Sea sediment storage proposed for carbon dioxide

It may be possible to fight global warming by burying carbon dioxide in reservoirs hundreds of meters below the ocean floor.

The developers of the concept contend that the global deep sea storage capacity of CO₂ is virtually unlimited, and that the area within the 200-mile economic zone of the US coastline is capable of storing thousands of years of current US CO₂ emissions.

Proposals for storing CO₂, generated by fossil fuel burning, in near-permanent geologic reservoirs have been discussed for more than a decade. These include injecting liquid CO₂ deep into the sea, as well as into land-based reservoirs in oil or gas fields and saline aquifers. A third proposal has been to transform CO₂ into thermodynamically stable minerals or bicarbonate brines.

But Kurt Zenz House, a geoscientist at Harvard University, Cambridge, Massachusetts, US, believes that these ideas suffer from crucial drawbacks - including the potential for eventual leakage into the atmosphere, and a limited total storage space.

Instead, House and his colleagues propose injecting CO₂ into sea floor sediments at depths of at least 3,000 meters, where the cool temperature and high water pressure would transform CO₂ into a dense liquid. At such high pressures, liquefied CO₂ could eventually turn into solid crystals at temperatures of 8-10?C, House explained to Chemistry World.

The concept is described in the current issue of Proceedings of the National Academy of Sciences of the USA¹.

Dense solution

House and his team argue that over thousands of years, the CO₂ will dissolve in the water that fills pores in the sea floor. This should form a dense solution that sinks below the surrounding liquid. Once in place, it should take million of years for the carbon dioxide molecules to diffuse out of the sea bed, they claim.

House told Chemistry World that the proposal could be implemented with current technology, similar to that used in deep sea oil and gas drilling. ’It’s the exact same thing,’ he said. ’But instead of sucking gas or oil out of the ocean, you are injecting CO₂. The technology is there. The question is who will pay for it.’

The injection process might cost between US $40-50 per tonne, he estimated. Current global anthropogenic CO₂ emissions are about 25 billion tonnes a year, a figure that some see rising to nearly 50 billion tonnes by 2050. Of that total, about 30-40% would need to be stored in order to avoid dangerous levels global warming, House said. "But we have a big arsenal of technology we can tap into," he added, and deep sea sediment storage could prove to be an important weapon against global warming.

Ned Stafford