| Title | Geothermal Development of Fracture Dominated Aquifers: the Case Study of the Deep Well Geretsried (Germany) |
|---|---|
| Authors | Michael DUSSEL, Inga MOECK, Markus WOLFGRAMM, Rene KAHNT, Robert STRAUBINGER, Andreas GAHR |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | geothermal exploration, deep fault zone, carbonate facies, role of diagenesis |
| Abstract | The German Molasse Basin hosts a high geothermal potential particularly in its southernmost part where the well-known Upper Jurassic carbonate Malm aquifer lies in 4.6-5 km depth. While the Malm aquifer is already exploited in the Munich area, the knowledge for sustainable geothermal development and economic exploitation of the southernmost part is still limited. The research project Dolomitkluft (i.e. dolomite fracture) addresses the geothermal development, testing and analysis of the first fracture dominated dolomitic aquifer in the deep Malm of the Bavarian Molasse Basin. The test site is located about 40 km south of Munich and east of the Lake Starnberg. The first drilling operation started in 2013 with the longest geothermal well in Europe with a 6036 m long well path. While the temperature was as expected with 155 °C, the productivity was significantly lower than expected and turned out as beyond any economic level. The dry well GEN-1 was preliminarily plugged in August 2013 and the negative results from this prominent project provoked a considerable draw-down in the whole geothermal market in Germany. The research project Dolomiktluft re-opened the well for Vertical Seismic Profiling (VSP) in 2016. After re-evaluation of the 3D seismic, a sidetrack was drilled from the dry well GEN-1 in 2017, targeting a graben type fault zone. Deep fault zones are hitherto scientifically understudied: the dimensions of damage zone, fault core, the permeability variability in these fault zones compared to intact rock are not well quantified. The interaction between fault kinematics, diagenetic processes and facies types affecting the reservoir quality criteria porosity and permeability in depth are not well known. Therefore, drill cores were taken from different sections between 5000 and 5300 m depth at a total of 20 m covering intact reservoir rock, damage zone and fault core for petrophysical, geomechanical and petrographic analysis. Total mud losses during drilling operation indicated permeable fracture zones in the reservoir section. However, a production test carried out after three acid jobs resulted in a high draw down and low productivity. The results indicate a tight fractured controlled reservoir rock where permeability is impaired primarily by diagenetic processes in a low-porosity carbonate facies. Further research is planned inferring the transition from hydrothermal to petrothermal methodologies. |