| Title | 3-D Geomechanical Modeling of the Stress Field in North German Basin: Case Study GeneSys-Borehole GT1 in Hannover Groß-Buchholz |
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| Authors | Ernesto MENESES RIOSECO, Jörn LÖHKEN, Rüdiger SCHELLSCHMIDT, Torsten TISCHNER |
| Year | 2013 |
| Conference | Stanford Geothermal Workshop |
| Keywords | stress field, North German Basin, recent stress field, salt rheology, far-field stress, poroeleastic model |
| Abstract | Understanding the behavior of geothermal reservoirs has increasingly gained in relevance to academic, public and environmental institutions in the last two decades. Among numerous projects operating worldwide, the project “Hydro-mechanical response of geothermal reservoirs in the stress field generated by complex geological structures” is a subproject of the interdisciplinary research association “Geothermal energy and high-performance drilling techniques” (Geothermie und Hochleistungsbohrtechnik “gebo”) in Lower Saxony, Germany. The aim of this subproject is to improve the knowledge and understanding of the hydro-mechanical behavior of a geothermal reservoir during and post- drilling and -stimulation. We focus in the present study on the stress field characterizing the reservoir, caused by the complex geologic structures surrounding it. Using COMSOL Multiphysics as finite element code, 3-D present-day geomechanical modeling of the stress field in a complex lithologically stratified region containing salt diapirs typical of the North German Basin have been carried out. In particular, the region surrounding the drilling demonstration site GeneSys-Borehole GT1 in Hannover Groß-Buchholz has been chosen as the study area. Geological/geophysical constraints constitute the real structure of sediments provided by the geotectonic atlas of NW Germany as well as measurements on density and seismic velocities performed within the framework of GeneSys-Borehole. Modeling results of the stress field are in good agreement with measurements of the stress field obtained from borehole breakouts and tensile fracture. In particular, the observed depth-dependent changes in the direction of the maximum horizontal stress component are reproduced remarkably well in the deeper section of the modeling domain. The obtained magnitude of the horizontal components of the stress seems to be lower than the values gained in an initial mini-frac test performed in the area under investigation. This suggests that additional far-field stresses may play a significant role in the study region. Moreover, model results show that depending on the rheological properties assumed for salt the resulting stress field in the targeted domain shows different patterns. Especially when allowing for far-field stresses, the impact of the different adopted rheological models for salt on the resulting stress field is even more striking. |