Finite element method for simulating coupled thermo-hydro-mechanical processes in discretely fractured porous media and application to enhanced geothermal reservoir analysis

We present our recent development of the finite element method (FEM) for simulating coupled thermo-hydro-mechanical (THM) processes in discretely fractured porous media and an application to geothermal reservoir modeling for the research test site Gross Schoenebeck in Germany operated by the GFZ German Research Centre for Geosciences. Numerical analysis of multi-physics problems in fractured rocks is important for various geotechnical applications. In particular for enhanced geothermal reservoirs where induced fractures and possibly natural fault systems dominate the system behavior, explicit modeling of those characteristic fractures (i.e. discrete fracture models) is essential to get more detailed understanding of in-situ processes and reliable estimations of heat extraction from those deep reservoirs. However, as fractures are mechanical discontinuities, it is difficult to solve the problems using continuity based numerical methods such as the FEM. Currently, equivalent porous medium or multiple continuum model approaches are often only the way to model fractured rocks with the FEM. The authors have recently developed lower-dimensional interface elements (LIEs) for modeling mechanics-involved coupled processes with pre-existing fractures (Watanabe et al. 2012 IJNME). The method does not require any double nodes unlike conventional interface elements. Moreover, for coupled problems, the approach allows for the use of a single mesh for both mechanical and other related processes such as flow and transport. All the code developments have been carried out within the scientific open source project OpenGeoSys (www.opengeosys.net) (Kolditz et al. 2012 EES). Using both traditional and new simulation techniques, a geothermal reservoir model for the research test site Gross Schoenebeck has been developed. Unstructured meshing of the complex faulted reservoir including both rock matrix and fracture elements has been conducted using recently developed automatic preprocessing software MeshIT (Cacace et al. 2012 IGC). This model will be used for lifetime prediction of the geothermal reservoir under different operational conditions (i.e. pressure regimes, injection-production rates). The model is able to describe the complex interaction of THM processes during heat extraction for real conditions in deep georeservoirs. The figure shows the flow regime (pathlines) as well as heat distribution after 15 years operation of the borehole doublet in the fractured rock system. Simulated flow regime (pathlines) as well as heat distribution after 15 years operation of the borehole doublet in the enhanced geothermal system