Numerical modelling of a carstic aquifer body as decision support tool for deep geothermal applications and their interference. The transboundary upper jurassic carbonates as a case study.

In the Lower Bavarian–Upper Austrian Molasse Basin, a regionally extensive and water-resources–relevant thermal groundwater system occurs within the Upper Jurassic carbonate rocks (Malm aquifer), extending from Regensburg to areas west of Linz. This resource has long been utilized on both sides of the national border for balneological purposes as medicinal and bathing water, as well as for geothermal energy production. It therefore represents a significant economic asset for the region and is of particular importance for regional water resource management.

Long-term monitoring and data obtained from pumping tests indicate hydraulic interference among some of these uses. To quantify and assess these interactions, a three-dimensional numerical model was developed and calibrated using observational data collected over several decades. The model will serve as a decision-support tool for future permitting processes, including new applications and modifications of existing uses.

Critical issues comprise the harmonization of heterogeneous datasets and the complex hydrothermal behavior of a steeply dipping aquifer, with localized geothermal gradient anomalies promoting thermally induced convection phenomena.The key innovations of the approach include full transient calibration of the whole reservoir over an analysis period of 100 years; consideration of thermal convection and density effects throuth uni-directional coupling;

Although the current application is restricted to geothermal systems, the modeling approach is methodologically transferable to other forms of subsurface utilization, such as carbon capture and storage (CCS), underground thermal energy storage (UTES), and related technologies as well as their interactions.

The results presented are based on a work under the commission of the Thermal Water Expert Group, acting on behalf of the Permanent Water Commission established under the Regensburg Treaty, and represented by the following institutions:
• Office of the Upper Austrian Provincial Government,
• Bavarian Environment Agency,
• Austrian Federal Ministry of Agriculture and Forestry.

The main publication is available for download (in German language) here: https://www.land-oberoesterreich.gv.at/files/publikationen/w_thermalwasser_bayern_ooe.pdf