Carbon dioxide stored underground in nature eventually ends up mainly in fizzy water, not rocks — and that could have implications for artificial carbon capture and storage projects.
A new study by British, Canadian and U.S. researchers, published in Thursday's edition of Nature, sheds some light on the conditions that allow carbon dioxide to be safely stored underground for thousands or millions of years, as well conditions under which it might leak back out into the atmosphere.
'CO2 has been stored in these fields, in some cases, for millions of years.'— Barbara Sherwood Lollar, University of Toronto geochemist
Carbon capture and storage projects have been touted as a possible way to reduce the emission of carbon dioxide into the atmosphere and reduce climate change, but the research is still in the very early stages.
"We haven't figured out as a planet yet how to do this," said Barbara Sherwood Lollar, a University of Toronto researcher who was one of the co-authors of the study. (CBC Radio's Quirks and Quarks will have an interview with Sherwood Lollar on April 4 ).
"But to do it, we have to understand how it works.… If you do put CO2 back into the Earth, what's going to happen to it and how long is it going to stay down there? How stable and safe is this as a solution?"
Sparkling water or limestone?
To gain some understanding, researchers from the U.K., Canada, the U.S. looked at the way nature stores carbon dioxide that gets pushed up from deep inside the Earth and trapped with natural gas in underground reservoirs. It has been suggested that such oil and gas fields can be used for carbon storage once the useful gas and oil have been pumped out.
In the past, researchers have suggested that the carbon dioxide could end up dissolved in water — turning it into sparkling water or club soda — or incorporated into minerals such as carbonates and limestone, but did not know which would be dominant and under what conditions.
University of Manchester doctoral student Stuart Gilfillan, who led the study with his supervisor Chris Ballantaine, decided to examine nine natural gas fields in North America, China and Europe. He and his colleagues discovered that carbon dioxide was largely dissolving into the water within a narrow, slightly acidic pH range, and less than one-fifth of the carbon dioxide was being incorporated into solid minerals.
'There are many places where CO2 gases bubble up through water naturally. We call them spas.'— Barbara Sherwood Lollar
This could be interpreted as bad news by some people, suggested a commentary by Heidelberg University researcher Werner Aeschbach-Hertig published in the same issue of Nature.
"Clearly, mineral trapping is the preferable pathway, as it promises to store the carbon over geological time scales," he said.
Sherwood Lollar acknowledged that carbon dioxide dissolved in water and bottled up underground can come up to the surface in different ways, such as through natural gas wells and other holes drilled by humans as well as through natural faults.
"So there are many places where CO2 gases bubble up through water naturally. We call them spas," she said with a laugh. She later clarified that such natural discharges are on a small scale and the implications for leakage from engineered carbon storage sites would require further study.
Safe carbon storage may still be possible: commentary
In addition, she highlighted some of the findings from the recent Nature paper.
"CO2 has been stored in these fields, in some cases, for millions of years," she said. "So obviously, the very fact that it's dissolved in water doesn't necessarily mean it's any less effective a storage mechanism than mineral precipitation."
Some of the water trapped deep underground in tiny pores in the rocks can remain there without moving for thousands of years, Sherwood Lollar added. In addition, dissolved carbon dioxide makes water denser and more likely to sink.
Aeschbach-Hertig had a similar opinion.
"Although dissolution in groundwater implies the possibility of CO2 transport and eventual leakage to the atmosphere … as is thought to occur in natural gas fields, this result does not mean that safe geological storage is impossible," he wrote.
Geology of storage sites needs study
However, he said the study highlights the importance of looking carefully at the geology of potential carbon storage sites and the way water is distributed underground in the area.
Sherwood Lollar's research specializes in the geochemistry of carbon isotopes, and her expertise was useful in the Nature study because isotope analysis was used to figure out that carbon dioxide had dissolved in the water.
Isotopes are forms of the same element that have different masses. For example, carbon-13 is heavier than carbon-12. Either may be found in carbon dioxide gas.
Helium, a gas found naturally underground that doesn't dissolve well in water, also helped researchers deduce whether the carbon dioxide had dissolved in water.
Within each reservoir, the ratio of carbon dioxide to helium-3 was expected to have started off uniformly the same, but to decrease in a certain area of the reservoir if carbon dioxide were either incorporated into the rocks or dissolved into the water in that area.
If carbon minerals were formed from carbon dioxide, more carbon-13 would incorporate into the rock than carbon-12 because it is heavier. That does not happen to the same extent when carbon dioxide is dissolved in the water.
By looking at the ratio of carbon dioxide to helium-3 at different sites within each reservoir, the researchers figured out that some of the carbon dioxide gas had been removed in certain areas. By looking at the ratio of carbon-13 to carbon-12, they were able to determine whether the carbon dioxide was dissolved in water or incorporated into the rocks.