Multidimensional modeling of the thermal and flow regime in the western part of the Molasse Basin , Southern Germany

The aim of this thesis is to obtain information on the groundwater flow regime at great depth within the Molasse Basin (SW Germany). For this purpose different numerical codes are applied. For an optimal geometrical representation of the basin geometry it was necessary to develop a coordinate transformation method. This work is presented in the first part of the thesis. Many popular groundwater modeling codes are based on the finite differences or finite volume method for orthogonal grids. In cases of complex subsurface geometries this type of grid either leads to coarse geometric representations or to extremely fine meshes. Therefore, a coordinate transformation method has been developed to circumvent this shortcoming. In computational fluid dynamics (CFD), this method has been applied successfully to the general Navier-Stokes equation. The method is based on tensor analysis and performs a transformation of a curvilinear into a rectangular unit grid, on which a modified formulation of the differential equations is applied. Due to this it is not necessary to reformulate existing simulation code in total. The coordinate transformation method was applied to the three-dimensional code SHEMAT, a simulator for flow and heat transport in porous media. The finite volume discretization scheme for the non-orthogonal, structured, hexahedral grid yields a 19-point stencil and a correspondingly banded system matrix. The implementation is straightforward and it is possible to use many existing routines without modification. The accuracy of the modified code is demonstrated for single-phase flow on a two-dimensional analytical solution for flow and heat transport and further on a thermal free-convection benchmark. Additionally, a simple two-dimensional case of potential flow is shown for a grid which is increasingly deformed. The result reveals that the error increases only slightly. In the second part the actual basin analysis is shown and discussed. Data relevant for a flow model at depth between 600 m to 1600 m below the surface are generally sparse in the western part of the Molasse Basin. However, a relatively large set of temperature measurements is available covering a large part of the area at a wide range of depths. Therefore, a thermal 3-D quasi steady-state model was set up with the aim of comparing modeled with measured subsurface temperatures. Additional to the temperature data, some values of rock thermal conductivity and heat production rate are available. Other data are too sparsely distributed to be useful. The purely conductive model reveals some strong thermal anomalies, especially along fault zones, and within stratigraphic layers with high hydraulic conductivity. While temperature in the upper crust is dominated by conductive heat transport, heat advection associated with groundwater flow may significantly alter the purely conductive regime. The thermal anomalies can be explained by various advective heat transport mechanisms yet most of them can be eliminated: The only constellation explaining the major positive thermal anomalies of 10 K and more is a fault zone intersected by an aquifer with flow parallel to the fault zone. This was validated by using a simplified type model. In spite of some shortcomings, the method presented here can be used to identify temperature anomalies, and in a second step, to identify possible explanations. Parts of this work have been published in the following two papers: 1. W. Rühaak (2006): A Java application for quality weighted 3-d interpolation, Computers & Geosciences 32,1: 43-51 (doi: 10.1016/j.cageo.2005.04.005). 2. W. Rühaak, V. Rath, A. Wolf & C. Clauser (2008): 3D finite volume groundwater and heat transport modeling with non-orthogonal grids using a coordinate transformation method, Advances in Water Resources 31,3: 513-524 (doi: 10.1016/j.advwatres.2007.11.002).