The North Sea bubble

Fossil fuels are likely to provide most of our power for the foreseeable future, but that does not mean that all the CO₂ produced has to go into the atmosphere. Norwegian company Statoil is pioneering attempts to store the gas elsewhere — deep below the seafloor. As Statoil's underground gas bubble grows, so do hopes that the technique could deal with more of the troublesome greenhouse gas.

The CO₂ is an impurity in the natural gas that Statoil extracts from the North Sea Sleipner gas fields. It is normally separated and released into the atmosphere, but since 1996 the company has been pumping the gas into a layer of sandstone around 1 kilometre below the seabed. Known as the Utsira formation, the layer traps the gas in a gigantic bubble which now contains some four million tonnes of CO₂.

Once separated from the natural gas, the CO₂ is compressed before being pumped into the reservoir, where the high pressure keeps it in a dense 'supercritical' state — a hybrid of gas and liquid phases. This limits the diffusion of CO₂ through the sandstone. A layer of shale, which is impermeable to the CO₂, sits on top of the sandstone, effectively sealing the reservoir.

Andy Chadwick of the British Geological Survey is part of a team of European scientists responsible for monitoring the project. The team's 1999 seismic survey showed that the CO₂ is trapped within the reservoir and will probably stay there indefinitely. Chadwick is impressed with the project so far, but warns that the CO₂ may start to compress, rather than dissipate through the reservoir, making it progressively harder and more expensive to inject more gas.

But the Sleipner project's success does not necessarily imply that similar schemes could be applied to CO₂ from power plants. At Sleipner, the CO₂ would have been extracted from natural gas anyway, regardless of any storage scheme. Separating the CO₂ produced by power plants would require new investment, increasing the price of electricity. Additional infrastructure would also be needed to transport the gas to the reservoir. But the potential for storage is huge: just 1% of the reservoir Statoil is using, says Chadwick, could hold three years' emissions from all of Europe's power stations.

For Statoil, the Sleipner project has actually saved the company money. Norway taxes offshore carbon emissions to the tune of $38 per tonne. Before January 2000, the price was $50 per tonne. By storing one million tonnes of CO₂ undersea every year and avoiding this tax, Statoil recouped its $80 million investment within two years. The company plans to run the project for 20 years, storing around one million tonnes of CO₂ every year — equivalent to 3% of Norway's current total annual emissions.