Assessment of Fault Reactivation Potential in a Deep Geothermal Reservoir of the NE-German Basin (Germany)

摘要

Stimulation treatments are a common method to increase the permeability of geothermal reservoirs especially in terms of Enhanced Geothermal Systems (EGS). Herein, one critical aspect is the understanding of in situ stress conditions and fault reactivation potential before and during hydraulic stimulation which infers a significant increase in fluid pressure by water injection. The increased fluid pressure can induce slip along faults referred to as induced seismicity. Especially in critically stressed geologic settings or in regions prone to earthquakes, the assessment of fault failure as a reaction to stimulation might be of high importance in geothermal field development. Slip tendency analysis has been successfully used to characterize fault slip likelihood and fault slip directions in any stress regime. In our study we apply the slip tendency analysis to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the Northeast German Basin, based on the notion that slip on faults is controlled by the ratio of shear to normal stress acting on the plane of weakness in the in situ stress field. The reservoir rocks, composed of Lower Permian sandstones and volcanic rocks, were stimulated by hydraulic fracturing. The surprisingly low microseismic activity was recorded with moment magnitudes M_w ranging from -1.0 to -1.8. The slip tendency analysis suggests a critically stressed reservoir exists in the sandstones, whereas the volcanic rocks are low stressed. Rock failure first occurs with an additional pore pressure of 20 MPa. Presumed failure planes are oriented as a conjugate set and strike NW and NE. Slip failure is more likely than tensional failure in the volcanic rock because high normal stresses prevent tensional failure. These results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity, and source characteristics indicate slip rather than extension along a presumed NE striking failurernplane. This match of fault reactivation potential assessment and induced seismicity analysis suggests the slip tendency analysis as an appropriate method to understand reservoir behavior under modified stress conditions.