The Groningen Gas Field in the Northern Netherlands is the largest gas field in Europe and has been producing since 1963. Small magnitude seismic events in this seismologically quiet region were first observed in the early 1990's and linked to gas production. The objective of the work described here is to advance the understanding of subsurface deformation induced by gas production by including hundreds of mapped faults and fault-offsets to (i) characterize subsurface behavior related to production-induced fault reactivation, (ii) evaluate alternate production strategies to help manage subsurface stresses to reduce fault slippage which can lead to seismicity, and (iii) integrate with a seismological model for prediction of seismic activity rate.
The multi-scale modeling framework includes a global model to capture full-field phenomena and three 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) Non-uniform reservoir rock properties based on porosity, iii) Non-uniform pressure depletion mapped from 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.
The geomechanical models are used to better understand subsurface behavior related to production induced compaction and fault reactivation. Several production scenarios are analyzed and compared on a relative basis based on the predicted dissipated energy. The slip and contact force data for all finite element (FE) nodes on the fault surfaces are used as input to the seismological models for prediction of seismic activity rate. Results show the fault offset is a key factor for production induced fault slip. A fault with offset can develop slip due to differential compaction on two sides even if the dip and azimuth are not favorable for fault slip. This leads to more slip on significantly more faults compared to that in previous models without offsets. Initial seismological model results based on slip data predicted by present models show good correlation with observed seismic activity rate.
Models rely on input parameters such as fault friction coefficient, rock properties and initial stress conditions that have some degree of uncertainty. To address the impact of these uncertainties, sensitivity analyses with a range of input parameters were undertaken which yield a range of outcomes. The use of quasi-static geomechanical models for predicting seismicity attributes is an evolving field and additional improvements or alternative correlations may be identified in the future. However, current model results are used to compare various production scenarios on a relative basis or for correlations in seismological modeling, and the parameters are expected to be consistent between scenarios.