Impact of groundwater flow on thermal response tests in heterogeneous geological settings

This study investigates thermal response tests (TRTs) in heterogeneous geological settings to assess the impact of groundwater flow on TRT interpretation and borehole heat exchanger (BHE) performance. Traditional TRT analysis relies on the infinite line source (ILS) model, which assumes homogeneous ground and negligible groundwater flow. However, these assumptions are often lacking in natural environments, resulting in an overestimation of the thermal conductivity. The analysis of four distributed thermal response tests (DTRTs) in high groundwater flow regimes reveals apparent thermal conductivities up to 40 times higher than expected, highlighting the limitations of the ILS method in such settings. To address this issue, a comparison between the ILS model and the moving infinite line source (MILS) model, which accounts for advective heat transfer due to groundwater flow, was conducted. Model fitting and parameter optimization were performed on 84 temperature perturbation time series distributed along four BHEs used for TRT. The MILS model (global RMS = 0.25 degrees C) outperforms the ILS model (global RMS = 0.48 degrees C) in TRT data fitting and better reflects actual thermal conductivity values obtained from laboratory tests and literature. The MILS model also estimates groundwater flow velocities up to 3.0 x 10-5 m/s. Considering the estimated thermal conductivity and groundwater flow velocity, it was found that advective heat transfer contributes to 35-44 % of the total thermal exchange potential for all BHEs. A correction procedure for the apparent thermal conductivity derived from the ILS model, considering Darcy flow velocity, is presented using nomograms. This correction is crucial for accurate BHE design in areas with significant groundwater flow, ensuring a better understanding of BHE performance and its implications for shallow geothermal energy applications.