There is increasing concern that storing carbon dioxide in underground rock formations could cause earthquakes that would allow it to escape, raising doubt that the strategy could play its expected role in slowing the harmful accumulation of the greenhouse gas in the atmosphere. But new research suggests that storing carbon in one particular type of underground rock could significantly reduce the risk.
A study published this month in Geophysical Research Letters suggests that storing carbon dioxide underground in a type of volcanic rock called reactive mafic rock could potentially present little seismic risk, because the surface of mafic rock reacts with carbon dioxide to form a solid mineral.
In the case of “mineral sequestration,” says David Bercovici, a professor of geology and geophysics at Yale University and an author of the new paper, “you are making rocks out of (the carbon dioxide). It’s not sitting there as a high-pressure fluid,” as would be the case with the storage rocks that are conventionally proposed. The newly formed minerals could help counter the conditions that lead to earthquakes.
Carbon capture and storage, or CCS, consists of techniques for capturing carbon dioxide emitted by fossil-fuel power plants and certain industrial facilities before it reaches the atmosphere, compressing it, and then burying it deep underground, where in theory it could be sequestered permanently in large geological formations. The International Energy Agency has said that CCS will be needed to achieve over one-fifth of the emissions cuts required by 2050 to maintain a decent chance that the global average temperature won’t rise more than 2 °C (see “The Carbon Capture Conundrum”).
The most commonly proposed storage receptacles are large porous rocks that lie deep below the earth’s surface. Candidates include depleted oil and gas fields, unmineable coal seams, and large formations called deep saline aquifers, so called because their pores contain brine.
However, a National Research Council report released last year said that while the earthquake risks are difficult to assess, “large-scale CCS may have the potential for causing significant induced seismicity.” Two Stanford University researchers went a step further, arguing in PNAS that the risk of induced earthquakes, even small ones, makes CCS a “likely unsuccessful” strategy for significantly reducing emissions (see “Researchers Say Earthquakes Would Let Stored CO2 Escape”).
Bercovici and his coauthor, Viktoriya Yarushina, a postdoctoral researcher at Yale, developed a relatively simple mathematical model that describes certain geophysical consequences of pumping carbon dioxide into underground porous mafic rock at different rates. The model, which the researchers designed to “cover a range of unknowns that we don’t have a handle on yet,” shows that “if you can balance it right so that you are causing the reactions to kind of keep pace with the rate that you’re pumping, then you could probably avoid an earthquake for a long time,” says Bercovici.
Mafic rock is the most common rock by volume on the planet, in large part because one type of it, basalt, makes up the seafloor. In fact, some researchers are investigating the possibility of storing carbon beneath the ocean. There are also a fair number of large underground mafic formations around the world which could be practical for CCS, though they are not as widely dispersed and are thus generally less accessible to power plants than are the reservoirs traditionally proposed.
But investigation into storage in mafic rock is at an early stage, and there are many unknowns regarding how it might work in practice. Much of the research now is aimed at better understanding the mineralization reactions that can occur naturally in the various types of mafic rock. “Any sort of engineering-level investigation of that process is in the future, and it’s absolutely crucial,” says Peter Kelemen, a professor of geochemistry at Columbia University.