B. Orlic

Integrated 3D geomechanical modelling for deep subsurface deformation: a case study of tectonic and human-induced deformation in the eastern Netherlands

Abstract Current advances in finite element computer codes and increase in computer power theoretically allow quantitative modelling of the geomechanical effects of hydrocarbon depletion from reservoirs. Here we show that it is technically possible to incorporate the full complexity of the 3D geological structure of a reservoir including faults into geomechanical models. In the workflow GOCAD is used for the structural modelling, integrated with DIANA for the geomechanical calculations. A case study on the Roswinkel gas field in the eastern Netherlands illustrates the working methodology. The case study clearly shows the strong dependency of gas depletion deformation effects on the prevailing tectonic stress field. Our models for gas depletion predict a stabilization of the stress field (further away from failure) for reservoirs in compressive and strike-slip regimes. On the other hand extensional stress regimes will result in failure of the reservoir, in agreement with observed earthquakes, provided that (a) the reservoir material or existing faults are weak and (b) the state of stress is close to failure of the material. The Roswinkel field, which is marked by a high abundance of earthquakes occurring after gas depletion started, is therefore most likely marked by an extensional tectonic regime and by geomechanically weak rock or pre-existing faults. According to base Tertiary fault displacements and the World Stress Map, the extension (minimum horizontal principal stress) is most likely NE-SW oriented.

Production-induced Fault Reactivation and Seismicity in The Netherlands - From Quick Scan to 3D Geomechanical Modelling

Recent induced seismic events in the giant Groningen gas field have raised concern on the safety of gas production and the risk of induced seismicity in the Groningen gas field in The Netherlands. Statistical analysis of past seismic events in the Groningen Field reveals that seismicity is non-stationary and seismicity rates are increasing, which leads to the conclusion that the maximum possible magnitude of seismic events, related to gas depletion, may be significantly higher than estimated before on the basis of statistics of past seismic events alone. The non-stationarity of seismicity may have implications for the seismic hazard of the small gas fields in The Netherlands as well.Formerly the maximum magnitude for these small onshore fields was estimated via the frequency magnitude distribution of the combined seismicity for all these fields, which resulted in an estimate of ML 3.9 for the maximum magnitude. However, as seismicity is non-stationary, only a small fraction of the gas fields is seismically active and the number of seismic events associated with them is generally small, past seismicity and related statistics cannot be used very well to study the future response of those fields and implications for fields with no recorded seismicity are unclear. In this presentation we show methods to assess the seismicity potential of these fields, which are based on techniques other than statistical analysis of past seismicity alone.

Detecting Remaining Gas Reserves Based on Coupled Modelling of Subsidence and Reservoir Depletion Processes

Depletion of large volumes of natural gas, oil and water from hydrocarbon reservoirs may lead to movements of the Earths surface (tilt or subsidence). Observations of these movements (Interferometric Synthetic Aperture Radar (InSAR), leveling measurements) can provide a better insight on subsurface processes like reservoir compaction or the aquifer strength. A simple inversion approach does not usually provide a sensible solution due to the non-uniqueness of the inverse problem and the sensitivity of the inverse problem to small fluctuations in the data. This necessitates the use of all available prior information (geologic model, reservoir model) in the inversion procedure to better constrain the subsurface parameters. A time-dependent inversion scheme is adopted for resolving the spatial and temporal reservoir pressure drop from the surface subsidence observations. The innovative inversion method is effectively applied to one synthetic and two field cases (one from The Netherlands, one from North Africa). The results suggest that incremental gas reserves due to reservoir compartmentalization can be detected successfully.