MR005-07
The Role of Geological Models and Uncertainties in Safety Assessments

Monday, 14 December 2020: 20:54
Virtual
Merle Bjorge1, Phillip Kreye1, Elisa Heim2, Florian Wellmann3,4 and Wolfram Ruehaak1, (1)Federal Company for Radioactive Waste Disposal, Site Selection Procedure, Department Safety Assessment, Peine, Germany, (2)Institute for Applied Geophysics and Geothermal Energy, RWTH Aachen University, Aachen, Germany, (3)Aachen Institute for Advanced Study in Computational Engineering Science, RWTH Aachen University, Aachen, Germany, (4)Computational Geoscience and Reservoir Engineering, RWTH Aachen University, Aachen, Germany
Recorded Presentation
Abstract:
Safety assessments are a central part in nuclear waste management and include the analysis of thermo-mechanical (TM) coupled processes. TM behavior of the host rock is, amongst others, dependent on the prevalent geological geometries. This study aims to evaluate the impact of 3‑D geological models and input data uncertainty on the TM behavior in claystone and the effect on the integrity of the rock.

To analyze the influence of geological geometries, a generic model of the region around the Mont Terri rock laboratory was created. First, a 3-D structural geological model was created with GemPy. Furthermore, uncertainties of the lithological contacts were quantified by means of stochastic simulations in GemPy, producing different model realizations. To quantify the uncertainties a location and orientation uncertainty was applied to the input data. The location uncertainty is defined data by a normal distribution with a standard deviation of 7 m, whereas the orientation uncertainty is characterized by a Von Mises-Fisher distribution with a concentration parameter of 50.

89 cross-sections of different 3-D model realizations were ported to a numerical model in Matlab. In this numerical model, 2-D TM coupled simulations are performed over a simulation time of 500 years using the Finite Element Method. To simulate a heat source of nuclear waste, a Dirichlet boundary condition of 100°C is set on the lower edge of the model while the remaining rock is initially in geothermal equilibrium.

The results of these simulations show mean temperature variations of 90.9 °C to 92.7 °C after 500 years, maximum stress varying between 0.019 MPa and 0.16 MPa elastic shear energy density and mean cumulative displacements ranging from 30 cm to 38 cm. The presented results indicate that different model geometries and differences in rock parameters lead to noticeable variabilities of the TM behavior of claystone.

Lastly, the results were compared with critical stresses of the deformation behavior of clay rock. This reveals, that at low lateral confining stress the maximum stress obtained in this study would exceed the stress of rock failure. This would increase the permeability and affect the rock integrity negatively. However, with increasing lateral confining stress rock failure is not reached.

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