More than 40% of CO2 emissions in Germany are generated in the heating sector. This contrasts with the expansion potential of deep hydrothermal geothermal energy, which accounts for around 25 % of the country's total heating requirements. One of the regions in Germany that can provide resources for this is the Upper Rhine Graben. There, fracture-bound deep geothermal systems are often associated with medium-depth hydrocarbon reservoirs. Examples are Soultz-sous-Forêts, Landau, but also the former Leopoldshafen oil field, below which a temperature of 170°C at 3,000 m depth has been proven. Heat networks in Germany typically show a low heat demand in summer and a high demand in winter. The base load supply in summer typically accounts for only a few to around 15 percent of the peak load in winter. Since the conversion of excess heat into electrical energy is associated with low efficiency levels, the storage of excess heat in medium-depth reservoirs is proposed here to optimize the overall system. The DeepStor research infrastructure at the KIT-Campus North represents an analog for such storage systems in the Upper Rhine Graben. With a planned production from the fractured geothermal system at depth of more than 3,000 m, heat can be extracted in sufficient quantity and temperature to cover the base and medium load. The surplus heat can be stored in the peripheral areas of the former Leopoldshafen oil field (with 70 % water saturation at the end of hydrocarbon production) and recovered in winter. A 3-D subsurface model for DeepStor and the city of Karlsruhe was created on the basis of 2-D and 3-D seismic data and information from more than 20 boreholes in former oil fields. Using temperature and hydraulic data from the corresponding boreholes, thermo-hydraulic simulations were carried out to 1) estimate the state of the deep geothermal system and 2) optimize the planned campus supply. In summary, our results show that the share of renewable geothermal heat energy can be increased from 25 % to about 65 % by storing the excess heat without heat losses to the atmosphere. Assuming that technical problems such as scaling can be solved, we are currently in the process of scaling up the storage system to the city of Karlsruhe with peak and base loads of around 250 and 30 MW.

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