Determination of thermal conductivities in the laboratory and the field: A comparison

The thermal conductivity of the subsurface is a fundamental parameter for the design of borehole heat exchangers in shallow geothermal energy systems. An average thermal conductivity value is usually assumed. Under real conditions, however, the thermal conductivity at depth can vary considerably depending on the local petrophysical and mineralogical properties of the subsurface (e.g. porosity). Hence, the aim of this study was to compare these properties of the subsurface with the thermal conductivities measured in the laboratory and in the field and to highlight possible correlations. For this purpose, a test field was established in the northern Black Forest (Germany) by obtaining an undisturbed drilling core of about 100 m length from sandstone of the Middle to Upper Buntsandstein formation and then installing a borehole heat exchanger (BHE). Various rock parameters were determined in the laboratory on 160 selected samples of the drilling core. Among other parameters, thermal conductivities under saturated and unsaturated conditions were measured and compared with values determined by depth-resolved classical and enhanced thermal response tests in the borehole heat exchanger (TRT). Furthermore, the porosity, permeability, grain density and pore diameter as well as mineralogical composition of the sandstone were intensively studied in the laboratory. The results do not show clear correlations between thermal conductivity, permeability and density. In contrast to those reported in literature, our results indicate a moderate correlation between porosity and thermal conductivity and a more pronounced dependence on grain size.

With regard to the depth profile of the thermal conductivity, the results between laboratory and field measurements were mainly consistent. The highest thermal conductivities (4.3 W/mK in the laboratory and 4.5 W/mK in the field) confirm the suitability of the Upper and Middle Buntsandstein formation for shallow geothermal installations. Most of these rocks represent typical fluvial deposits, so that the results obtained can be easily transferred to other regions with similar sandstone deposits.