Project-Reference - City of Freiburg

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A Conductive Geothermal Model of the Upper Rhine Graben

Improved understanding of subsurface geothermal potentials

By order of the Baden-Württemberg Geological Survey, Geophysica constructed a large scale 3-D numerical model. The objective of this study was to characterize the subsurface properties as precise as possible using all available drill hole information (core, logs and cuttings) and to construct a numerical model which predicts temperatures down to 5 km depth. The geothermal model is based on a geological 3-D model which was provided by the Baden-Württemberg Geological Survey. This structural model has been built up within the EU-project “Geopotentials of the deep Upper Rhine Graben (GeORG).

The GeORG project group provided all necessary information of the subsurface. Logging data and borehole reports including information to lithology and stratigraphy were allocated for 28 boreholes. Additionally, a very few laboratory measurements of porosity and thermal conductivity were made available. Finally, we worked on 11 key wells to derive specific thermal-hydraulic values for the model. These boreholes were selected with respect to the best geographic and stratigraphical coverage of the studied area and due to the amount and the quality of available log and core data. 

Using this information, a regional numerical model was set up covering the geothermal sites of Bruchsal, Landau and Soultz-sous-Forets. We performed heat transport simulations by a finite difference code for temperature predictions, assuming a fully conductive regime. Bottom hole temperatures and a few temperature logs were available, which allowed to calibrate the model. The ends up in a geothermal model which allows to visualize the steady-state temperatures within the Upper Rhine Graben. The strong contrast between thermal properties of tertiary sediments and the crystalline bedrock yields large lateral temperature variations within one depth layer. For 4 km depth differences of up to 50 K were calculated. This might be important for geothermal exploration. 

By comparison of model results with measured data in boreholes it is possible to highlight zones of thermal anomalies and thus to better characterize the geothermal potential of the area.

Temperature distribution at the top of tertiary basis, calculated under conductive conditions.