Norwegian Sea Proposed As Storage Site For Carbon Dioxide

Drs. Helge Drange and Guttorm Alendal and Prof. Ola M. Johannessen at the Nansen Environmental and Remote Sensing Center in Bergen will publish their study in the 1 July issue of Geophysical Research Letters, published by the American Geophysical Union. They 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 say, be viewed as an acceleration of a natural process. This option would be successful only if certain environmental and economic considerations can be satisfied, they note.

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 modeling study assumes 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 various depths from 350 to 950 meters [1,150-3,120 feet]. They conclude that if the initial size of the carbon dioxide particles is four millimeters [0.2 inches] or less, the plume would rise no more than 100 meters [330 feet] 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. This is an area the researchers say needs further study. They say 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 [16-33 feet] apart in the cross-stream of the prevailing current.

The model predicts 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 [2,000 feet] is the minimal safe depth, and 800 meters [2,600 feet] still safer. At the depth of 950 meters [3,100 feet], 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 will enter the northern Atlantic Ocean as bottom water and remain isolated from the atmosphere for centuries.

Aside from the question of possible effects on deep ocean organisms, the process of sequestering carbon dioxide in the Norwegian Sea would have to be economically viable, the researchers say. They find that the technology is presently available, and the cost of implementing the project might actually be lower than the tax the Norwegian government now imposes on emissions of carbon dioxide from offshore oil and gas fields.

Drange and colleagues emphasize 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. Among the issues to be addressed are the impact on marine organisms and the independent effect of increasing acidification of ocean surface waters, due to higher atmospheric carbon dioxide levels.

The study was funded by Saga Petroleum AS, the Norwegian Research Council, the Nordic Council of Ministers, and the EC Environmental and Climate Programme.