Calibrated 3D Geomechanical Model for Forecasting Gas Storage Seismicity

The Bergermeer reservoir is one of the largest gas storages in Western-Europe, with excellent reservoir properties. However, during original depletion seismicity was observed with magnitude up to 3.5 at a pressure below 58 bar. Therefore, it was deemed prudent to monitor the gas storage reservoir with a permanent, downhole micro-seismic array. The monitoring is embedded in a so-called Traffic Light System for managing seismic activity. The array observed 400 micro-seismic events with a magnitude below 0.9 during refill of the reservoir. Most activity was induced by the Midfield fault that also induced depletion seismicity. This activity is far below the threshold for felt or damaging earthquakes. A calibrated geomechanical model was developed that matches the micro-seismicity observed during refill and storage cycles. The geometry was obtained from the seismic interpretations using an efficient method to convert the faults and horizons to parametric surfaces that can be used in a Finite Element Method (FEM) model. The model was populated with Leak-off Test (LOT), minifrac, core and log data. The model was calibrated on observed depletion and refill seismicity, stress measurements and the observed surface displacement from geodetic and GPS surveys. The continuous records of surface displacements showed a much stiffer reservoir during refill, compared with depletion. Also, the response was delayed by 0.25 year with respect to pressure, indicating time dependence in the rock deformation. Fault slip during depletion and stress hysteresis explains the refill micro-seismicity, since the peak shear stress is redistributed during slippage, giving higher shear stress at the edges of the slip area. The model predicts higher seismic activity for higher injection rate, but the maximum magnitude is limited in the worst case to 2.2, which is expected to cause no damage.