Carbon dioxide to sink in the drink
19 Jun 2001
Researchers in Norway have proposed a large-scale demonstration project, in which carbon dioxide would be pumped directly from offshore oil and gas fields into the Norwegian Sea.
The project would test the conclusions of a theoretical study, using computer models, that suggests the Norwegian Sea, through transport to the Atlantic Ocean, would provide safe, long term storage for carbon dioxide.
Researchers Dr Helge Drange, Dr Guttorm Alendal and Professor Ola M.Johannessen at the Nansen Environmental and Remote SensingCentre in Bergen note that the oceans already absorb carbon dioxide from the atmosphere, but the process of mixing the gas at deep levels can take up to 1,000 years.
Purposeful storage could, they said, be viewed as an acceleration of a natural process.
The Norwegian Sea is a deep basin off Norway's northwestern coast, beyond Haltenbanken, a region on the continental shelf where oil and gas fields produce carbon dioxide as a by-product.
The modelling study assumed the annual carbon dioxide emissions from various size gas power plants over a ten-year period. Drange and his colleagues considered the effect of releasing carbon dioxide, collected at the source, at depths ranging from 350 to 950 meters.
They concluded that if the initial size of the carbon dioxide particles is four millimetres or less, the plume would rise no more than 100 meters from the point it enters the ocean.
Once the injected carbon dioxide has dissolved in the seawater, it tends to sink lower and eventually transport to the Atlantic Ocean through passages between Iceland and Scotland.
Its acidity, higher than that of the ambient seawater, could affect deep sea organisms, which are used to a relatively constant chemical environment. They said the level of acidity can be reduced by not pumping all of the carbon dioxide to one point, but using rather an array of ports located 5-10 meters apart in the cross-stream of the prevailing current.
The model predicted how much carbon dioxide would rapidly reach the surface and enter the atmosphere, based on the depth at which it was originally released.
The researchers say that 600 meters is the minimal safe depth, and 800 meters still safer. At the depth of 950 meters, virtually no 'outgassing' occurs, and the carbon dioxide-enriched water stays well below the level at which it might mix with upper ocean water.
Following normal flows from the Norwegian Sea, this water enters the northern Atlantic Ocean as bottom water and remains isolated from the atmosphere for centuries.
Drange and colleagues emphasise that their theoretical conclusions must be tested in real world conditions, including the cumulative effects of instituting many such sequestration projects, rather than just one.
