Modeling Effects of Thermo-geology on Boreholes and on the Shallow Geothermal Gradient using Distributed Temperature Sensing in Rantoul, Illinois

Using fiber-optic distributed temperature sensing (FO-DTS) to study a geothermal exchange system requires great precision. The ease of use and capability to determine the temperature of different media (water, air, soil, etc.) at higher temporal and spatial resolutions compared with other technologies allows for its use in a range of applications and environments. In this study, FO-DTS was conducted in two adjacent boreholes:

  1. a cased groundwater monitoring 40-m deep and
  2. a grouted, uncased borehole extending 100 m below ground. The fiber optic cable was installed vertically in both boreholes, attached to the PVC pipe in the cased borehole and sealed against the sidewall with bentonite grout in the other.

Thermal profiles were measured over a 2- year period from 2015–2017. Following the data collection, heat transfer in the subsurface around both boreholes under both saturated and unsaturated conditions were numerically evaluated. According to previous studies in Central Illinois, the transient water table in the surficial zone fluctuates within the upper 10 to ~15 m. The temperatures measured in this zone are susceptible to seasonal fluctuations in climate conditions at the ground surface. Below this zone, temperatures are mainly controlled by the local geothermal gradient. In this research, we will show how heterogeneities in the thermo-geology and well construction materials impact the natural geothermal gradient in the shallow subsurface.

  • PI: Yu-Feng Lin
  • PI Institution: University of Illinois
  • June 18, 2018 – September 17, 2019