Page last updated at 22:11 GMT, Wednesday, 3 September 2008 23:11 UK
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The final stage of the carbon capture and storage (CCS) chain involves storing the CO2 deep underground in locations where it will remain locked away for thousands of years.

Graphic explaining carbon capture graphic (BBC)

In order to transport carbon dioxide (CO2) most efficiently, it needs to be liquefied. This is achieved by pressurising the gas.

"If you increase the pressure to about 70 atmospheres, the CO2 becomes a dense liquid - about the same density as crude oil," explained Stuart Haszeldine from the University of Edinburgh's School of GeoSciences.

"This means that you can transport much more carbon dioxide in a small space."

In this state, the gas is then ready to be moved, either by tanker or pipeline, to the location where it will be buried deep underground.

Some people are trying to solve all storage problems all at once before taking the first step of the journey
Professor Stuart Haszeldine,
University of Edinburgh

A storage site 800m or more underground will ensure that the pressure will be great enough to keep the CO2 in its liquid state.

When the CO2 is pumped into the subterranean storage site, it spreads out into the surrounding porous rock (eg limestone).

"Although they may appear solid, most of these rocks are sedimentary and made of sand or carbonate pieces," explained Professor Haszeldine.

"About a third of the rock is space made up of microscopic pores, and this is where the CO2 seeps into."

Because the CO2 is buoyant, the storage area needs to have a geological "cap" to prevent the molecules escaping.

This means the location requires an impermeable layer, such as mud or clay, above the area where the CO2 is being pumped.

There are three main geological options that are being assessed for their suitability:

Oil and gas fields: Considered to be "secure vessels" because the sites have safely stored oil or gas for millions of years. A natural porous reservoir with an impermeable seal above it.

Saline aquifers: The injected CO2 physically pushes the salt water out of the way before dissolving into the water over time, forming fizzy salty water.

Unmineable coal seams: Works in a slightly different way to the previous two options. The liquid CO2 chemically "sticks" on to the small fractures and pores in the coal. This option is a less attractive option at the moment, because it is still at an experimental stage.

Current estimates show that we could store up to 40 years of global CO2 emissions in the world's known oil and gas fields.

Saline aquifers are 100 times bigger than oil and gas fields, which could mean that there is potential to lock away carbon dioxide for at least several centuries.

One problem is that while some nations are well-endowed with oil fields or saline aquifers, such as the UK, others have few.

India and Japan, for example, have very little in the way or oil or gas fields.

This means that they will have difficulty finding sites in which they can lock away their liquid CO2.

"These nations may have to export their CO2 to other storage sites around the world," Professor Haszeldine suggested.

He added that another hurdle to overcome was the use of saline aquifers to store CO2 on a large industrial scale. This still needed further investigation, he said.

"There are possibilities that the increased pressure to push in the CO2 could cause complex interferences between different boreholes used to inject the CO2.

"It is possible to overcome this, but this has not been physically demonstrated yet."

Lessons to come

Professor Haszeldine highlighted another potential problem: "When you are storing CO2 under a landmass with a large human population, we will have to be extra vigilant.

"We need to make sure that when the CO2 is pushed in, we know where the salty water that is pushed out of the way ends up.

"If the salty water is pushed upwards within the layering within the sedimentary basin, it could enter freshwater aquifers used for agriculture or drinking water.

"Although it is not very likely, it does mean that there is an extra factor that needs to be demonstrated."

However, he said that this should not be used as a reason not to push ahead with carbon capture and storage technologies.

"Some people are trying to solve all storage problems all at once before taking the first step of the journey.

"We don't need to answer all of those questions right now; we can get on and use the best sites now and progressively learn more as we go along."

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