Numerical calibration of rock thermal parameters based on thermal response test data

Shallow geothermal energy is one of the renewable energy sources that, when properly utilized, has a minimal impact on the environment. The relatively high investment cost of installing the system can be reduced if the local natural conditions are well defined. Knowledge of local soil properties and understanding of heat transfer in the natural environment are essential. One way to evaluate thermal properties is to perform the thermal response test (TRT). The main advantage of the method is that it provides the actual average thermal conductivity in the vicinity of the well, taking into account local hydrogeological factors and physical properties of the rock. The thermal conductivity determined in this way may differ significantly from laboratory or field measurements because they do not take into account all the factors that affect thermal conductivity in the subsurface.

The study presents the estimation of the thermal parameters of the subsurface using the results of the thermal response test carried out for the purpose of designing the geothermal system for heat storage (eastern part of central Slovenia).

In one of the borehole heat exchangers (BHE), we performed the TRT, which formed the basis for the calibration of the numerical model. First, we created a static numerical model, based on the average annual data of subsurface temperature, heat flow, thermal conductivity, and volumetric heat capacity of the subsurface. Further, we created a transient numerical model in which we included one-minute data of the BHE inlet temperature and flow rate, which were used to calibrate the thermal conductivity and volumetric heat capacity of the subsurface. In the next step, we will validate the numerical model with the operating data of the BHE field to determine the efficiency of the borehole thermal energy storage.