In recent years public concern about earthquakes induced by gas production has increased in the Netherlands. This has mainly been caused by numerous seismic events related to gas depletion in the Groningen gas field, the largest gas field in Western Europe. Induced seismicity has also been observed in 31 smaller gas fields located on land (onshore) or in the area close to the Dutch coast. Earthquakes with magnitudes as high as ML = 3.5 have occurred in Roswinkel and Bergermeer causing damage to buildings.

In 2016 State Supervision of Mines (SSM), with input from the geological survey of the Netherlands (TNO) and the onshore operators, proposed a guideline for a qualitative seismic risk analysis for depletion induced seismicity arising from gas production in the small fields in the Netherlands. The guideline follows international practices for risk assessment using a risk matrix approach. This paper elaborates the seismic risk guideline and reports on the application of the guideline to the gas fields in the Netherlands.

Risk is a combination of hazard and consequences. The result of the seismic risk analysis is qualitative and gives a relative scoring of the producing gas fields in the Netherlands in terms of risk. In order to obtain more information on the quantitative assessment of the risk, more detailed studies are needed. The Groningen gas field clearly poses a much larger seismic risk than that obtained for the other, smaller gas fields, most of which fall into the lowest risk category. Because of the large difference in risk between the Groningen field and the other smaller gas fields, the guideline of SodM deems it sufficient to carry out a qualitative risk analysis for the other gas fields in the Netherlands, as performed in this paper. Based on the combination of the hazards and consequences, the risk can be further interpreted and, if necessary, appropriate measures can be implemented.@en

Halite solution mining and natural gas production in the west of the province of Fryslân in the Netherlands cause subsidence significantly exceeding original predictions. Subsidence at the centre of the salt subsidence bowl reached 33 cm in 2015 while only 7 cm had been predicted in the 1995 forecast. Maximum subsidence above the nearby Harlingen gas field was around 33 cm in 2015 compared to 10 cm in the 2003 forecast for the end of field life. In addition, subsidence accelerated over time. From the regulator's point of view, the development of the discrepancies was unacceptable and led to an early reassessment of the salt mining project and to suspension of the gas production by the operator in 2008. Extensive research into the causes of the exceedances was able to provide a proper explanation for both occurrences. For the salt solution mining, cavern convergence rates at the in-situ stresses and temperatures at the depth of the project are much higher than initially estimated. This leads to a different production mechanism (squeeze mining), which gives much more subsidence. After convergence and subsidence modelling were corrected for this in 1998, predicted and observed subsidence are in much better agreement. The additional and accelerating subsidence above the Harlingen gas field can only be explained by pore-collapse of the gas-bearing reservoir chalk. During the laboratory compaction tests used for the original subsidence predictions, pore collapse only occurred at stress levels beyond those possible in-situ. The explanation for the inconsistency was found to be the strong impact of loading rate on the pore-collapse stress level. The applicability of an existing rate type compaction model for the Harlingen chalk was confirmed by new variable loading rate laboratory experiments. Using the model to translate the laboratory measured pore-collapse stresses into values applicable under field conditions, a much better agreement with the observed field behaviour was obtained. The Harlingen case thus provides further evidence that the large influence of loading rate on the chalk pore collapse stress seen during laboratory experiments applies down to field loading rates and that corrections for this need to be applied in compaction and subsidence forecasts.@en

The Wadden Sea is a shallow tidal sea in the north of the Netherlands where gas production is ongoing since 1986. Due to the sensitive nature of this area, gas extraction induced subsidence must remain within the "effective subsidence capacity" for the two tidal basins (Pinkegat and Zoutkamperlaag) affected. We present a probabilistic method to monitor the "effective subsidence capacity" and ensure that subsidence is below the long term (18.6 years) volumetric rate for relative sea level rise that can be accommodated by the tidal basins without environmental harm. The role of sedimentation volume rate, relative sea level rise and subsidence volume rate due to gas depletion are taken into account including their uncertainties. The probability of exceeding the acceptable subsidence limit for the period 2012 to 2050 is 2.8% for the tidal basin called Zoutkamperlaag and 1% for the tidal basin of Pinkegat for climate scenarios that fit the current relative sea level rise observations on the Dutch coast. The values are shown to be dominated by the effect of relative sea level rise, and not due to subsidence induced by gas depletion in the Wadden Sea. To current knowledge no harm is done to nature.@en

One of the limitations of InSAR is that it is only capa-ble of measuring a 3D projection of a real deformation vector on the radar line of sight. Therefore it is not possi-ble to retrieve the full displacement vector from a single InSAR measurement. The optimal solution for this limi-tation is a combination of InSAR measurements from dif-ferent imaging geometries. However, this solution is not applicable in areas where InSAR data are available in one particular geometry only. In this paper, a new approach will be discussed for 3D decomposition of InSAR defor-mation measurements for the case of subsidence. We use the hypothesis of a physical relation between horizontal displacement and vertical deformation. Using this hy-pothesis, we propose an iterative strategy for 3D decom-position of InSAR subsidence measurements. The pro-posed approach is tested on the PSI results of a complex subsidence field in Friesland, the Netherlands. Finally, the results are validated by comparison with available lev-eling data.@en