by Nicole Branan Thursday, January 5, 2012
Burying carbon dioxide in underground geologic formations is an attractive option for dealing with increasing carbon dioxide in the atmosphere. But before employing such schemes, researchers need to be sure that the greenhouse gas will actually stay put. Scientists have done everything from computer modeling to pumping vast amounts of carbon dioxide into the subsurface to find out if — and how — the gas might be trapped, but gauging how sealed the formations are over geologic time scales is difficult. Now a team of researchers has tried to trace the fate of carbon dioxide in naturally occurring carbon dioxide fields. They found that rather than being stored in the rock itself, the gas mainly dissolved in the formation’s water, a trapping mechanism that should keep the gas securely underground, the researchers say.
Stuart Gilfillan, a geochemist at the University of Edinburgh in Scotland, and his colleagues looked at gas samples from nine carbon dioxide fields in North America, China and Europe. These enormous underground formations are naturally filled with hundreds of trillions of cubic feet of carbon dioxide along with trace amounts of other gases. Through isotope analyses, the team determined that the carbon dioxide slowly migrated into the formations from deep in Earth’s mantle, which means that the fields provide a preview of what the underground formations that scientists are considering as possible carbon dioxide storage sites will look like millions of years from now. Today, oil producers remove carbon dioxide from these natural underground deposits and use the gas for enhanced oil recovery, operations that involve pumping carbon dioxide into depleted oilfields to coax the remaining oil out of the ground. To get their samples, Gilfillan and his colleagues hooked up their flasks to the multitude of oil producers' wells that are scattered across these fields.
Carbon dioxide trapped in underground formations can take one of three paths: It can remain in the pore spaces between the rocks and sand as a gas — the gas phase; it can dissolve in the groundwater that moves through the formation; or it can get incorporated into rock. Through an analysis of the ratios of carbon dioxide and helium-3 concentrations, the team estimated the amount of carbon dioxide that originally occupied the formations as a gas. When they analyzed samples from different areas of each gas field, they found that large amounts of it had disappeared from the gas phase in various areas of the fields.
This could have happened by two different mechanisms, Gilfillan says: dissolution in formation water or precipitation as carbonate minerals. An analysis of the carbon-13 to carbon-12 isotopes and noble gas isotope ratios revealed that most of the gas actually dissolved in the formation water rather than precipitating as carbonate minerals (thus being stored in the rock), the team reported in Nature. When carbon dioxide is tied up in minerals, “the carbon-13 gets more readily incorporated because it is heavier,” Gilfillan says. So when carbon dioxide reacts to become carbonate minerals, it leaves a larger portion of its lighter (carbon-12) component bouncing around as gas. But the team didn’t see this characteristic fractionation, which hints that the gas went mainly into water. Noble gas isotope ratios pointed to the same mechanism, he says.
The study is “an excellent start” to figuring out how and how long carbon dioxide could stay in the subsurface, says Yousif Kharaka of the U.S. Geological Survey in Menlo Park, Calif. “The method is very good and the conclusions are reasonable.” What the team found is also consistent with what one would expect based on modeling studies, he adds. However, he cautions that a more detailed look at the chemistry involved in the different fields might be warranted. For example, the pH of the formation water influences the isotopic fractionation of carbon dioxide. In certain pH ranges, Kharaka says, “you can’t really distinguish mineral precipitation from solution [by looking at isotope ratios] anymore.” That’s why it’s important to know what the pH in the water is.
Gilfillan and his team only theoretically estimated the pH rather than measuring it, Kharaka says. In addition, the chemical composition of the rock inside the formations can influence the chemistry and pH of the water. “I would like to see somebody do a much more detailed study, looking at these isotopes the way [Gilfillan’s team] did, but then also going after the water and the mineral composition at the same time.”
Even though trapping the carbon dioxide in carbonate minerals would be preferable — the “golden ticket,” Gilfillan says — storing the carbon dioxide in water should also work well for keeping the gas underground, he says. “Whenever you dissolve carbon dioxide in water, it becomes more dense than the water surrounding it, so it sinks to the bottom of the reservoir, which means that we store it pretty securely.” In addition, dissolving carbon dioxide allows us to store larger amounts, he says, because mineral trapping fills up a lot of pore space, limiting the amount of carbon dioxide that can be sequestered.
But the findings also mean that “you need to take into account what actually happens to the water in your potential storage sites, because it could migrate out of them,” Gilfillan says. “So, the key is making sure that you know where the water is going and that the hydrology of the storage site is well-known.”
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