2.1 Sleipner

Statoil's Sleipner discovery in the Norwegian sector of the North Sea contained 6.3 Tcf of natural gas [1]. The discovery contains about 9% CO2 which needs to be removed before the gas can be sold [2]. During the project's development phase, in 1991 the Norwegian government introduced an offshore CO2 tax and so Statoil proposed to inject rather than emit the CO2 [3]. This makes Sleipner the first commercial geological CO2 storage project (see Figure 1). Since 1996, the project has been injecting about 1 Mt/yr of CO2 into the Utsira Formation. Statoil estimates that there is sufficient structural closure within 12 km of the injection site to store 20 Mt of carbon dioxide over the life of the project [4].

Figure 1 – A schematic depiction of the Sleipner Vest development [5]

The gas produced from the Ty formation is pre-treated before the CO2 is removed and further treated on the Sleipner T platform. Most of the treated natural gas is exported through a pipeline to the European mainland. Some of the gas is reinjected into the Sleipner East discovery to improve condensate production [6].

The CO2 is captured from the produced gas stream with a conventional amine process using Methyl diethanolamine (MDEA) as the solvent [7]. Before injection [8], CO2 is converted to a supercritical state, requiring compression to 8 MPa and cooling to 40°C. This is achieved using a compressor train consisting of 4 units, each with a fluid knockout drum to remove water, compressor, cooler and gas turbine driver.

The recovered CO2 is injected into the Utsira Formation, a 50 m-to-250 m thick sandstone unit approximately 1,000 m below the Sleipner discovery [8]. One horizontal injection well is used to inject 1 Mt of CO2 per year into the storage reservoir [8]. The Utsira formation has very favourable geological characteristics. Permeabilities are in the range 1 to 10 Darcy and porosities are greater than 30%. These properties, together with the sizeable net pay mean that injectivities in the Utsira formation are quite high. In practise, very few CO2 storage formations would have such high injectivities.

There has been extensive monitoring of the project as part of the IEA Greenhouse Gas R&D Programme's (IEA GHG) Saline Aquifer CO2 Storage (SACS) project [4]. The SACS Best Practice Manual [6] reports the results of seismic surveys of the Utsira formation over the period 1994 to 2001. Again, because of the particular characteristics of the Utsira formation, the images produced show the location of injected CO2 in very clear bands.