 W/m, the in-situ heating process was recorded at high depth resolution. This information serves to infer the thermal conductivity distribution along the borehole. The presented field experience reveals that the temperature rise in the early phase of the test should not be used due to initial heat accumulation caused by the outer jacket of the CMHC. The comparison of these results with those of a conventional thermal response test (TRT) and a distributed thermal response test (DTRT) in the same borehole confirmed that the ATRT result is reliable (with a difference less than 5% and 1%, respectively), since this novel method affords much less energy and test time. Additionally, the ATRT result agrees well with ground thermal conductivities tested in the lab, which supports its potential as an advanced geothermal field investigation technique in the future.