Planet Earth Online

North Sea perfect candidate for carbon capture and storage

15 January 2009

Politicians are committed to cutting levels of the greenhouse gas in the atmosphere to slow down climate change. Carbon capture and storage is accepted by scientists as a realistic way to cut atmospheric levels of the gas considerably in the short term.

But risks around storing millions of cubic metres of the gas in depleted oil or gas fields have met with some concern, not least because of the possibility of the gas seeping through the rocks and eventually back into the atmosphere over a short period of time, in effect cancelling out any benefits the technology would bring.

In a paper published this month in Geology, researchers from the University of Edinburgh and the Scottish Universities Environmental Research Centre describe how carbon dioxide trapped naturally beneath a mudrock seal in the North Sea has remained intact for at least 70 million years. The evidence indicates that carbon dioxide has moved upwards just 12 metres in all that time.

Highly impermeable

Mudrocks are highly impermeable, meaning that fluids or gases - such as oil or carbon dioxide - leak through them over very long timescales, in the region of tens of millions of years.

The Miller oilfield, which lies four kilometres beneath the bottom of the North Sea contains unusually high levels of natural carbon dioxide. Lying directly above the oilfield is the Kimmeridge Clay Formation - a classic mudrock seal. It has been exposed to the high levels of carbon dioxide for millions of years, so provides a natural parallel for carbon capture and storage in an underground reservoir.

Jiemin Lu from the University of Edinburgh, who led the study, compared samples from the Kimmeridge Clay Formation above the Miller oilfield with samples from an identical Kimmeridge Clay Formation in a low carbon dioxide area. He wanted to test samples from both areas for long-term movement of carbon dioxide.

By analysing the ratios of the stable isotopes of the carbonate minerals within the mudrock, Lu was able to identify the carbon dioxide source.

Safe as houses

'The carbon dioxide penetrated only 12 metres through mudrock over a period of 70 to 80 million years, which shows that this type of mudrock has a considerable safety margin,' said a member of the research team, Professor Stuart Haszeldine from the University of Edinburgh's School of Geosciences.

'As the Miller oilfield is made up of a well-known reservoir and a mudrock seal, it would be a perfect vessel for engineering the secure storage of 70 million tons of carbon dioxide - that's about 25 years of storage for a small coal-fired power station.
'There will be tens of depleted oilfields in the North Sea similar to this with many, many, hundreds of million tons of storage. The UK has one of the world's best carbon dioxide storage opportunities,' Haszeldine added.

Germany and Norway are already experimenting with carbon capture and storage - albeit on a relatively small scale. Denmark and Holland will soon follow. The UK is unlikely to see the technology in use until 2013 at the very earliest.

Ironically, this very same Miller oilfield was the first-choice site for a carbon capture and storage project proposed by BP in 2005. It would have been a world first and in operation in 2009, but was turned down by the UK government.

Levels of atmospheric carbon dioxide now stand at 387 parts per million. This is a rise of almost 40 per cent since the 280 parts per million of the industrial revolution and the highest for at least 650,000 years.

Keywords: Environmental change, Europe, Geology,

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