One way to combat global climate change is to directly capture carbon dioxide, the main greenhouse gas, as it is being emitted, and store it safely. But methods of carbon dioxide sequestration, notably, pumping the gas into underground geologic structures such as exhausted oil reservoirs, are not practical in many areas, and raise fears that the stored carbon dioxide will escape.
Now researchers at Harvard University and Columbia University have proposed a new method for trapping nearly limitless amounts of carbon dioxide – a technique they say will be secure, as well as a practical option for areas located far from underground reservoirs.
The researchers, in an article posted online this week in the Proceedings of the National Academy of Sciences, propose that carbon dioxide be pumped into the porous sediment a few hundred meters into the sea floor in deep parts of the ocean (greater than 3,000 meters deep), in what one of the researchers, Dan Schrag, professor of geochemistry at Harvard, calls “a fairly simple, permanent solution.”
The key was finding a “sweet spot,” where the pressure and temperature of the surrounding environment make carbon dioxide more dense than surrounding fluids, thereby trapping it in place. This situation occurs at the bottom of the ocean because of a combination of high pressure and low temperatures – a fact others have also noted in proposals to store carbon dioxide in deep parts of the ocean.
But such injections would kill ocean life, and, unless sequestered in deep trenches, the carbon dioxide could be carried by currents to shallow areas, where it could reenter the atmosphere.
The researchers’ insight was that injections into the sea floor could take advantage of the pressure and temperature of the ocean, while avoiding the negative side effects of earlier proposals. The carbon dioxide, in liquid form, would be brought to the sequestration site by ship or pipeline, and piped into the sea floor with equipment like that used by the oil industry for drilling deep-sea wells. Once beneath the sea floor, the carbon dioxide would interact with the surrounding fluids and produce hydrate ice crystals, which would plug the rock pores, serving as a secondary cap on the carbon dioxide. Over hundreds of years, the carbon dioxide would dissolve in the surrounding water, and then would only have the potential of leaking out by diffusion, a slow process that would take millions of years, the researchers say. Within the next five years they hope to run a large-scale field test of this new approach.