Every year since 1996, we have captured one million tonnes of carbon dioxide from natural gas production on the Sleipner field and stored it in an aquifer more than 800 metres below the seabed. On Sleipner, carbon dioxide is captured using a conventional amine process and stored in geological layers.
During the planning stage, it was a challenge to make the processing equipment compact enough for there to be space for it on a platform out in the middle of the North Sea, 250 kilometres from land.
The extra costs in connection with the compression and injection of carbon dioxide amounted to about USD 100 million.
As of year end 2007, roughly 10 million tonnes of carbon dioxide have been stored. The subterranean migration of the carbon dioxide has been mapped in various research projects, which are partly financed by the EU.
It started in 1990 with the choice of the conceptual solution for the Statoil-operated Sleipner West gas and condensate field in the North Sea, when it was still at the planning stage. Tests showed that the natural gas in the reservoir contained around 9% carbon dioxide. This exceeded the customers' requirements and the carbon dioxide content therefore had to be reduced.
In 1991, the Norwegian authorities introduced a carbon dioxide tax as a measure aimed at reducing emissions offshore. The tax now amounts to around USD 50 per tonne,
The carbon dioxide tax was one of the reasons for Statoil's plans to separate the carbon dioxide offshore and to inject it into geological layers deep beneath the Sleipner platform.
The carbon dioxide will probably remain stored in the geological layer for thousands of years. The layer contains porous sandstone filled with saline water, and it is called the Utsira formation. The carbon dioxide is enclosed beneath an 800-metre-thick layer of cap rock, and it will therefore not be able to escape to the atmosphere.
The Utsira reservoir is continuously monitored using seismology, and comprehensive models have been developed for calculating how the carbon dioxide moves in the reservoir. SINTEF and NTNU are involved in addition to several international research institutions.
A number of articles have been published on the basis of data from Sleipner, and StatoilHydro has proposed guidelines for a legal framework for how the carbon storage is to be monitored. These proposals are currently under consideration by the EU.
