| Title | Constraining of Focal Mechanisms of Induced Seismicity Using Borehole Logging Information |
|---|---|
| Authors | Yusuke MUKUHIRA, Kazumasa FUSE, Makoto NAOI, Michael FEHLER, Takatoshi ITO, Hirokazu MORIYA, Hiroshi ASANUMA, Markus O. HÄRING |
| Year | 2018 |
| Conference | Stanford Geothermal Workshop |
| Keywords | induced seismicity, focal mechanism, borehole logging, in-situ stress, existing fracture |
| Abstract | Monitoring of microseismicity is essential for successful hydraulic stimulation in Enhanced/engineered geothermal development. Hypocenter location of the microseismic events is the indicator of the fracture whose permeability has been enhanced by shear slip. In addition to the location of microseismicity, knowledge about the geometry of the fracture network is also important for better understanding of fractured reservoir, flow, and design of the heat extraction system since geometry of the fracture is directly correlated to the permeability. However, due to the small number of monitoring station in geothermal microseismic monitoring, it is often difficult to constrain the focal mechanism of microseismicity from first motion information. We propose a novel method to overcome this limit and estimate focal mechanism using borehole logging information such as in-situ stress and information about existing fractures. We estimate the range of fault planes which can have a shear slip under a given stress state and increase of pore pressure. By referring to that region, 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 of focal mechanisms. By referring to this, we can reject less unlikely forcal mechanisms and chose more likely ones. Thus, we can constrain the focal mechanisms considering not only seismogenic information but also other geophysical information. We apply our method to real data from Basel, Switzerland and examine the performance and the feasibility of our method by comparing with the well constrained focal mechanism of larger eventsrecorded by a regional network. |