| Keywords |
Geothermal reservoir exploration, characterization and modeling, 3D thermal-hydraulic modeling in porous, fractured, faulted and karstified Upper Jurassic carbonate reservoir, Bavarian Molasse Basin, 3D seismic interpretation, deep geothermal energy, opti |
| Abstract |
Deep geothermal reservoir exploration and characterization are indispensable when it comes to reservoir modeling, reservoir management and reservoir optimization of a multi-well geothermal field. In particular, the Upper Jurassic carbonate aquifer – also known as Malm aquifer - in the Munich region of the South German Molasse Basin has experienced extensive exploration and thermal water use in the last two decades. One of the most prominent geothermal projects set up by the municipal energy supplier of the city of Munich (Stadtwerke München-SWM) and currently in progress is called GRAME - optimized and sustainable reservoir development of deep geothermal plants in the Bavarian Molasse Basin. Following the SWM’s district heating vision of turning Munich by 2040 into Germany’s first large city to develop its district heating completely from renewable energy, this project aims at exploring, characterizing and optimizing potential reservoir sites for deep geothermal energy utilization in the southern part of the city of Munich. SWM’s district heating vision implies the development of 400 MWth for the Munich district heating provided by the optimized placement of around 40 deep geothermal wells by 2040. The present work lies within the scope of the GeoParaMoL project, which is part of the GRAME project and focusses on the estimation of geophysical parameters to determine facies of the Malm, structural and stratigraphic geological features and the modeling of the thermal-hydraulic long-term behavior of the Malm affected by geothermal multi-well arrays. This work concentrates on reservoir modeling of the Upper Jurassic carbonate aquifer in the southern part of the city of Munich. The aim is to predict the long-term thermo-hydraulic behavior of such fractured and karstified reservoir affected by different geothermal doublet and triplet arrays as well as smart multi-well patterns under different operational conditions. This includes the entire workflow starting from the seismic interpretation through building the geological and petrophysical model up to flow and heat transport modeling in order to optimize geothermal energy recovery. The optimization of geothermal energy production as well as reservoir management of multi-well patterns involve the study of possible positive and negative thermal-hydraulic interferences that such multi-well systems may have within the system and with neighboring geothermal wells already in operation in the immediate surroundings of the study region. Carbonate sedimentation leads to typical sedimentary patterns which can be visualized by seismic imaging. Seismic reflections show seismic impedance contrasts and have to be translated into sedimentary and lithologic boundaries by the seismic interpreter. According to the rise and fall of sea level, a carbonate platform grows and calcareous sediments are precipitated or distributed on ramps or within lagoons and faunal activity leads to build-ups of different size. These build-ups themselves control the sedimentation process in their neighborhood. These processes result in a heterogeneous distribution of different carboniferous facies. Seismic sequence stratigraphy is a tool, which helps identify sedimentary environments in a seismic section or a seismic cube. During this workflow, typical seismic patterns are linked to carbonate features, sedimentary sequences are identified and the development of the carbonate platform is reconstructed. Without borehole information, the determination of geophysical parameters for facies interpretation is challenging. The integration of shear waves could be helpful to limit the range of lithological and petrophysical parameters. For this reason, shear wave measurements were carried out during the regular 3D seismic survey in Munich. Conventional CMP processing shows shear wave reflections from the top of the Upper Jurassic carbonate platform on both horizontal components. An observed travel time ratio of ts/tp=1.55 indicates a VP/VS ratio of 2.05 for a P-to-S converted wave. Furthermore, the combination of P- and S-waves enables the derivation of geophysical parameters (e.g., VP/VS) to support the facies interpretation. Apart from carbonate facies, faults play an essential role in geothermal exploration. The prediction of potential fluid pathways in urban Munich started with the interpretation of a 3D seismic survey (170 km2) that was acquired during the winter of 2015/2016 in Munich. The stratigraphic horizons, Top Aquitan, Top Chatt, Top Bausteinschichten, Top Lithothamnien limestone (Top Eocene), Top and Base Malm (Upper Jurassic), together with the detailed interpretation of the faults in the study area were used to construct a 3D geological model. The study area is characterized by synthetic normal faults that strike parallel to the alpine front. Most major faults were active from Upper Jurassic up to the Miocene. The Munich Fault, which is the biggest fault in the study area, has a maximum vertical offset of 350 meters in the central part. Increased fluid flow along the faults has led to massive sinkhole structures with up to 60 m displacement. The long-term thermal-hydraulic behavior (~ 50 years) of the Malm aquifer affected by diverse geothermal doublet arrays and multi-well configurations has been numerically modelled with FEFLOW 7.0. Several scenarios with varying geometrical and operational conditions were implemented and numerically simulated. Preliminary thermal-hydraulic modeling results show that, for the simulation time considered, geothermal doublet arrays with a lattice spacing between 1 and 2 km and flowrates between 80 and 120 l/s are promising scenarios. In addition, model results indicate that geothermal multi-well configurations of 4 to 6 wells are under particular geothermal and hydrogeological conditions more appropriate. This later model result relates to the role of hydraulically active faults. Finally, modeling results suggest thermal and hydraulic advantages and disadvantages of geothermal doublet arrays over a single doublet. For instance, the use of geothermal doublet arrays leads to a significantly slower advancing thermal front (i.e., thermal breakthrough) but once the thermal breakthrough is reached, the temperature in the production well drops more rapidly. |