Geomechanics response and induced seismicity during gas field depletion in the Netherlands

In this paper we present a review of controlling geological, tectonic and engineering factors for induced seismicity associated to gas depletion in the Netherlands and we place experiences from extensive Dutch geomechanical studies in the past decade in the context of generic models for induced seismicity. Netherlands is in a mature gas production phase, marked by excellent subsurface structural and stratigraphic characterization. Over 190 gas fields of varying size have been exploited. No more than 15% of these fields show seismicity. Geomechanical studies show that, similar to the EGS stimulation phase, largest seismicity is localized on pre-existing fault structures. However, the prime cause for seismicity in gas depletion is differential compaction, whereas in EGS stimulation related pressure build-up and fluid pressure diffusion along the faults form the prime mechanism. On the other hand, our study has a close theoretical analogy to reservoirs where the fluid volumes extracted are significantly larger than the re-injected volumes, and which can result in (differential) reservoir compaction.The observed onset of induced seismicity in the Netherlands occurs after a considerable pressure drop in the gas fields. Geomechanical models show that both the delay in the onset of induced seismicity as well as the non-linear increase in seismic moment observed in the induced seismicity, can be explained using a model of differential compaction, if the faults involved in induced seismicity are not critically stressed at the onset of depletion. The presented model serves to highlight key aspects of the interaction of initial stress and differential compaction in the framework of induced seismicity in Dutch gas fields. It is not intended as predictive model for induced seismicity in a particular field. To this end, a much more detailed field specific study, taking into account the full complexity of reservoir geometry, depletion history, mechanical properties and initial stress field conditions is required.