Numerical Investigation of Fault and Fracture Stability in Horstberg Geothermal Reservoir in the North German Basin

A numerical study was performed in order to analyze the fault stability and induced hydraulic fracture deformation in an enhanced geothermal system, Horstberg in the North German Basin. The Horstberg geothermal site is based on a mono-borehole concept, i.e. the circulation of cold and warm water occurs only within one single well. This well is completed in a multilayer sandstone reservoir at a depth of approximately 3700 meter. Here, an induced hydraulic fracture has been created by massive water injection at flow rates up to 50 l/s, resulting in a hydraulic connection between the upper and lower reservoir formations (Solling and Detfurth). Laterally, the reservoir is constrained by inverted faults. A finite element software is used to apply Biot theory of poroelasticity, i.e. fluid flow in a deformable porous media. The resulting increase in pore pressure and shear stress and decrease in normal stress are assumed to be the main mechanisms for changes in Coulomb stress as well as fault instability. The faults and fracture stay stable under described operational conditions.