Analytical methods for detecting and locating groundwater flow in Borehole Heat Exchanger fields using distributed temperature sensing

Conductive thermal fields are influenced by convective intrusions. Although small-scale flow intrusions may not have immediate effects, they can significantly impact the thermal behaviour of the system in the medium and long term. In particular, the presence of aquifers near Borehole Heat Exchangers introduces an advective heat transfer component that alters both the undisturbed ground temperature distribution and the thermal relaxation process following heat injection or extraction. When the field is inactive, convective flow alters the temperature distribution within the field. Conversely, when the thermal field is perturbed, it tends to revert to its original state through a process of thermal relaxation, which is enhanced or hindered depending on the extent of convective intrusions. This study investigates the impact of convection under conditions of inactive and active fields. An experimental monitoring system, equipped with Distributed Temperature Sensing, recorded temperature variations in the field under both inactive and operative conditions at ENEA geothermal site. Two complementary analytical methodologies are proposed to detect and locate convective intrusion on conductive thermal fields. Under undisturbed conditions, aquifer-induced heat removal was quantified by solving the heat conduction Eq.. The daily volumetric heat generation rate matches the presence of groundwater flow intrusion. Under active conditions the location of groundwater flow was assessed by evaluating deviations in the thermal relaxation rate of individual stratigraphic layers. Layers exhibiting relaxation rates higher than the average are characterized by aquifer-assisted relaxation. Results from the proposed methodology are validated through piezometers that confirm the presence and depth of the aquifer.