Ocean floor could store century of US carbon emissions

Alok Jha, The Guardian 14 Jul 08;

A century's worth of future carbon dioxide emissions from the US could be stored securely in a layer of undersea rocks within easy reach of the west coast of the continent, according to a new study.

The Juan de Fuca plate, which comprises the ocean floor a few hundred kilometres from the coasts of Washington and Oregon, contains layers of basalt that geologists think might be suitable for long-term sequestration of CO2 as part of a carbon capture and storage (CCS) system.

CCS is a set of technologies to trap, transport and bury the carbon dioxide from power stations and factories in underground locations such as abandoned oil fields or deep aquifers.

Estimates suggest the technique could prevent up to 90% of the greenhouse gas emissions from major emitters being locked away, so preventing global warming.

But "the effectiveness of these methods for CO2 sequestration depends strongly on the reservoir capacity, retention time, stability, and risk for leakage," notes David Goldberg of the Lamont-Doherty Earth Observatory at Columbia University in New York, in the latest edition of the journal Proceedings of the National Academy of Sciences.

Using data from previous geological surveys and drilling studies, Goldberg led a team that estimated there was an area of 78,000 km2 of suitable undersea aquifers on the Juan de Fuca plate.

The team said that the basaltic geology of the Juan de Fuca plate meant that two methods could be used to store CO2 in the long term: physical trapping and geochemical trapping. The first involves burying the gas under layers of rock that have low or zero permeability: this means the gas is physically blocked from bubbling back to the surface. The second method, also known as mineral trapping, involves CO2 reacting with the rocks into which it is injected to make stable, solid minerals such as carbonates.

Goldberg's criteria for suitable burial sites included the aquifers being under at least 2,700m of water and covered by 200m or more of sediment. At this deep level CO2 liquifies and is denser than sea water, so even if a leak were possible, it should not rise to the surface.

The region identified could potentially store around 208bn tonnes of liquefied CO2, the researchers said, a figure that could rise to 250bn tonnes depending on how much of the gas reacted with the rocks to form carbonates.

At the current annual emission rate of 1.7bn tonnes of carbon a year by the US, the researchers estimated that the Juan de Fuca plate would provide sufficient capacity for 122–147 years.

Andy Chadwick, team leader for CO2 storage research at the British Geological Survey, said that, though the basalt that makes up the Juan de Fuca plate was technically suitable for storing carbon dioxide, there were several unknowns that needed investigation before scientists could conclude it was a secure place for CO2 storage.

He said that, for example, Goldberg's team was relying on finding fractures in the basalt to physically pump in and store the CO2. "These fractures they're depending on for the storage capacity may also be connected to the surface or the seabed and would provide leakage pathways," said Chadwick.

It would be technically very difficult to analyse the basalt in sufficient detail to find all the fractures and ensure they were secure. For now, he said, depleted oil and gas fields were a better option for CCS systems. "They are the easiest targets because they are well-characterised and we know they are good geological fields.

"The only problems are they have lots of wells made through them and it's the man-made wells that would provide the greatest hazards of leakage. But they are certainly preferable at the moment to putting CO2 into basalt."

Goldberg himself notes that, even if further study confirms the Juan de Fuca plate as a suitable location for storing carbon, challenges remain in establishing the necessary infrastructure. Pipelines would be needed to transport the CO2, they would need to be maintained and monitoring systems would need to watch for leakages – all of which remains to be costed.