Can rock core thermal conductivity data replace thermal response tests?

Borehole heat exchanger (closed-loop) systems coupled to a ground-source heat pump are applied for space heating and cooling, using the ground as energy source or storage medium. For accurate dimensioning of a ground-source heat installation, knowledge of the thermal conductivity of the subsurface is vital. Thermal response tests (TRT) are widely used to measure the in situ thermal conductivity in a well. Alternatively, the thermal conductivity is approximated from rock core samples based on lab measurements. Rock core thermal conductivity data are financially more attractive for planning purposes than expensive TRTs. The value of both approaches was tested statistically. Effective thermal conductivities measured via TRTs show a clear trend towards higher thermal conductivity values in comparison to lab measured thermal conductivity values from rock cores (about 80 % of cases) as an effect from groundwater in non-grouted boreholes. The deviation from the rock core samples, however, varies strongly as several geological layers may be represented in one single well. Furthermore, the thermal conductivity of the rock core samples varies strongly within individual geological units. The comparison of both techniques shows that the in situ thermal conductivity at a location cannot be predicted from rock core data of a rock type.