Unwanted carbon dioxide from Statoil's Sleipner West field in the Norwegian North Sea is being stored 1,000 metres beneath the seabed. This solution won the chief executives health, safety and environmental prize for 2000.


Process technician Sverre Andreassen at the carbon dioxide plant on Sleipner T. (Picture: Øyvind Hagen)

About 2,800 tonnes of carbon dioxide are separated daily from Sleipner West's gas production and injected into the Utsira sandstone formation (aquifer), rather than released to the air.

This solution has been in use since the field came on stream in the autumn of 1996. But 2000 is when the saline aquifer carbon dioxide storage (Sacs) project demonstrated that the injected gas remains in place rather than leaking out.

The Statoil chief executive's HSE prize was awarded to the carbon dioxide separation and injection project on 14 December 2000.

Recipients included Sigmund Helland, Knut Røed and Dag Petter Berg, who initiated this development in Statoil. Others were the Sacs project, represented by manager Tore A Torp and Olav Kårstad, and the Sleipner operations organisation, represented by Rasmus Haugland.

One aim of the Sacs work has been to document what happens to the carbon dioxide after it is injected below ground.

Statoil has hosted this project, which is funded by the European Commission as well as a number of major energy companies and national governments around the North Sea.

"The core of Sacs has been to arrive at a reasoned view of whether carbon dioxide remains in the Utsira sand and whether developments in this formation can be monitored," explains Mr Torp.

Seismic
The spread of carbon dioxide through the aquifer is recorded by seismic surveys. One was conducted before injection started, and a another took place in the autumn of 1999.

Many of the specialists working on geological issues doubted whether liquid carbon dioxide could be distinguished on seismic maps from the brine already present in Utsira.

But staff at Statoil's research centre in Trondheim have confirmed their theory that sound waves reflect differently from carbon dioxide and salt water.

Comparing seismic data collected before and after injection started has allowed researchers to show how carbon dioxide deep inside the Utsira formation migrates.

It is held under the layer of shale cap rock, 80 metres thick, which covers the whole formation. This extends for several hundred kilometres in length and about 150 kilometres in width.

Store
With a thickness of 250 metres, the formation can store 600 billion tonnes of carbon dioxide. That compares with a mere million tonnes being injected annually from Sleipner West.

"The entire carbon dioxide emissions from all the power stations in Europe could be deposited in this structure for 600 years," says Mr Torp.

One question asked by both researchers and other people when they hear about the Sleipner solution is how long the greenhouse gas will remain underground.

Mr Torp admits that the researchers cannot promise it will stay in store for ever. But a duration until the next ice age, in 5-10,000 years, must be good enough, he says.

He believes that the formation is highly unlikely to leak for several hundred years. By then, the "carbon age" will be over and humans will have found cleaner energy solutions.

All in all, the signs are that carbon dioxide leaks from the Utsira formation will not be greater than seepage from natural carbon dioxide deposits elsewhere.

Only
Sleipner West is currently the only place in the world where large volumes of carbon dioxide are injected for underground storage.

It represents a relatively expensive approach. Generally speaking, a coal- or gas-fired power station which converted to this disposal method would see its costs rise by 50-80 per cent.

However, the Sleipner West licensees would have had to pay NOK 1 million per day in Norwegian carbon dioxide tax had they released the greenhouse gas to the air.

Injecting the carbon dioxide costs about the same, and the solution is more environment-friendly.

Many researchers believe that carbon dioxide injection could also prove a preferred solution elsewhere. Formations similar to Utsira are found in many parts of the world.

"This approach isn't the first people are likely to turn to, but it will be one of the key solutions if emission reductions are really going to bite," says Mr Torp.

He believes the injection method will first be adopted when an international tax system makes it attractive for energy producers to remove carbon dioxide and harder for customers to buy cheaper, more polluting power.


Published: 18.12.2000

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