Statistics of seismic events at the Groningen field

In the Netherlands, over 190 gas fields of varying size have been exploited, and 15% of these have shown seismicity. The prime cause for seismicity due to gas depletion is stress changes caused by pressure depletion and by differential compaction. The observed onset of induced seismicity due to gas depletion in the Netherlands occurs after a considerable pressure drop in the gas fields. Geomechanical studies show that both the delay in the onset of induced seismicity and the nonlinear increase in seismic moment observed for the induced seismicity in the Groningen field can be explained by a model of pressure depletion, if the faults causing the induced seismicity are not critically stressed at the onset of depletion. Our model shows concave patterns of log moment with time for individual faults. This suggests that the growth of future seismicity could well be more limited than would be inferred from extrapolation of the observed trend between production or compaction and seismicity. The geomechanical models predict that seismic moment increase should slow down significantly immediately after a production decrease, independently of the decay rate of the compaction model. These findings are in agreement with the observed reduced seismicity rates in the central area of the Groningen field immediately after production decrease on 17 January 2014. The geomechanical model findings therefore support scope for mitigating induced seismicity by adjusting rates of production and associated pressure change. These simplified models cannot serve as comprehensive models for predicting induced seismicity in any particular field. To this end, a more detailed field-specific study, taking into account the full complexity of reservoir geometry, depletion history and mechanical properties, is required.

Section: Reduction Of Seismicity After Production Stop: the Mechanicasupporting

confidence: 80%

Geomechanical models for induced seismicity in the Netherlands: inferences from simplified analytical, finite element and rupture model approaches

Wees

Fokker

Thienen-Visser

et al. 2017

“…For the induced seismicity, a plausible causal mechanism linking it to the pressure fluctuations in the reservoir could be identified (Pijpers, 2016). Support for the reduction in seismicity is also provided by the statistical analysis reported by Nepveu et al (2016). To our knowledge this is the first time that operational controllability of gas-production-induced seismicity (at least in the short term) has been corroborated.…”

Section: Recent Developmentssupporting

confidence: 57%

From checking deterministic predictions to probabilities, scenarios and control loops for regulatory supervision

Waal¹,

Muntendam-Bos²,

Roest³

2017

Prediction of gas-production-induced subsidence and seismicity is much more difficult and uncertain than generally recognised in the past. It is now widely accepted that uncertainties in predicted subsidence and seismicity are large prior to and during the initial stages of production. At later stages, predictions remain highly uncertain for periods more than three to five years into the future. This requires a different regulatory framework to ensure that associated risks remain within accepted boundaries. Previously, single-scenario operator predictions were checked against field measurements.When subsidence or seismicity started to deviate beyond claimed uncertainties, the operator was asked to provide prediction updates. The practice was long considered acceptable, as structural damage to buildings and infrastructure or personal risk to people was not expected. This all changed following the 2012 Huizinge seismic event, necessitating better identification, assessment and ranking of risks, the use of scenarios, probabilistic forecasting and a much intensified field monitoring and control loop. It requires that the regulator becomes actively involved in assuring the integrated control loop of risk identification, predictions, monitoring, updating, mitigation measures and the closing of knowledge gaps, to ensure that subsidence (rate) and induced seismicity remain within acceptable limits. And it requires that this increased involvement of the regulator is supported in the mining law and by appropriate conditions in the Production Plan assent.

“…Nepveu et al (2016) came to a similar conclusion and proposed a delay of 2-8 months between the change in production and the change in activity rate.…”

Section: Temporal Variationsmentioning

confidence: 69%

Development of seismicity and probabilistic hazard assessment for the Groningen gas field

Dost

Ruigrok

Spetzler

2017

124

The increase in number and strength of shallow induced seismicity connected to the Groningen gas field since 2003 and the occurrence of a M L 3.6 event in 2012 started the development of a full probabilistic seismic hazard assessment (PSHA) for Groningen, required by the regulator. Densification of the monitoring network resulted in a decrease of the location threshold and magnitude of completeness down to ∼ M L = 0.5. Combined with a detailed local velocity model, epicentre accuracy could be reduced from 0.5-1 km to 0.1-0.3 km and a vertical resolution ∼0.3 km. Time-dependent seismic activity is observed and taken into account into PSHA calculations. Development of the Ground Motion Model for Groningen resulted in a significant reduction of the hazard. Comparison of different implementations of the PSHA, using different source models, based on either a compaction model and production scenarios or on extrapolation of past seismicity, and methods of calculation, shows similar results.