Ten years' experience of monitoring CO2 injection in the Utsira Sand at Sleipner, offshore Norway

Underground storage of carbon dioxide (CO2) as a measure to reduce the amount of greenhouse gases in the atmosphere, and thereby to slow down global warming, has been studied and discussed widely over the last two decades (IPCC, 2005). Although considerable experience had been gained on CO2 injection for enhanced oil recovery before the start of the Sleipner storage project, very little was known about the effectiveness of underground storage of CO2 over very long periods of time. A number of demonstration sites have been initiated in the past few years, mainly for research purposes to investigate the feasibility of CO2 injection in different types of reservoirs and to study the chemical and flow behaviour of CO2 in the subsurface. The first, longest running and largest demonstration of CO2 injection in an aquifer up to now is at Sleipner, in the central North Sea.

Since October 1996, Statoil and its Sleipner partners have injected CO2 into a saline aquifer, the Utsira Sand, at a depth of 1012 m below sea level, some 200 m below the reservoir top. The CO2 is separated on the platform from natural gas produced from the deeper lying Sleipner Gasfield and injected into the aquifer through a deviated well at a lateral distance of about 2.3 km from the platform (Figure 2). This article outlines the experiences gained at this site, especially with respect to monitoring of CO2 migration in the subsurface.