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Saline Aquifers Could Hold CO2 for a Century

Capillary trapping and solubility trapping simulated with glass beads
Capillary trapping and solubility trapping—which the researchers simulated with glass beads—will occur as the CO2 plumes migrate along the underside of the capstones that top the saline aquifers. Courtesy of Michael Szulczewski, Juanes Research Group, MIT 

A carbon capture and storage solution proposed by MIT researchers could hold 100 percent of the CO2 emissions produced by U.S. coal- and gas-fired power plants during the next century. 

April 3, 2012—A carbon capture and storage solution that would inject CO2 emissions into saline aquifers located deep underground could hold 100 percent of the emissions produced by coal- and gas-fired power plants in the United States during the next century, according to a study published last week in the Proceedings of the National Academy of Sciences.

More than 40 percent of the CO2 generated worldwide is produced by electrical power generation, according to the study. Of that, coal- and gas-fired plants have accounted for 97 percent of the total CO2 emissions produced by electricity-generating power plants in the United States since 2000.

The study identified 11 significant, deep, saline aquifers as potential locations to capture and store these plants’ emissions. Each of the aquifers is located between 1 and 3 km underground, is laterally continuous over long distances, and is geographically distant from existing large earthquake faults.

“A saline aquifer does not have economic value, per se,” wrote Ruben Juanes, Ph.D., an associate professor in energy studies in the civil and environmental engineering department at the Massachusetts Institute of Technology (MIT), in response to written questions submitted by Civil Engineering online.

“These aquifers have salinities that are much higher than seawater and therefore would never be used for drinking-water purposes,” Juanes said. Salinity also equates with depth, and at these depths the pressure will be high enough that the injected CO2 will be in a “supercritical state,” meaning it is technically neither a liquid nor a gas, although it “can be thought of as a liquid, with a density much higher than that at atmospheric conditions,” Juanes noted.

Once the CO2 is injected into the aquifers, it will be trapped in two ways, according to Michael L. Szulczewski, A.M.ASCE—a graduate student in the civil and environmental engineering department at MIT—who explained the process in a video released by the university. As the CO2 is injected into the aquifer, it will migrate along the underside of the capstone as it seeks the highest elevation. Juanes said that two mechanisms—capillary trapping and solubility trapping—will occur along the underside of the capstone in the wake of the CO2 plume as it migrates, capturing some of the CO2. Convective dissolution will also occur as the CO2 combines with the saline groundwater within the aquifer. This CO2 and saline mixture will be heavier than the saline groundwater alone, and thus sink to the bottom of the aquifer, where it will remain.

Although detailed studies must be undertaken before any site is selected and ground-level monitoring for any escaping CO2 will be crucial, the researchers anticipate that the saline aquifers will be able to trap all the CO2 before it is able to migrate to a fracture or rock fissure in the capstone, from which it could potentially escape. “The storage must be not only safe, but also effective,” Juanes said. “The CO2 must be sequestered underground for periods of thousands of years to ensure that it is an effective way to mitigate atmospheric CO2 emissions.”

The disparity in previous researchers’ estimates of the United States capacity for carbon capture and storage (CCS)—which ranged from a storage capacity of 2 years at current levels of emissions to 5,000 years—motivated the study. “We felt that this range was too large to inform whether CCS could be a viable climate-change mitigation technology,” Juanes said. “Much of the controversy came from the role that overpressures would play in limiting capacity.” As a result, the study examined both the overpressures that would be created on the 11 aquifers due to the injection of CO2, as well as how the CO2 would behave and migrate once it was injected into the aquifer.

The study also pointed out that the absence of a comprehensive policy to drive CSS adoption, guide its safety, and monitor regulations is the key barrier to its widespread use. “If CCS is to be implemented at large scales and be an effective mitigation technology, it would benefit from coherent, federal policy,” Juanes noted.

The study, The Lifetime of Carbon Capture and Storage as a Climate-Change Mitigation Technology, was funded in part by grants from the U.S. Department of Energy and the MIT Energy Initiative. It was written by a group of researchers working at MIT that included Szulczewski and Juanes as well as Christopher W. MacMinn, Ph.D., who recently completed his doctoral work in the mechanical engineering department, and Howard J. Herzog, a senior research engineer at the MIT Energy Initiative.



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