Abstract

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.

1. INTRODUCTION

Subsidence caused by extraction of hydrocarbons is a sensitive issue in the Netherlands due to its proximity to sea level. The Wadden Sea in the north of the Netherlands is a shallow tidal sea behind a chain of coastal barrier islands. It has been inscribed on the UNESCO’s World Heritage list since 2009 because of its unique morphodynamic features and its wildlife. Also, it is one of the most notable nature conservation areas in the Netherlands protected under the European Birds and Habitats Directives. In the Wadden Sea, gas production is ongoing since 1986 from the Ameland gas field (Fig. 1). Since 1990 the subsidence caused by depletion of the Ameland gas field is compensated by sand suppletions under a dynamic ‘maintain the coast line’ preservation policy [1]. In 2006 additional gas production commenced in the Nes, Moddergat, Lauwersoog and Vierhuizen fields (Fig. 1). Due to the sensitive nature of this tidal sea the gas fields in the area (Ameland Nes, Lauwersoog, Vierhuizen, Anjum, Ezumazijl, Mestlawier and Moddergat) are being produced within the so-called "effective subsidence capacity" [2].

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