Abstract. At the start of gas production its effects on land
subsidence are not certain. There are uncertainties in mechanisms, models
and parameters. Examples are non-linear deformation of reservoir rock, fault
transmissibility, behaviour of overlaying salt and aquifer activity. Looking
back at historical cases in the Netherlands, a factor two or three
difference between initial prediction and final outcome is quite common. As
the Dutch regulator, SSM is tasked with assuring proper management by
operators of the risks associated with land subsidence from natural gas
production in The Netherlands. Large initial uncertainties can only be
tolerated if operators can demonstrate that timely actions can still be
taken when predefined subsidence limits are at risk of being exceeded now or
in the future. The applied regulatory approach is illustrated by the case
history of gas production induced subsidence in the Dutch Wadden Sea area. This
environmentally highly sensitive UNESCO World Heritage Site is a natural gas
province. Extensive legal, technical and organisational frameworks are in
place to prevent damage to its natural values. Initial uncertainties in the
predicted subsidence (rate) were later exacerbated by the detection of
strong non-linear effects in the observed subsidence behaviour, leading to
new concerns. It was realised that – depending on the underlying physical
cause(s) – there will be a different impact on future subsidence. To assure
proper management of the additional uncertainty by the operator, several
improvements in the regulatory approach have been implemented. Possible
underlying mechanisms had to be studied in depth and improved data analysis
techniques were requested to narrow down uncertainties as time progresses.
The approach involves intensified field monitoring, scenario's covering the
full range of uncertainties and a particle filter approach to handle
uncertainties in predictions and measurements. Spatial-temporal double
differences, production data and the full covariance matrix are used to
confront scenario predictions against measurements and to assess their
relative probability. The regulator is actively involved in assuring this
integrated control loop of predictions, monitoring, updating, mitigation
measures and the closing of knowledge gaps. The regulator involvement is
supported in the Mining law and by appropriate conditions in the production
plan assent. With the approach it can be confidently assured that subsidence
(rate) will remain within the allowed range.
Regulating Subsidence and its uncertainty in the Dutch Wadden Sea