Forward Induced Seismic Hazard Assessment (FISHA) Based on a Hydromechanical Coupled Fracture Mechanics Earthquake Model

Abstract

The occurrence of an Mw 3.2 induced seismic event in 2006 during the injection of fluid at the Basel geothermal site in Switzerland initiated an on-going discussion on the potential hazard and risk of hydraulic stimulation in Europe. So far, development of mitigation strategies of induced Seismic Events of Economic Concern (SEECo) has become an important issue. Statistical methods have been developed to understand the induced seismicity from which appropriate strategy for mitigating effect of SEECo can be established. However, all these methods require catalogues of monitored seismicity. Therefore, they cannot deliver a priori strategies for stimulation treatment. Here we present the Forward Induced Seismic Hazard Assessment (FISHA). This procedure converts the output of a hydromechanical coupled and fracture mechanics based model into probabilistic seismic hazard of induced seismicity in terms of time-dependent occurrence rate of induced seismic events. FISHA is applied to the simulation results of various stimulation scenarios, where injection flow rate, duration and style of injection, as well as reservoir parameters are varied. The geothermal reservoir is modelled using a discrete element fracture network that is coupled to a routine of viscous fluid flow in porous media generating flow-driven failure of rock matrix and pre-existing fractures in Mode I (tensile) and Mode II (shear). The output is a synthetic seismicity catalogue. This catalogue includes hypocentre, occurrence time and magnitude of induced seismic events. The synthetic catalogue is then used to determine the magnitude completeness Mc and to estimate the time-dependent parameters a and b of the Gutenberg-Richter frequency-magnitude relation using statistical methods. These parameters are then applied to estimate the time-dependent occurrence rate of SEECo for each stimulation scenario. In this way, we can evaluate the stimulation scenarios in terms of hazard according to the corresponding rate of SEECo. The advantage of FISHA is then the assessment of the occurrence rate of SEECo resulting from various stimulation scenarios, which gives a priori information to operator how to mitigate the risk of occurrence of SEECo.