Groningen gasfield

The Groningen field was discovered in 1959 and at that time turned out to be the largest natural gas field in the world. Production started in 1963 and NAM (Nederlandse Aardolie Maatschappij), the joint venture between Shell and Exxon Mobil which operates the field, estimate the initial recoverable reserves at 2900 billion cubic metres (Nm3). By 2015, approximately 2000 billion Nm3 gas had been produced. As Groningen gas is relatively rich in nitrogen (14%), its calorific value is lower than that of most other gas fields in the Netherlands. Owing to the Groningen field’s huge volume, most domestic appliances (e.g. for heating and cooking) in the Netherlands, Germany, Belgium and Northern France have, since the 1960s, been adapted to use this low-calorific gas.

Lithology and reservoir characteristics
The Groningen gas field reservoir is the Upper Rotliegend Group of Early Permian age (Figure 1). Figure 2 shows a cross-section through the entire reservoir. The reservoir is located at depths of between 2600 and 3200m. The total thickness of the Rotliegend in the Groningen field ranges from approximately 100m in south-southeast to approximately 300m in north-northwest. From east to west the Rotliegend thickness is fairly uniform. The Upper Rotliegend Group has been subdivided into the Slochteren Formation (reservoir rock) and the Silverpit Formation (non-reservoir rock). The Slochteren Formation is a sequence of conglomerates and course-grained sandstone layers deposited in sheet floods and fluvial braiding channel fills, interbedded with fluvial medium-grained sandstone layers deposited in a desert plain and finer-grained aeolian dune and sand-flat deposits. The Slochteren Formation is overlain by the Ten Boer and Ameland Members of the Silverpit Formation. The claystone of the Ameland Member is restricted to the northern part of the Groningen field, where it separates the Slochteren Formation in the Upper and Lower Slochteren Formations, and forms a potential barrier to flow. The Ten Boer and Ameland Members were deposited in the southern Permian Basin, which at the time comprised a desert lake environment. The gas now contained in the Slochteren Formation is migrated from the coal layers in the underlying Carboniferous. The thick impermeable salt and anhydrite layers of the overlying Zechstein Group form the seal that traps the gas in the Rotliegend reservoir rock. 


Figure 1   Structure map of Groningen gas field. The depth contours represent respectively the top of the Slochteren Formation (left) and the top of the Rotliegend (right)

Structural setting
The gas-water contact in the Groningen field is encountered at depths between 2971m in the north-east of the field to 3016m in the south-west. The Groningen field is cut by several fault systems, subdividing the field into a large number of fault blocks. Most fault systems trend either east-west or north-northwest to south-southeast, and the throw can be up to 200m. Permeability is generally not affected by the faults that intersect the field. The Groningen field is mainly delimited by faults, and in a few places by ‘dip closures’.


Figure 2  Geological cross section of the Groningen gas field (Production plan 2016, NAM) Note that the vertical scale is exaggerated 7 times compared to the horizontal scale

Production
Gas production from the Groningen field started in 1963 and is expected to end around 2080. The initial reservoir pressure was 347 bar, but by 2016 it has dropped to approximately 95 bar as a result of reservoir depletion. The reservoir’s expected abandonment pressure is approximately 10 bar.
The Groningen field is currently (spring 2016) being produced by means of 258 wells at 22 production locations. Treatment facilities are present at twenty of these production locations, and the gas of the other two well sites is transported by pipeline to the nearest gas-treatment location. There are also 28 observation wells for reservoir management and a number of injection wells to inject the produced water back into the reservoir.
In view of the increased induced seismicity, the volume of gas to be produced from the Groningen gas field has, since 2014, been determined by the Minister of Economic Affairs in a decree on the Groningen production plan. For the 2015-2016 gas year, the maximum production has been set at 27 billion Nm3. The preliminary decree of July 2016 proposed a further reduction to 24 billion Nm3 per year for the next five gas years, with extra gas being allowed to be produced only in the event of very cold winters in the Netherlands. The Dutch government also publishes News on the decision-making process on its website.
The following table lists the various production-limiting measures taken by the Minister of Economic Affairs since 2014.

Date

Reduction measure by the Minister of Economic Affairs

17 January 2014

 

  • A maximum production of 42,5 bcm for 2014
  • Maximum 3 bcm for the Loppersum clusters (Leermens, Overschild, de Paauwen, Ten Post, Het Zand)

December 2014

  • A maximum production of 39,4 bcm for 2015
  • Maximum 3 bcm for the Loppersum clusters (Leermens, Overschild, de Paauwen, Ten Post, Het Zand)
  • Maximum 9,9 bcm for the clusters close to Hoogezand-Sappemeer (Kooipolder, Slochteren, Zuiderveen, Spitsbergen, Tusschenklappen, Froombosch, Sappemeer) for the period 1st October 2015 until 30th September 2016
  • Maximum 2 bcm for the Eemskanaal cluster

 

February 2015

  • Maximum production of 16,5 bcm for the first six months of 2015

14 April 2015

  • Limit production from Loppersum clusters if necessary for the security of supply

23 June 2015

  • Maximum production of 13,5 bcm for the last six months of 2015

18 November 2015

  • Maximum production of 27 bcm for the gas year 2015/2016

24 June 2016

  • Maximum production of 24bcm for the coming 5 gas years (provisional decision)

Seismicity
The first event in the Groningen gas field occurred on 4 December 1991 and had a magnitude ML=2.4. By 31 December 2015 a total of 900 events have been registered, 256 of which exceeded ML=1.5 in magnitude (Figure 3). The event with the highest magnitude registered to date, with a magnitude of ML= 3.6, occurred near Huizinge on 16 August 2012. Figure 4 shows the number of events in the Groningen field with magnitudes exceeding ML> 1.5, plotted against annual production, while the map in figure 5 shows the locations of these earthquakes in the Groningen field. Technical reports on induced here. The websites of the Royal Netherlands Meteorological Institute (KNMI) and Kennislink (in Dutch only) also provide information on earthquakes induced by gas production.

 

 

 


Figure 3 & 4  (Left) Histogram of the events registered in the Gronigen field. (Right) Induced seismicity (  >1.5) of the Groningen field in time sorted by magnitude vs. the annual production.

Figure 5  Location of induced events (ML≥1,5) in the Groningen field

Subsidence
Ground subsidence above the Groningen field is recorded by a monitoring network of bench marks. The maximum amount of subsidence measured in early 2016 is 33 cm. Since May 2014, twelve additional GPS stations have been installed. Continuous measurements are reported monthly. These can be viewed on the page of geodetic measurement records under the heading of ‘Monitoring Plan – Northern Netherlands’ (including continuous GPS measurements in the Groningen gas field since 2014).