Introduction

The carbon dioxide injection at the Sleipner field in the North Sea (Baklid et al. 1996), operated by Statoil and the Sleipner partners, is the world's first industrial scale CO2 injection project designed specifically as a greenhouse gas mitigation measure. CO2 separated from natural gas is being injected into the Utsira Sand (Fig. 1), a major saline aquifer of late Cenozoic age (Chadwick et al., 2004a; Zweigel et al., 2004). The injection point is at a depth of about 1012 m bsl, some 200 m below the reservoir top. Injection started in 1996 and a total of more than 7 million tonnes of CO2 are presently in situ at the time of writing.

Figure 1. Location map showing Sleipner and the extent/thickness of the Utsira Sand aquifer.

Since 1998 the injection operation has been linked to a number of research projects, notably SACS, SACS2 and CO2STORE. These projects, funded by the EU, industry and national governments, aim to show that underground storage is a safe and verifiable technology. Specifically they have carried out scientific research into the geological aspects of the Sleipner injection operation by monitoring and modelling the injected CO2 plume.

Key aims of the monitoring programme at Sleipner are outlined below:

1. To show that the CO2 is being confined safely within the primary storage reservoir.

2. To image the distribution and migration of CO2 throughout the reservoir and, should it occur, into adjacent strata.

3. To provide early warning of any potentially hazardous migration towards the seabed.

Baseline 3D seismic data were acquired in 1994, prior to injection, with repeat surveys in 1999 (2.35 million tonnes of CO2 in the reservoir), 2001 (4.26 Mt) and 2002 (4.97Mt). In addition, to complement the information available from the seismic datasets, a seabed gravimetric survey was acquired in 2002.

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