Cold storage could improve carbon capture

Current carbon capture techniques see CO2 extracted from major sources, such as the gas flue of a power station, before being piped or shipped to a suitable site where it is injected into deep, warm reservoir rocks and capped by low permeability seals such as shale or clay.

But researchers from the University of Leicester and the British Geographical Survey have suggested that storing the CO2 in a denser ‘frozen’ state would allow a greater volume of the gas to be stored once a site had been identified and reduce the chance of it escaping or travelling through porous rock.

PhD student Ameena Camps told edie that in conventional storage operations the CO2 is injected relatively deeply, at a depth of more than 800m.

“This depth is important because in many geological situations it marks the point where pressures and temperatures go beyond the point where CO2 becomes supercritical,” she said.

Supercritical CO2 is not a gas or a liquid, but shares characteristics of both, so that it can diffuse through solids as a gas might while retaining a liquid-like density.

“The main reason for wanting supercritical CO2 is that it is relatively dense compared to CO2 gas – and so much more of it can be stored in a given volume of rock,” said Ms Camps.

Initially the CO2 will be stored in its free state, but over time it will dissolve into formation pore waters, and over even longer time scales the dissolved CO2 will react with minerals within the rocks, storing the CO2 in an immobile form for geological timescales.

The research team, however, believes that rather than storing the CO2 in a supercritical state it would be more efficient to store the compound as a liquid pool under a layer of hydrated CO2, or simply to store the lot as a hydrate.

“Hydrates are ice-like crystalline minerals that look like normal ice and form when gas and water freeze together at low temperature and high pressure,” said Ms Camps.

“They are made of a cage of frozen water molecules with the gas molecules trapped inside.”

According to the researchers this approach offers many advantages over conventional storage methods, as it opens up a wealth of potential storage sites which would be inappropriate for deep-rock supercritical reservoirs.

“Within deep-water sediments or below permafrost regions, pressures may be equally high as in a deep aquifer storage system, but temperatures will be much lower,” said Ms Camps.

“Under these conditions the stable phase of CO2 will be a liquid, which is likely to have a higher density to that of supercritical CO2, therefore greater volumes of CO2 could be stored in similarly sized volumes of suitable rock formations.

“The increased density of liquid CO2 compared to supercritical CO2 would also reduce buoyancy forces driving upward migration, meaning a thinner caprock, or thinner zone of CO2 hydrate may be sufficient to contain the stored CO2.

“Cooler temperatures would also increase the viscosity of CO2, causing it to move slower, and therefore slowing possible migration.”

By employing geophysical techniques and computer modelling, Ms Camps has identified a number of sites in Western Europe with the potential to store carbon dioxide by this method.

She is also exploring further implications of her research that may benefit geologists’ understanding of the stability of deep submarine slopes and contribute to improvements in global water supplies through further understanding of desalination processes.

Sam Bond