One of the common expressed worries about carbon capture is that injected CO2 will eventually leak from its resting spot, offsetting climate benefits or creating health risks. With offshore CO2, there is speculation about the effect of any potential leak on acidity, among other things.

But in a first-of-its-kind study, European and Japanese researchers deliberately injected CO2 offshore knowing it would leak, to see what would happen. They found that although there was small leakage into the water, it was confined to a minority of the injected CO2, and the effects on the surrounding ecosystem were negligible. Furthermore, the area quickly recovered and water chemistry returned to its original state within a month.

"If CO2 leaked from storage and reached the seafloor, then the environmental impact will be measurable, but very restricted in area and not catastrophic," said Jerry Blackford, a marine system modeler at Plymouth Marine Laboratory and author of the paper, published yesterday in Nature Climate Change. "No one has previously released CO2 into shallow sediments."

Although the scientists injected a small amount of CO2—4.2 metric tons over 37 days—Blackford said the leakage amount was "realistic" for what might be expected from an injection well in that period in the rare instance of a leak.

The team injected the CO2 about 40 feet below the seafloor in the Ardmucknish Bay off the coast of Scotland. "The site is not one targeted for carbon storage but has similar ecosystems, making results transferrable," Blackford said. The researchers detected CO2 bubbling very quickly from the site, presumably from existing microfractures in the sediment.

However, they never witnessed more than 15 percent of the injected CO2 actually escaping from the seafloor. The rest remained dissolved in the sediment, where it caused dissolution of seashells, as would be expected from the acidity. The CO2 that did escape as bubble plumes dissolved rapidly, although there was a detected change in water chemistry over a few tens of meters—a change that disappeared within about 21 days, with the help of strong currents.

Offshore scars that healed
After that time, the chemistry and biology recovered to "conditions similar to nearby unimpacted sites," Blackford said. The CO2 that remained in the sediment moved laterally before containment.

Offshore sequestration sites for carbon dioxide, while not major players with proposed capture projects on power plants yet, are considered a potential reservoir for the greenhouse gas because of their size and geology.

They could eliminate some of the common "not in my backyard" issues raised by opponents of CO2 storage, for instance, and reduce concerns about underground pressure of stored gas in some cases. Some analysts suggest the Gulf of Mexico could be a prime spot for CO2 storage in the United States, with a decline in cost of CO2 capture.

The issue of CO2 leakage with offshore sequestration came to the forefront last year, after a Nature article raised concerns about unexpected fractures above Statoil ASA's Sleipner natural gas project in the North Sea, where 1 million metric tons of CO2 has been injected annually below the seafloor since the mid-1990s. That article about "seabed scars" quoted European officials as saying it is likely that some CO2 would leak eventually.

After that, the European Commission's ECO2 research project—which has been examining the likelihood and impact of CO2 leakage—released a comment this year emphasizing that it had "not found any indication that CO2 has leaked from Sleipner in its 17 years of operation." It noted that the detected fracture was 15 miles north of the Sleipner injection site.

Stuart Haszeldine, a professor of carbon capture and storage at the University of Edinburgh who did not participate in the study, called the new work encouraging, as it indicates escaped CO2 is not likely to have a perceptible impact, even in a worst-case scenario of poor site selection and leaky wells.

"Consequently, the real-world risks resulting from CO2 storage deep beneath the bed of the North Sea are likely to be extremely small, and many thousand times less than the impacts of continuing to dump CO2 into the global atmosphere," he said. The tested area was shallow as well—a real storage site would inject CO2 more deeply and provide more protection, he said.

Onshore, the questions are different
Previous experiments relied on natural CO2 seeps close to volcanoes, which were not indicative of captured and stored CO2 because of contamination from hydrogen sulfide gas, he noted. "Experiments in laboratories, even with very large tanks, can never really replicate the real natural ecosystem complexity," he said.

Similarly, Blackford said that even if CO2 were to leak, it likely would be on a small scale, as chemical processes would act on the greenhouse gas in rocks and sediments before it reached the surface. Yet he emphasized that his study measures not the likelihood of a leak, but the impact of one.

If a leak were 1,000 times the size, then the impact would "scale accordingly," he said.

"We caution that impacts are likely to increase step-wise if a greater proportion of CO2 is emitted in the gaseous phase, either through fractures or as pore waters become super-saturated. ... Without operational evidence, realistic leakage scenarios can be only approximated," the study adds.

And in its rebuttal this year, ECO2 said its assessment of Sleipner was ongoing. "We don't know how likely or unlikely it is that leakage may occur in the future," ECO2 said.

Monitoring of stored CO2 offshore will be crucial and is not easy, according to the paper. Seabed "pockmarks," for example, are a good indicator of leakage, but they are often difficult to distinguish from some natural features. The researchers recommend monitoring via mobile autonomous underwater vehicles with sensors measuring water chemistry and acoustics to detect gas bubbles.

The paper also does not apply to onshore CO2 storage, where scientists have raised concerns about injected CO2 causing earthquakes (ClimateWire, Nov. 5, 2013). The chemistry and water dynamics are entirely different, Blackford said.

"I wouldn't want to extrapolate this study to land," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500