Constraining of Focal Mechanisms of Induced Seismicity Using Borehole Logging Information

摘要

Monitoring of microseismicity is essential for successful hydraulic stimulation in Enhanced/Engineered geothermal development. Hypocenter locations of the microseismic events are indicators of fractures whose permeability has been enhanced by shear slip. In addition to the locations of microseismic events, knowledge about the geometry of the fracture network is also important for better understanding of the fractured reservoir, flow, and design of the heat extraction system since fracture geometry is directly correlated to permeability. Focal mechanisms contain first order information about the fracture network. However, due to the small number of monitoring stations in geothermal microseismic monitoring, it is often difficult to constrain the focal mechanisms of microseismicity from first motion information. We propose a novel method to overcome this limitation and estimate focal mechanism by including borehole logging information such as in-situ stress and information about existing fractures. We estimate the range of fault planes which can have shear slip in a given stress state and increase of pore pressure. By referring to that range, we eliminate candidate of focal mechanisms which are consistent with first motion information but not consistent with in-situ stress. As another approach, we use existing fracture information from borehole logging. We establish a statistical model of existing fractures and calculate the likelihood of shear slip from all candidate focal mechanisms. By referring to this model, we can reject less unlikely focal mechanisms and chose more likely ones. Thus, we can constrain the focal mechanisms considering not only seismological information but also other geophysical information. We apply our method to field data from Basel, Switzerland and examine the performance and the feasibility of our method by comparing with the well constrained focal mechanisms of larger events determined using data from a regional network.