by Kim Luke
Carbon capture and storage (CCS) has the potential to reduce harmful fossil-fuel related emissions to the Earth's atmosphere by recapturing CO2 and storing it beneath the Earth's surface. The technique holds promise for reducing global warming but key questions remain about safe long-term storage of CO2, and specifically about the means by which CO2 is trapped underground.
An international team of scientists from the Universities of Manchester, Edinburgh and Toronto have uncovered a phenomenon in nature that could point the way to successful CCS. Their findings were published as the cover story in the prestigious international journal Nature on April 2.
"To solve the problem of carbon capture and storage, it is important to understand the possible storage mechanisms," said Barbara Sherwood Lollar, a specialist in carbon isotope and gas geochemistry in the Department of Geology at the University of Toronto. "One way is via CO2 reaction with the reservoir rock to form new minerals, a process termed mineral trapping. Another involves CO2 dissolving into water in the same way that CO2 is dissolved in sparkling water in a process known as solubility trapping."
Much of the research that is currently underway on CCS involves artificially pumping CO2 into the ground, but the Manchester-Toronto team took a different approach.
"Throughout the world, there are natural geologic "analogue" sites," explained Sherwood Lollar. "These are underground gas fields where CO2 originally generated by deep processes in the Earth was trapped and has remained in place over thousands or even millions of years."
By investigating the way in which CO2 was trapped in these ancient natural gas fields, the research team gained valuable insight into the underground processes that would need to be harnessed to successfully undertake carbon capture and storage. They identified strong correlations between two different types of gas tracers -- the noble gases (helium and neon) measured at Manchester, and carbon isotope variations in the CO2 gases determined at the U of T's Stable Isotope Laboratory.
"What we found was remarkable," said Sherwood Lollar. "At sites throughout the world, we found that the major way CO2 is stored is by dissolution into the underground water, rather than by mineral trapping."
"Water containing dissolved CO2 is more dense than water without CO2," explained Stuart Gilfillan, lead author of the study. Gilfillan was a PhD student at the University of Manchester when the work was completed and is now with the Scottish Centre for Carbon Storage at the University of Edinburgh. "The CO2-rich water sinks to the bottom of the reservoir where it is more likely to be securely stored."
The insight was only possible because of the collaboration between the two universities.
"Manchester and Toronto are international leaders in different aspects of gas tracing. By combining our expertise we have been able to invent a new way of looking at old gas fields," said Chris Ballentine, a University of Manchester professor and primary director on the project. "This new approach will also be essential for monitoring and tracing where carbon dioxide goes when we inject it underground. It will be critical for future safety verification."
Research was funded by the Natural Environment Research Council in the United Kingdom and the Natural Sciences and Engineering Research Council Discovery Program in Canada.