What is the risk that CO2 stored underground after carbon capture will escape again?

Oil and gas have stayed trapped underground for millions of years, and so can carbon—if we carefully choose the right sites to store it.

Updated February 23, 2024

Setting up a large-scale “carbon capture and storage” system to stow our planet-warming carbon dioxide (CO₂) emissions safely underground is a major challenge of engineering, policy and economics. But keeping the CO₂ underground once it’s injected is relatively simple—you just need to inject it carefully and put it in the right place, says Bradford Hager, Associate Director at MIT’s Earth Resources Laboratory. Storing carbon will require detailed geologic tests before sites can get up and running, and careful monitoring once it’s stored, but scientists can borrow “fairly standard techniques” to conduct those tests, Hager says.

CO₂ can be injected into a few different types of underground formations, including saline aquifers, which are deposits of briny water, or past oil and gas formations. Geologists look for a particular sequence of different rock types to hold CO₂ in place. The lower rock layer needs to be porous, like sandstone or limestone, allowing injection and capture of CO₂ in the rock’s “pore spaces.” Above, there must be a non-porous “cap rock” that will seal in the carbon dioxide and keep it from escaping. CO₂ also needs to be injected more than 3,000 feet below the surface. This ensures the CO₂ will stay at the high temperatures and pressures needed to keep it in a fluid form, which makes it very dense and causes it to take up less space underground. It also places it deeper than deposits of groundwater used for drinking.

Much of the information we have about where carbon storage may work comes from oil and gas exploration. To consider possible sites for extraction, oil and gas companies run seismic tests, characterize the properties of the rock, and drill exploratory wells to learn more about what lies beneath the earth’s crust.

Site characterization for carbon storage involves similar techniques, like sending seismic waves into the earth, which then bounce off of underground formations. That creates what Hager calls a “seismic image” that can be verified with drilled rock samples, which provide information about how well the rock can hold liquid and how easily liquid flows through it. We think of all rocks as being completely solid, but some are more malleable than others, and these are best for storing CO₂. Hard and brittle rocks make it harder to inject CO₂ because they’re more apt to crack—potentially releasing CO₂—if injection creates high pressures underground.

Oil and gas drilling has also provided useful experience with injecting liquids into the earth’s subsurface, Hager says. Along with hydrocarbons, oil and gas extraction produces water, which companies have historically pumped back into the ground. In some areas, that’s provided information about rock permeability and how easily fluids can flow through them. “We know a lot about conditions under which fluids are trapped stably underground,” says Hager. Storing carbon dioxide in a “supercritical” liquid state is fairly similar.

When considering storage locations, scientists must also consider the pressure that carbon injection may build up underground, and whether any active faults are nearby. In some areas, like Oklahoma, injection of oil wastewater close to the rock “basement” has occasionally spurred earthquakes,¹ but in other areas, injection does not appear to cause any temblors.²

Areas where oil and gas have already been pulled from the ground may work well for storing carbon, because hydrocarbons previously hid away in those layers of rock for millions of years. But it’s also important to make sure previous drilling operations didn’t leave the ground compromised, because CO₂ could escape from uncapped wells. “You would want to make sure that you're not in an area which had been turned into Swiss cheese by previous drilling operations,” says Hager.

A 2013 study from the U.S. Geological Survey showed that 65 percent of technically accessible carbon storage locations in the U.S. are found in “coastal plains,” mainly along the Gulf Coast, where younger sediments make it easier to inject CO₂.³ Since 2021, when a new law provided funding for carbon capture programs, the U.S. government has plowed billions of dollars into developing carbon storage technology and projects. Before large-scale deployment happens, though, Hager says we need specific, detailed studies of which sites are suitable for safe, long-term storage.

Thank you to Christopher Berry of the United Kingdom for the question. You can submit your own question to Ask MIT Climate here.

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