Sanz, Pablo F. (ExxonMobil Upstream Research Co. (EMURC)) | Lele, Suvrat P. (EMURC) | Searles, Kevin H. (EMURC) | Hsu, Sheng-Yuan (EMURC) | Garzon, Jorge L. (EMURC) | Burdette, Jason A. (ExxonMobil Development Co.) | Kline, William E. (EMURC) | Dale, Bruce A. (ExxonMobil Production Co.) | Hector, Paul D. (ExxonMobil International Ltd.)
The Groningen Gas Field in Northern Netherlands is the largest gas field in Europe with production starting in 1963. Seismic events were first observed in 1986, but these were generally small with minimal damage. A government study concluded in early 1990’s that tremors were linked to gas production. The objective of the work described here is to utilize advanced geomechanical modeling to (i) characterize subsurface behavior related to production-induced fault reactivation, and (ii) evaluate alternate production strategies to help manage subsurface stresses to reduce fault slippage which can lead to seismicity.
Multi-scale 3D geomechanical models were developed using a non-linear quasi-static finite element method. This modeling framework includes a global model to capture full-field phenomena and two sub-models for regions with observed seismic activity which honor conditions of the global model, but also include explicit modeling of multiple faults. This approach considers the following features: i) Irregular stratigraphy and fault surfaces, ii) Variable reservoir rock properties according to porosity changes, iii) Non-uniform pressure depletion derived from field data and reservoir simulations, iv) Relaxed deviatoric salt stresses at start of production, v) Salt creep effects during production, vi) Biot coefficient effects for reservoir rocks, and vii) Coulomb friction behavior to capture slippage along faults. Models are verified by comparing predictions for the production history period (1964 – 2012) with corresponding field data. The model predictions for production forecast period (2012 onwards) are used for relative comparison of various production scenarios. Subsidence and reservoir strains calculated from the full-field global model during production history match well with corresponding field data without the need for calibration of material properties.
Model results show that the fault frictional dissipated energy correlates well with the radiated energy from observed seismic events, and that the energy scaling factor associated with this correlation is constant and the same for both sub-model 1 and 2. The dissipated energy during frictional sliding is a scalar quantity that provides a representative measure of fault activity for a given area of interest. Furthermore, because the dissipated energy correlates well with observed radiated energy, the models can be used for relative comparison of production scenarios to identify strategies that reduce fault loading.
Several production forecast scenarios are analyzed and evaluated based on predicted frictional dissipated energy to assess fault slippage. These results indicate that curtailment of production alone is not an effective alternative for mitigation of energy dissipation and related seismic activity. This study shows that advanced geomechanical models are a powerful tool that can provide valuable insight into the overall trend of cumulative radiated energy, are useful in understanding seismic activity, and can be used to identify production scenarios that mitigate seismic activity.