Depth-specific thermal conductivities of Triassic sandstones

Triassic sandstones of the Middle and Upper Buntsandstein are highly suitable for ground source heat pump (GSHP) systems. Thus, knowledge of their thermal properties, which can be measured or estimated by theoretical models, is crucial. However, the transferability of estimated thermal conductivities to the field scale has not yet been thoroughly examined. Therefore, in this study, the thermal and lithological properties of 156 core samples from a borehole in the Buntsandstein are analysed in the laboratory. Various theoretical models are applied and compared to the laboratory-derived thermal conductivities. The best agreement is achieved with the Voigt-Reuss-Hill model with an average thermal conductivity of 4.5 W m⁻¹ K⁻¹ and an RMSE of 0.7 W m⁻¹ K⁻¹ (T = 20 °C). The results of this model are compared to depth-specific, effective thermal conductivities from an enhanced thermal response test (ETRT). These effective thermal conductivities range between 2.3 and 6.1 W m⁻¹ K⁻¹ with an average of 4.7 W m⁻¹ K⁻¹. We demonstrate that some theoretical models can provide an initial estimation of the effective thermal conductivity of sandstones when groundwater flow is negligible. However, the accuracy of the estimation is limited by sample quantity and model assumptions.