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 were to leak, 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 says. ‘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 cyclopentane or tetrahydrofuran, then the ice forms at room temperature. Smaller molecules, like CO₂ and methane, need higher pressure for the hydrates to form at ambient temperatures.

The seabed provides perfect conditions for CO₂ hydrate formation. At a typical seabed temperature of 4°C, they form at a pressure of 20 atmospheres – equivalent to a depth of 200m. Within the seabed itself, the hydrates can fill the cavities between the sediment, reducing permeability and preventing escape into the ocean.

‘We have designed a specialist experimental set-up for this project,’ Tohidi explains. ‘We have a cylindrical cell with two sections, where the lower part is warmer than the upper. This is similar to the sediments on the seabed, because they get warmer as you go deeper – a rise of 3°C 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.’ He adds that he’s expecting the first results within three months. It should also help to establish what conditions are required when identifying sites for the storage of 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 under way in, for example, Japan and the US, to investigate the possibility of extracting gas from these reservoirs because of worries about the security of the fuel supply.

Carbon dioxide 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 says. ‘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 many thousands of years, so why can’t the CO₂ hydrates be similarly stable?’ Tohidi wonders. ‘If we could prove that CO₂ hydrate is stable, then 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.’