[Left: Artist’s illustration of carbon dioxide being emitted into the atmosphere and injected into the ocean. Storing carbon dioxide in the ocean could reduce the amount in the atmosphere. Image credit: Tor Sponga, Bergens Tidende]
The slow-mixing abyss of the North Atlantic may be the future storehouse for greenhouse gases, say scientists from Bergen, Norway. They report in the July issue of Geophysical Research Letters that carbon dioxide emitted as a byproduct of gas power plants in Norway could be released hundreds of meters down in the Norwegian Sea. And, the authors say, it could cost less than the tax Norway charges its power plants ($32 per metric ton of emitted carbon dioxide). “Norway is a place where environmental concerns are taken quite seriously,” says Franklin Orr Jr., Dean of Earth Sciences at Stanford University. “The carbon tax is an indication that they are willing to take steps to limit CO2 emissions.”
Since 1996, the Norwegian company Statoil has been pumping carbon dioxide, emitted from the Sleipner West gas reservoir in the North Sea, into a sandstone aquifer 800 meters below the seafloor. Aquifers like this one can hold carbon dioxide for thousands of years. But with the gas already being successfully pumped into seafloor sediments, why investigate releasing it into deep water? Helge Drange, the paper’s primary author, explains that sequestering carbon dioxide in deep water could cheaply dispose of the gas when aquifers are too deep or far away.
[Left: Artist’s illustration of carbon dioxide
being emitted into the atmosphere and injected into the ocean. Storing
carbon dioxide in the ocean could reduce the amount in the atmosphere.
Image credit: Tor Sponga, Bergens Tidende]
Drange, Guttorm Alendal and Ola M. Johannessen, all of the Nansen Environmental and Remote Sensing Center in Bergen, used a numerical modeling system to calculate the ideal conditions for releasing carbon dioxide into deep water. It includes an ocean circulation model, plume model, a 3-D large eddy simulation and a 3-D Eulerian advection-diffusion model. The authors predicted how currents and water chemistry in Haltenbanken, a region in the Norwegian Sea, would affect the release of pressurized, liquid carbon dioxide pumped from nearby, offshore gas fields.
As carbon dioxide bubbles into high-pressure and low-temperature seawater, a hydrous, crystalline solid called hydrate forms. Thin skins of hydrate can encapsulate carbon dioxide bubbles as they rise through the water, preventing the bubble from dissolving. The authors’ modeling system predicts that if small carbon dioxide droplets are released at 950 meters, 99.6 percent of the gas will remain in ocean water after 70 years. However, if bubbles are larger, at least one centimeter across, they can rise more than 200 meters in the water, Orr says. At 600 meters, the authors calculate that half the injected carbon dioxide will outgas after 70 years.
At depths of 400 to 500 meters, the bubbles become gas and “will rise immediately to the surface and outgas.” But, says Alendal, “we can make hydrate work with us instead of against us.”
He explains that pure hydrate particles sink to the ocean floor and slowly dissolve, prolonging the gas’s residence time. But pure hydrate forms only when carbon dioxide is released as very small droplets, and neither this technique nor the kinetics of hydrate dissolution are well understood.
Injecting carbon dioxide into deep water also may harm marine
organisms, because the dissolving gas acidifies seawater. Sessile, benthic
fauna are most sensitive to pH changes and are therefore at the greatest
risk. The authors say further study must assess the potential dangers
to marine life, which, Alendal says, “may be the biggest drawback of the
whole procedure.”
To dilute injected carbon dioxide, Drange and his colleagues
recommend releasing the liquefied gas from numerous, dispersed plumes.
To maximize the gas’s residence time they suggest bubbling it into the
Norwegian Sea at intermediate depth, where it will flow into the Atlantic
Ocean, becoming bottom water. Still, “we foresee a 10 to 15 year time frame
before the option can be used operationally,” Alendal says.
Later this summer, the United States plans to take part in this area of carbon dioxide sequestration research near Keahole Point, Hawaii. Forty to 60 metric tons of liquid carbon dioxide will be released at different flow rates for 40 hours, 800 meters down. The United States, Norway, Canada, Japan and Australia are sponsoring this field experiment and the Pacific International Center for High Technology Research is working with scientists to conduct it.
Seawater pH and hydrate formation will be monitored while a submersible digital camera captures footage of benthic fauna and bubbling carbon dioxide. Scientists will compare their observations with the models Drange and his colleagues developed to assess the plausibility of deep-water carbon dioxide sequestration.
Jann Vendetti
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |