Enough capacity exists in deep saline aquifers in the U.S. to store about a century’s worth of carbon dioxide emissions from the nation’s coal-fired power plants, a new study from researchers at the Massachusetts Institute of Technology (MIT) shows.
According to the study, previous estimates of the carbon capture and storage capacity in deep saline aquifers more than half a mile below the surface ranged from enough to store just a few years’ worth to many thousands of years’ worth.
The disparity in these estimates is because deep saline aquifers have no commercial value. There has been little exploration to determine their extent, MIT says. Second, the fluid dynamics of how concentrated, liquefied carbon dioxide would spread through such formations is complex and difficult to model. Most analyses have simply estimated the overall volume of the formations, without considering the dynamics of how the carbon dioxide would infiltrate them, the study suggests.
The MIT team modeled how the carbon dioxide would percolate through the rock, accounting not only for the ultimate capacity of the formations but the rate of injection that could be sustained over time. “The key is capturing the essential physics of the problem,” study co-author Michael Szulczewski says, “but simplifying it enough so it could be applied to the entire country.” That meant looking at the details of trapping mechanisms in the porous rock at a scale of microns, then applying that understanding to formations that span hundreds of miles. The researchers started with the full complicated set of equations for the fluid flow, and then simplified it.
Howard Herzog, a senior research engineer with the MIT Energy Initiative and a co-author of the paper, says this study “demonstrates that the rate of injection of CO2 into a reservoir is a critical parameter in making storage estimates.”
When liquefied carbon dioxide is dissolved in salty water, the resulting fluid is denser than either of the constituents, so it naturally sinks. It’s a slow process, but “once the carbon dioxide is dissolved, you’ve won the game,” co-author Ruben Juanes says, because the dense, heavy mixture would almost certainly never escape back to the atmosphere.
The MIT team’s analysis was led by Juanes, the ARCO Associate Professor in Energy Studies in the Department of Civil and Environmental Engineering, and was part of the doctoral thesis work of graduate students Christopher MacMinn PhD ’12 and Szulczewski. It is published this week in the Proceedings of the National Academy of Sciences.
The research was supported by grants from the U.S. Department of Energy, the MIT Energy Initiative, the Reed Research Fund, the Martin Family Society of Fellows for Sustainability and the ARCO Chair in Energy Studies.