| Title | Exploration of a Geothermal Reservoir in Carbonate Rocks for a Metropolitan Area |
|---|---|
| Authors | Hartwig VON HARTMANN, Jennifer ZIESCH, Britta WAWERZINEK, David C. TANNER, Hermann BUNESS, Ernesto MENESES RIOSECO and Rüdiger THOMAS |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | carbonate platform, district heating, seismic interpretation, German Molasse Basin |
| Abstract | Production of hot water from deep carbonate rocks is still a challenging task. The very heterogeneous distribution of permeability increases the exploration risk. Nevertheless, water from an Upper Jurassic carbonate platform at depth of 2 – 3 km within the Alpine Molasse Basin should feed the district heating network of Munich, the capital of Bavaria in Germany. Up to 2040, a pattern of geothermal wells distributed throughout the city should supply the district heating of half a million inhabitants. To reduce the exploration risk and support the well planning a 170 square kilometer seismic survey was acquired recently. Shear wave measurements accompanied the main survey with special source geometries and three component geophones. The Federal Ministry of Economic Affairs and Energy funded a scientific program supporting the efforts of the local energy supplier (Stadtwerke München) for this geothermal project. It comprised a detailed structural interpretation, which allowed a retrodeformation of structures. Facies analysis based on seismic attribute maps and seismic pattern gives insight into the development of the carbonate platform. The analysis of shear wave velocities helps to detect dolomitized areas. At least a reservoir model shows the cooling of the reservoir the during the production phase. A large fault zone divides the area into a northern and a southern part. Toward the east, this fault splits into two main branches, which further splits into smaller fragments. The faults within the surveys belong to a regional fault system, which runs parallel the alpine front. In the footwall, a field of dolines resides, whereas in the hanging wall some very large dolines exist at the termination of faults. The retrodeformation shows deformation along the fault, increasing from west to east and with highest values one kilometer apart from the fault. The facies distribution of the carbonate platform shows local variations. Four sequences subdivide the carbonate platform vertically. They indicate prominent changes in the sedimentary environment. Several mounds lead to a topography within each sequence. At the northern and southwestern borders, topographic highs are linked by smaller ramps to the troughs. The seismic pattern show a decrease in water depth by vertical and lateral grow of mound like structures. Calculating the relation of shear-wave and compressional-wave velocities indicates that the mound like structures are dolomitized in contrast to the trough sedimentation surrounding them. The development of a geothermal reservoir with numerous wells in a confined area may result in unexpected interaction of the wells during production and injection of water. A detailed analysis of the reservoir therefore helps to avoid this by an optimized distribution of the wells. Each applied method concentrates on special aspects of the quality of a carbonate reservoir: structural and karst development, facies distribution and dolomitizing. This information will be evaluated during the ongoing drilling within the area. |