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.

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.

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Thw paper was prepared for presentatmn al tfw Western Regional Meeting held m Anchorage, Alaska, 22-24 May 199S This paper was selec!ed lor presentahon by the SPE Prcgram Comm!tlee folfowmg revmw' of Informalton cnntamed In an abstracf submmed by the author(s) Contenls of the paper as presented, nave nO[ been rewewed by the Soctety of Petroleum Engnwers and are subjecf to correcton by the authcr(s) The maternal, as presented, does not necassanly reflecf any POSIIIO"of the Society of Petroleum Engtneers or idsm-mbersPapers presented a[ SPE meetlcgs are subject 10 publlcatum rewew by Edional Comm!ttee of the SomBly of Petroleum Engineers Permwsmn to COPY!s restricted to an abstract of not more than 3C0 works Illustrations may not be coped The abstract should contain conspicuous acknowledQmam of where and by whom the IXILI.X was presemed WWo L\brarvan, SPE, P O 833836 Rlchardso", TX 75083-3836 USA, fax 01 -214-952:!+435 ABSTRACT Production of fluids from the shallow, thick, and low-strength Diatomite reservoir in the South Belridge field has resulted in reservoir compaction, surface subsidence, and numerous well failures. The most severe subsidence problems occurred while the field was under primary recovery, prior to implementation of waterfloods for pressure support, Consequently, a significant number of wells in the field have incurred casing damage, which proportionate y limits the utility of these wellbores for cent inued production and routine wellwork operations. The nature of casing damage varies significantly depending on the location in the field, however casing shear ("kinks") generally produces the most severe problems in retaining full wellbore utility since it Iim its tool passage (e.g., packers, scrapers, etc.) and is not economically repairable using conventional milling tools. Since the mid 1980s, surveillance activities have been in place to monitor and assess the magnitude and progression of surface subsidence, reservoir compaction, and wellbore damage in Section 19 of the South Beh-idge field. Surveillance activities include the use of ground elevation markers and a variety of production logs --several of which involve novel application and log interpretation. These data subsequently were used to develop and verify hybrid geomechanics and wellbore geometric models for assessing casing damage, identifying well operability and wellbore utility limits, and forecasting remaining wellbore life from which a reservoir management plan and wellbore management operational strategies were established. A key ingredient in the implementation of this plan and operational strategies involved a cooperative tool development with a major oil tool service company to repair casing "kinked" wellbores, as an alternative to re-drilling wells. This paper provides a case history describing the various synergistic reservoir and wellbore management activities designed to effectively mitigate subsidence-induced operational problems and the accompanying field benefits resulting from their implementation. INTRODUCTION The Sou...