Evaluating Darcy Flux Through Enhanced Thermal Response Testing

The efficiency of underground thermal energy storage (UTES) and ground source heat pump (GSHP) systems depends strongly on subsurface thermal properties and groundwater dynamics. Groundwater flow introduces an advective component to heat transport, which can significantly affect thermal conductivity measurements and system performance. Enhanced Thermal Response Testing (ETRT) provides high-resolution depth profiles of effective thermal conductivity (λ_eff), enabling detection of geological heterogeneities and zones influenced by groundwater flow.

In this study, ETRT data from boreholes up to 100 m depth were analyzed using the Infinite Line Source (ILS) method, while the Péclet number (Pe) approach was used to estimate Darcy fluxes and assess the relative contribution of advection versus conduction. Calculated Darcy fluxes were compared with regional groundwater flow data for three Slovenian sites representing distinct hydrogeological settings: Ljubljana (a Quaternary aquifer), Veliko Črnelo (fractured dolomites), and Brod v Podbočju (a silty aquitard). Results reveal a clear gradation of regimes: strong advective influence in Ljubljana, mixed conditions with localized advective zones in Veliko Črnelo, and predominantly conductive transport in Brod v Podbočju.

The Péclet-based analysis proved robust for differentiating these regimes, provided that conductive thermal conductivity (λ_cond) is accurately determined from conductive segments or laboratory measurements. These findings highlight the importance of incorporating advective processes into geothermal system design and modelling, particularly in high-flow areas.

Acknowledgements

The research borehole was drilled within project V1-2213 GeoCOOL FOOD – Cold storage of food using shallow geothermal energy, funded by the Slovenian Research and Innovation Agency and the Ministry of Agriculture, Forestry and Food under the Targeted Research Program "Our Food, Rural Areas and Natural Resources." Research was a combination of work at ARIS projects within the framework of the Young Researcher Program of K. Borko, research core funding No. P1-0020 Groundwater and Geochemistry, CRP GeoCOOL FOOD, DP Geo-OPT and UNESCO IGCP project 636. The study focused on the analysis of Enhanced Thermal Response Test (ETRT) data from boreholes up to 100 m depth, interpreted using the Infinite Line Source (ILS) model.