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Is there a danger that pumping liquid carbon dioxide underground could have the same negative impacts as fracking?
Pumping carbon dioxide deep underground can cause earthquakes under certain conditions, but there are ways to prevent this.
April 21, 2021
Because the main cause of climate change is too much carbon dioxide (CO2) in the atmosphere, one idea to fight it is to capture CO2 and store it underground. There are a few operations in the world that already do this, by capturing CO2 through a chemical process, condensing the gas into liquid form, and then injecting it deep underground. This process is known as “geologic sequestration,” because the CO2 is stored (“sequestered”) in rock.
While geologic sequestration of CO2 is not yet common, it is similar to a more common process in the oil and gas industry: wastewater injection. Fracking, or “hydraulic fracturing,” is used to collect oil or natural gas contained in rocks underground. To frack, drillers inject a mixture of water, sand and chemicals into a rock layer and break it apart, but some of the water-sand-chemical mixture flows back up the well. This wastewater is separated from the oil and is often injected back underground at a different site.
Under certain conditions, injecting this wastewater underground can cause earthquakes. Oklahoma, for example, experienced a surge in earthquakes in the 2010s due to a large amount of wastewater being injected underground.1 Once injected into an underground well, the fluid raises the pressure in the surrounding rocks and aquifers, reducing the friction on fractures in the rocks. If these wells are near a fault line, the pressure within the fault can also increase, moving around the rocks and causing an earthquake—just like the added pressure of air on an air hockey table can move around the puck.2
Geologic sequestration is basically the same process as wastewater injection, and it has also caused earthquakes at several sites around the world.
But while both of these injection processes can and have triggered earthquakes, it’s not common. "Most wastewater is injected without causing any earthquakes at all," says Brad Hager, a professor of earth sciences at MIT who researches manmade earthquakes. "[Earthquakes] can be a big problem, but they are a tractable problem. The big thing which determines whether injecting fluids causes earthquakes or not is basically the geology that you're injecting into."
Hard rocks—like granite, for example—are hard and brittle, making them prone to break when liquid is injected into them. These rocks are more likely to break than softer rock, potentially triggering earthquakes. According to Hager, the most successful geologic sequestration operations inject into soft sedimentary rock formations, like shale or sandstone, which are more permeable and can absorb the added liquid without breaking.
Other types of geological formations can also increase the risk that the trapped CO2 leaks back out. These leaks tend to occur at sites where a brittle layer of rock sits above the aquifer that contains the CO2. As the pressure from the injected liquid builds, the brittle rock layer—known as a caprock—can crack, allowing the CO2 to seep out back into the atmosphere. It’s also possible that CO2 could leak into drinkable groundwater if injected at shallow depth near an aquifer. According to Hager, both earthquakes and leaks from CO2 injection can be managed by avoiding sites near faults in caprocks and by properly mapping sites near drinking water.
"I want to be clear that there are risks," he says, "but there are risks to everything, and the risks for continuing to emit carbon dioxide into the atmosphere without taking it out far outweigh the risks of putting the CO2 underground."
Thank you to Barbara Ann Wilder of Wilmington, North Carolina, for the question. You can submit your own question to Ask MIT Climate here.
1 United States Geological Survey: "Oklahoma has had a surge of earthquakes since 2009. Are they due to fracking?" Accessed April 21, 2021.
2 United States Geological Survey: "How does the injection of fluid at depth cause earthquakes?" Accessed April 21, 2021.