Change of the THM properties of a Malm geothermal reservoir under present and future exploitation schemes

Sustainable and efficient use of the geothermal resource sitting below the city of Munich (Germany) is a topical issue for the local energy providers. Indeed, by 2035, Munich city expects to supply 100 % of its electricity and heat demand from renewable energies. Hence, the exploitation of the highly conductive aquifers will be intensified with the development of numerous new geothermal power plants. This is a challenging task, which requires better understanding of the coupled processes taking place underground and of their effects, especially in terms of possible induced seismicity and ground uplift or subsidence. These last two concerns are at the heart of the INSIDE research project. The present study focuses on the thermo-hydro-mechanical (THM) modelling of the deep geothermal site in Pullach im Isartal (close to Munich), which constitutes one step to reach the overall project goal.

At the Pullach site, since 2005, a deep geothermal reservoir in the Mesozoic Malm is successfully operated and supplies heat to the local grid. A finite-element (FE) reservoir model is developed consisting of eight different lithologies ranging from the crystalline basement to the Earth’s surface. Focusing on the Malm geothermal reservoir, the lithology of interest is subdivided into the three potential and permeable reservoir rocks of the lower, middle and upper Malm. The three geothermal wells are included with their real trajectories and open-hole sections as lower-dimensional elements. The FE model is used to simulate the fully-coupled THM processes taking place under continuous operation using the TIGER open-source application, which is based on the Multiphysics Object-Oriented Simulation Environment (MOOSE).

The 3D numerical model was calibrated using hydraulic tests conducted at the wells and the continuous operation of the plant, running for many years, gave the opportunity to assess the simulation results. Consequently, predictions of the long-term changes of the THM characteristics of the geothermal reservoir for the next 50 years are expected realistic. The results show that, under our assumptions and the current operational conditions, exploitation will remain sustainable in the future. Furthermore, near the injection well, a long-term increase in porosity and permeability should be expected due to thermo-poroelastic effects. At last, millimetre-scale uplift could be observed at surface above the injection well.

In addition to the present exploitation schemes, several scenarios to increase the exploitation using additional wells are investigated. A similar approach is taken, but particular attention is paid to possible interferences between existing and future neighbouring wells due to enhanced reservoir utilization.