Three-dimensional thermohaline numerical investigations within the Northeast German Basin
Abstract
The identification of geothermal target reservoirs for potential energy supply requires a regional understanding of the thermal setting of sedimentary basins. The Northeast German Basin (NEGB, in northern Germany) hosts a significant amount of groundwater and heat resources. However, the main physical transport processes responsible for these resources are still not completely understood. In Germany, the NEGB represents a complex heterogeneous geological system characterized by several stratified aquifer complexes of regional relevance. The sedimentary structure of the NEGB is affected by a thick Zechstein salt sequence structured in numerous salt pillows and diapirs piercing the overlying Mesozoic aquifers. At shallower depths, an embedded aquitard consisting of fine clayey deposits (Rupelian Clay) separates the Quaternary to late Tertiary freshwater aquifer from the underlying Mesozoic saline aquifers. An important feature is that discontinues within this aquitard exist in areas where the Rupelian clay was not deposited or has been eroded leading to hydraulic connections between the upper and lower aquifers (hydrogeological windows). As a consequence of both salt diapirism and due to existing geological discontinuities in the Rupelian aquitard (hydrogeological windows) the depths and thicknesses of the major Mesozoic aquifers vary widely on the basin scale. Moreover groundwater in the vicinity around salt structures may be subjected to lateral fluid density gradients triggered by abrupt variations in the fluid temperature and salinity. As a result, a complex groundwater flow field is expected within the basin system which is affected both by the complex geometry of the different horizons (aquifers and aquitards) as well as by external (hydraulic gradient) and internal (buoyancy) driving forces. Within the project GeoEn (Joint Research Project on GeoEnergy Research) three-dimensional thermohaline numerical simulations are carried out to discriminate interrelated double diffusive convective processes with regard to hydrogeological windows within the Rupelian Aquitard. The governing equations for coupled fluid, heat and mass transport are solved by using the commercial Finite Element based simulator FEFLOW. The results demonstrate that hydrogeological windows enable inter-aquifer connectivity favoring strong heat and mass transport which causes a mixing of warm and saline groundwater with cold and less saline groundwater within both aquifers. In areas where the Rupelian aquitard confines the Mesozoic aquifer, dissolved solutes from major salt structures are transported laterally giving rise to plumes of variable salinity content ranging from few hundreds of meters to several tens of kilometers. Additional thermal convective currents triggered by buoyancy forces may overwhelm counteracting stabilizing salinity induced forces offside of salt domes. This may result in buoyant upward groundwater flow transporting heat and mass to shallower levels within the same Mesozoic Aquifer.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H51D1226K
- Keywords:
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- 1034 GEOCHEMISTRY Hydrothermal systems;
- 1832 HYDROLOGY Groundwater transport;
- 1828 HYDROLOGY Groundwater hydraulics;
- 1847 HYDROLOGY Modeling