FISHA - Forward Induced Seismic Hazard Assessment, Application to Synthetic Seismicity Catalog Generated by Hydraulic Stimulation Modeling

Authors: Amir Hossein HAKIMHASHEMI, Jeoung Seok YOON, Oliver HEIDBACH, Arno ZANG, Gottfried GR√úNTHAL
Keywords: discrete element modeling, induced seismicity, geothermal reservoir, occurrence probability, large magnitude event, Gutenberg-Richter relation
Conference: Stanford Geothermal Workshop Session: Modeling
Year: 2013 Language: English
Abstract: After the occurrence of perceptible induced events during stimulation experiments in geothermal reservoirs the development of strategies to reduce the impacts of induced Seismic Events of Economic Concern (SEEC) became a hot topic. Here we present a general workflow to assess the hazard of induced seismicity in terms of occurrence Rate of SEEC (RSEEC). The workflow is called Forward Induced Seismic Hazard Assessment (FISHA) as it combines the results of forward hydro-mechanical-numerical models of fluid injection with methods of time-dependent probabilistic seismic hazard assessment. The advantage of FISHA is that it can quantify hazard from numerical experiments of various injection scenarios in order to recommend the optimum in terms of RSEEC. The geomechanical numerical model that we use to exemplify the FISHA workflow represents the geothermal reservoir with a discrete element fracture network (code PFC2D from Itasca). Additionally a hydro-mechanical coupled routine is implemented that enables us to simulate viscous fluid flow in porous media and flow driven failure of rock matrix as well as pre-existing fractures in Mode I (tensile) and Mode II (shear). From these we can compute seismic magnitude. We present here the results of the FISHA workflow using four fluid injection scenarios with variable injection parameter, i.e. injection flow rate, duration and style of injection. The resulting synthetic catalogs of induced seismicity for these four scenarios, including locations, time of occurrence, magnitude, etc., are used to calibrate the magnitude completeness (Mc), the Gutenberg-Richter (GR) a and b parameters, and the seismicity rate for each scenario in different time intervals. Using these parameters, the time-dependent RSEEC is calculated for each scenario. The workflow of FISHA is rather general, not limited to the geomechanical model used in this study, but it should be based solely on the resulting synthetic catalog or spatiotemporal stress changes. In contrast to other mitigation strategies that are based on real-time data or on monitored induced seismicity of stimulation experiments, we can perform various synthetic experiments with the same starting conditions. This enables us to estimate a priori the best stimulation strategy of the generic geothermal reservoir.
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