Thermal rock properties of geothermal reservoirs and caprocks in the Danish Basin – prerequisites for geothermal applications

The Danish subsurface provides a large potential for the use of low-enthalpy geothermal heat and, thereby, to change the national district heating structure by providing a base load power to the system. In the past decade, new exploration and research campaigns were performed to remove geological, technical and commercial obstacles for a significant use of these geothermal resources. One of the obstacles is the lack of knowledge on the thermophysical rock properties. Subsurface thermal conditions as well as the production capacity and lifecycle of geothermal district heating plants largely depend, among other, on these properties. For the Danish Basin only few published data sets are available and mostly limited to thermal conductivity. Values of thermal diffusivity and specific heat capacity are mostly unknown. In order to overcome this gap, new laboratory measurements were conducted. Thermal conductivity and thermal diffusivity were measured on drill cores sections, while specific heat capacity was calculated based on these values and on rock density. Geological targets for the study are Mesozoic reservoir sandstones (Gassum Fm., Frederikshavn Fm., Haldager Sand Fm.), but also mud-/claystones and limestones of seal rocks (Fjerritslev Fm., Vedsted Fm.). The rock suite of 43 specimens studied was sampled in six wells. The measurements are performed under dry and saturated conditions using the optical scanning method. Furthermore, the values of conductivity and thermal diffusivity of the matrix were obtained by geometric mean averaging. Therefore, the ranges of characteristic values for each lithology were identified and a qualitative survey on the mineralogical composition of the samples on the basis of the matrix data was assembled. Further observations on the behaviour of thermal diffusivity and the relative application of the geometric mean model are also provided. This study was possible thanks to the "GEOTHERM" project, funded by the Innovation Fund Denmark.