How fluid flow and heat transport models impact predictions of deep geothermal potentials: the "heat in place" method applied to Hesse (Germany)

Abstract

<p>One key aspect in the energy transition is to use the deep geothermal energy stored in sedimentary basins as well as igneous and metamorphic basement rocks. To estimate the variability of deep geothermal potentials across different geological domains as encountered in the Federal State of Hesse (Germany), it is necessary to understand the driving processes of fluid flow and heat transport affecting subsurface temperature variations. In this study, we quantify the stored energy in geothermal reservoirs with the method of “heat in place” (sensu Muffler & Cataldi, 1978, as one proxy for geothermal potential) for different geological units as integrated in a series of coupled 3D thermo-hydraulic numerical models of Hesse. This way we can show how conductive, advective and convective heat transport mechanisms influence the thermal field and thereby the predicted “heat in place”.</p><p>In this work, the “heat in place” is calculated for each grid point in the model for a volume of one cubic meter times the reservoir thickness. The temperature is extracted by interpolation along the middle surface of each reservoir unit and thus considered as an average representative value of the integrated energetic input for each layer.</p><p>The heterogeneous geology of Hesse ranges from outcropping Paleozoic basement rocks to up to 3.8 km thick Cenozoic sedimentary deposits in the Upper Rhine Graben. This geological complexity is expressed by areas of different hydraulic and thermal configurations, strongly influencing the geothermal potentials.</p><p>The “heat in place” was quantified for five sedimentary layers: Cenozoic, Muschelkalk, Buntsandstein, Zechstein und Rotliegend as well as for the underlying variscan basement for the entire volume down to a maximum model depth of 6 km. In the Cenozoic, Buntsandstein and Rotliegend reservoir units, “heat in place” is predicted to be highest in the Upper Rhine Graben, where the modelled temperatures are higher than 90 °C in the reservoirs.</p><p>In general, we can assess geothermal potentials across the study area more comprehensively with the prediction of “heat in place” that is based on the thermal field derived from thermohydraulic simulations. For example, the conductive thermal field points to the Upper Rhine Graben as the most proliferous region due to highest predicted temperatures. With the convective simulations the area with high temperatures is reduced from the entire Graben to local zones of upwelling hot fluids. At the end, with the prediction of “heat in place”, we integrate even more parameters like the reservoir thickness and therefore suitable regions for geothermal utilization are even more restricted.</p><p> </p>