The watery fate of carbon dioxide
If we're going to bury carbon dioxide underground to reduce greenhouse gas levels in the atmosphere, then we need to know it won't shoot out again in a couple of generations time.
Experience from naturally formed underground reservoirs of CO2 shows it's quite possible to keep the gas trapped in bubbles underneath impermeable 'cap' rocks. A study in Nature [subscription] has now sampled a bewildering number of isotopes in nine reservoirs to fill in some more details on exactly where CO2 goes in these sedimentary systems after millions of years.
It turns out, say researchers from Edinburgh and Manchester, in the UK, that much of the CO2 ends up dissolved in water. Only a very small amount - at most, 18 per cent in some areas - gets transformed into minerals such as calcium carbonate.
That doesn't surprise any experts in the field - the limited amount of free calcium cations in these conditions would tend to limit mineralisation. If you read the Intergovernmental Panel for Climate Change's 2007 report on carbon capture, however, you might have thought the potential for mineralisation was somewhat greater.
Although it appears CO2 won't get tied up in minerals much, dissolving it in water is still a good way to trap the gas (which otherwise remains as a buoyant, supercritical fluid phase). In fact, some researchers are actively investigating mixing CO2 with water as it's injected, in order to encourage dissolution. The solution created like this is denser than water and should sink downwards.
The slight worry is that injecting a lot of gas into water creates acidic carbonic acid, which could etch out holes in the surrounding rock. And a surge of CO2 might push otherwise stable water into an unexpected flow.
What the paper reinforces is that we really need to know where water might flow in order to decide the long-term fate of carbon dioxide trapped underground. While geologists might be fully aware of this point, will the companies preparing to dump CO2 from their powerplants into the nearest hole also be aware? Only time will tell.
Richard Van Noorden
