Getting cagey about CO2

Petroleum engineers are developing technology designed to remove a potential obstacle to under-sea storage of carbon dioxide emissions.

Carbon capture and storage were high on the agenda in last week's Budget when Gordon Brown launched a consultation process to identify barriers to its widespread use. Prof Bahman Tohidi and his team at Heriot Watt University's Institute of Petroleum Engineering believe a natural physical process could solve the problem of CO2 leakage after it has been buried.

Instead of allowing CO2 emissions from power stations to escape to the atmosphere, it has been suggested they could be trapped and delivered by pipeline to a suitable underground storage site, such as an exhausted oil or gas field beneath the sea. However, if the cap rock holding the CO2 leaked, this would pose a problem: the escaping gas would make the sea more acidic, raising the threat of an environmental disaster.

Tohidi believes that, if the conditions were right, any leaking CO2 could form hydrates with the water, blocking the gaps between particles in the sediment and providing a secondary seal. 'Even if the cap rock breaks, this would prevent the CO2 escaping into the ocean,' he said. 'It would provide security, and reduce worries about changing the ecology of the ocean.'

Hydrates are a combination of water and gas molecules. The hydrogen bonds in water hold the molecules together in 'cages', with empty cavities between them that gas molecules fit into. The attractive forces between the gas and water molecules can stabilise the cage structure, causing ice to be formed at a higher temperature than usual.

If the molecules are quite large and round, such as in cyclopentane or tetrahydrofuran, then the ice forms at room temperature. Smaller molecules, as in CO2 and methane, need higher pressure for the hydrates to form at ambient temperatures. The bottom of the ocean provides perfect conditions for CO2 hydrate formation. At a typical seabed temperature of 4^oC, they form at a pressure of 20 atmospheres, equivalent to a depth of 200m. In the seabed the hydrates can fill the cavities between the sediment, reducing permeability and preventing escape.

'We have designed a cylindrical cell with two sections, where the lower part is warmer than the upper,' Tohidi explained. 'This is similar to the sediments on the seabed, because they get warmer as you go deeper — a rise of 3^oC for every 100m. We will inject CO2 at the base, and monitor it as it moves up the cell with a series of sensors that measure resistivity.

'This way we will be able to look at hydrates forming, and sample the fluid at the top to see if CO2 is coming out. At the moment we are relying on the cap rock to keep the CO2 in. We hope to show that if the water is sufficiently deep there will be a secondary seal.'

First results are expected within three months. They should also help to establish what conditions are required for sites to store CO2, and provide a method of monitoring for leakages.

A supplementary objective of the project is to look at methane hydrates. These exist in huge quantities underground — particularly in the coastal shelf — and some estimates put the figure at twice the amount of fossil fuels. Various projects are underway in, for example, Japan and the US, to investigate extracting gas from these reservoirs because of worries about the security of the fuel supply.

CO2 could be used as a safe way to extract the methane. As CO2 hydrates are more thermodynamically stable than methane hydrates it should, in theory, be possible to inject CO2 into the reservoir, where it displaces the methane in the hydrate structure.

'Several techniques have been proposed for removing the methane, but they all require it to dissociate into methane gas and water, thus replacing a solid with two fluids,' Tohidi said. 'As the methane hydrate is usually found in sediments, the sediment would be expected to subside. I'm not sure this is very safe — you might get a gas leak, or it could even destabilise the sea floor. If we can replace the methane with CO2, then it should be a safer way of producing methane.'

Another thought is that hydrates themselves could be used for CO2 storage. Although the ratio between water and CO2 in the hydrate is only around 6:1, in practice this means one volume of hydrate is formed from 176 volumes of gas. 'The methane hydrate deposits have been stable for thousands of years, so why can't the CO2 hydrates be similarly stable?' said Tohidi.

'If we could prove CO2 hydrate is stable, it could be used as a sink for CO2 storage in itself. The places that generate CO2 are not necessarily close to reservoirs where it could be stored, so the more places you can store it, the better.'