The combination of fault throw analysis from a 3D structural model and the empirical approach based on the frictional equilibrium provides a limitation of the in-situ stress regime in the Lower Permian of the Groß Schönebeck geothermal aquifer system. This limitation is explained by the throw of faults in the Rotliegend where no reverse faulting can be observed indicating a stress regime between normal to transition to strike slip faulting regime. The minimum horizontal stress evidenced by hydraulic fracturing tests is Shmin~53 MPa (Legarth et al. 2005), the vertical stress Sv~105 MPa is estimated from overburden rock density and thickness. The coinciding values of Shmin from the calculation from the frictional equilibrium and from hydraulic fracturing shows that the reservoir rock in Groß Schönebeck is in frictional equilibrium. The allowable stress values for SHmax range between 74 MPa (in a normal faulting regime) and 105 MPa (in a transitional stress regime between normal and strike slip faulting). Compared with the results from a geomechanical analysis of borehole breakouts these stress regimes and their magnitudes seem reasonable. The geomechanical study incorporated core tests and numerical modelling, indicating similar stress values of the maximum horizontal stress SHmax~95-100 MPa.
Furthermore it is demonstrated how pre-existing data can be re-used in order to develop a 3D structural model with modern 3D vizualisation software. The 3D model describes both the morphology of the geological formations and the fault pattern that is decoupled by the Upper Permian Zechstein salt in a subsalt and suprasalt faultsystem, respectively. The Lower Permian Rotliegend fault system is dominated by major NW-SE and minor NE-SW trending faults. The NE-SW oriented faults bear the highest shear stresses related to the current stress field, and as critically stressed faults they are supposed to act as hydraulically conductive structures. The suprasalt fault pattern is characterized by various normal fault directions that are related to hinges of salt antiforms. The dominating structure of the suprasalt is given by the Zechstein morphology of NE-SW trending saltridges and the location of salt synforms. The Tertiary layers equalizes the salt movement induced morphology of the suprasalt successions. Despite its uncertainties the 3D model remains the most detailed geological model of the Groß Schönebeck area until more or newly generated seismic data are incorporated to a new model. Since additional 2D seismic data from former gas exploration are available for the area of northern Brandenburg, future work will focus on the integration of more seismic data to extend and refine the existing geological 3D model.


Literature
LEGARTH B., HUENGES E. & ZIMMERMANN G. (2005) Hydraulic fracturing in a sedimentary geothermal reservoir: Results and implications. Int J Rock Mech & Mining Sci 42: 1028-1041
PEŠKA, P. & Zoback, M.D. (1995) Compressive and tensile failure of inclined well bores and determination of in situ stress and rock strength. J of Geophy Res, 100, B7: 12,791-12,811

Vilnius, Lithuania