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Seismic Interpretation and Geomechanical Reservoir Modeling – a Case Study from the Upper Rhine Graben

Abstract

Faults and lithological changes can affect the tectonic stress regime in the subsurface. Thereby, significant deviations in stress magnitude and orientation from the regional tectonic trend can occur. In order to minimize exploration risk and optimize the drilling of hydrocarbon and deep geothermal reservoirs, a reliable prediction of such stress variations is desired. A corresponding reservoir model has to incorporate data such as reservoir geometry, regional tectonic stress field as well as mechanical properties of the different lithologies and faults. Numerical techniques, e.g., the Finite Element (FE) method, are well suited for geomechanical reservoir modeling. If available, stress data actually observed (e.g., borehole breakouts, fracture closing pressure) can be used to calibrate the numerical model. Results of the numerical computations comprise, among others, the 3D stress tensor for each part of the model as well as the slip and dilation tendency of faults. These results can be used for a wide range of application, e.g. borehole stability, hydraulic frac planning and permeability anisotropies in stress-sensitive reservoirs and the prediction of tectonic stresses in unexplored reservoir areas. The workflow described above is applied to a study area located in the central part of the Upper Rhinegraben, northwest of Karlsruhe / Germany. The geometry of the geomechanical reservoir model is based on a 3D geological subsurface model created from a 3D seismic survey, covering an area of 7.5 × 9.5 sq km. The model ranges from the earth surface through the Tertiary graben fill, Mesozoic and Paleozoic sediments to the crystalline basement at about 4 km depth. Key challenges are the geometry transfer from the geological modelling to the FE software and the calibration of the geomechanical reservoir model. The complex subsurface model incorporates 9 lithostratigraphic horizons and 29 faults, including truncating and cross-cutting faults. No wells and, hence, no measured stress data presently exist in the immediate study area. Thus, neotectonic data is evaluated to link the faults observed at depth in the seismic data with topographical features at the earth's surface. Neotectonic fault activity is then compared to the faults for which modeling predicts the highest slip tendency in the present-day stress regime.