CO2 storage — Sleipner field beneath the North Sea

Figure 1: Sleipner summary showing location map, cartoon of injection operation, and scanning electron microscope image of the porous Utsira Sand reservoir rock(images courtesy of Statoil and BGS)(click to enlarge).

With the release of the DTI's Carbon Abatement Technology Strategy, monitoring and learning from the existing underground carbon dioxide (CO2)storage project at Sleipner in the North Sea becomes even more important (Figure 1).

Natural gas produced at the Sleipner West field naturally contains about 9.5% CO2, which has to be removed to get the gas to saleable quality. Instead of venting the separated CO2 to the atmosphere, where it would add to the greenhouse problem, Statoil, the operators of the field, and their partners decided to inject it down a 3 km-long well and store it in a porous and permeable reservoir rock called the Utsira Sand.

About a million tonnes of CO2 per year is prevented from entering the atmosphere in this way, and a total of more than 11 million tonnes has been injected so far.

First CCS demonstration site

Started in 1996, the Sleipner project was the world’s first demonstration of carbon dioxide capture and underground storage. It is of great interest to the international community because if the concept can be applied to power stations it holds out the promise of making deep cuts in global CO2 emissions — without having to abandon fossil fuels.

BGS is among the organizations involved in monitoring and modeling the distribution of injected CO2 in the Utsira Sand to check that it is behaving as predicted and is not migrating out of the intended storage site. This type of demonstration will do much to satisfy future legal verification requirements and allay public concerns about safety issues.

Figure 2:3D seismic survey, showing acquisition vessel and subsurface ‘cube’ of data (Images courtesy of Statoil, Veritas and BP)(click to enlarge).

Time lapse 3D (4D) seismic data (Figure 2) were acquired in 1994, prior to injection, and again in 1999, 2001, 2002, 2004, 2006 and 2008 with around 11 million tonnes of CO2 in the reservoir at the time of the last survey.

Spectacular seismic images have been obtained, with the plume of injected CO2 imaged as a number of bright sub-horizontal reflections, above and around the injection point (Figure 3). The reflections are interpreted as wavelets from thin (just a few metres thick) layers of CO2 trapped beneath intra-reservoir beds of shale. The data shows the precise subsurface location of the CO2 plume and confirms that, so far, the CO2 is confined securely within the storage reservoir.

In addition to the 3D seismic, other monitoring surveys have been deployed, including high resolution 2D seismic (in 2006), seabed gravity (in 2002, 2005 and 2009), seabed controlled source electromagnetics (CSEM) (in 2008), and seabed imaging and bathymetry (in 2006). Current research work at BGS is focussed on understanding detailed migration processes within the plume by relating the seismic signals directly to CO2 distributions and amounts in the reservoir and history matching numerical flow simulations with the observed data.

What is the storage capacity of the Sleipner field?

Looking ahead, the Utsira Sand has an estimated pore-space volume of about 6 x 1011 m3. If only about 1% of this were utilised for CO2 storage, this would be sufficient to store 50 years emissions from around 20 coal-fired or nearly 50 gas-fired 500 MW power-stations.

Figure 3 : Time-lapse seismic images from Sleipner showing vertical sections (along the top) and maps of the expanding plume in 1994, 2001, 2004 and 2006 (along the bottom). The total height of the plume is about 250 metres, with a total width in 2001 of around 2 km. (Images courtesy of BGS involvement in the SACS, SACS2 and CO2STORE projects). (click to enlarge).

The Utsira Sand is by no means an unusual geological formation in terms of its storage potential, and the Sleipner operation represents just one of many subsurface storage scenarios. The possibility of storing CO2 in exhausted oil or gas bearing structures, which form proven long-term traps for buoyant fluids and gas, is another option. Underground CO2 injection is routinely used by the oil industry to assist with enhanced oil recovery (EOR), in the effective exploitation of oilfields.

Further information

Contact Dr Andy Chadwick for more information about carbon capture and storage (CCS).