Coal is the most abundant and cheapest fossil fuel on the planet, but it’s also the dirtiest in terms of how much heat-trapping carbon dioxide (CO2) it spews into the atmosphere when you burn it. One possible way of dealing with coal’s globe-warming effect is to capture the CO2 from coal exhaust and bury it deep underground in a process known as carbon capture and sequestration, or CCS. Opponents of the idea have argued, however, that among other potential dangers, CCS could trigger earthquakes.
And for years, proponents have said, “tell us something we didn’t know.” Geologists have been aware since the 1960’s that pumping liquids and gases into underground rock formations can trigger earthquakes by adding just a little extra pressure to existing faults in a sort of straw-that-broke-the-camel’s-back effect.
In 2011 alone, subsurface injection of wastewater from mining operations was blamed or suspected in quakes that shook Arkansas, Colorado, Ohio and Oklahoma. But they were small earthquakes, causing minimal damage and no injuries at all, and if that’s the worst consequence of keeping a lid on global temperatures, it might well be worth it.
Or maybe not.
In a new analysis published last week in Proceedings of the National Academy of Sciences, Mark Zoback and Steven Gorelick of Stanford University point out that in order to be effective, CCS projects need to keep CO2 out of the atmosphere for thousands of years – and that earthquakes too small to endanger life or property could nevertheless create leaks that would make the whole thing a waste of time. The bottom line, according to Zoback: “CCS is a risky proposition. Not that it’s impossible, or even inappropriate. It should be done. But at a global scale, it’s not likely to reduce CO2 emissions significantly.”
Zoback and Gorelick base their analysis on a phenomenon called “critically stressed crust,” a consequence of plate tectonics. In places where vast, continent-sized plates of crustal rock attempt to slide past each other (as in California) or where one plate dives under another (as in Japan), the movement proceeds in fits and starts, with long periods of no motion at all as the stress gradually builds, and then the sudden jerk of a major earthquake.
In the interiors of continents, by contrast, far from a plate’s edge, the stress is more or less constant, and it’s usually relieved in less dramatic fashion through myriad small earthquakes, like the 5.9-magnitude quake that hit Virginia last summer. (They’re not always small, though, as the massive quakes near New Madrid, Missouri in the early 1800’s and a killer quake in Tangshan, China, in 1976, make clear.)
Because of this constant stress, any significant extra underground pressure could push a fault that was on the verge of rupturing. “Obviously,” Zoback explained in an interview, “any faults of significant scale would be avoided in choosing a location for CCS.” But it’s the faults of insignificant scale, which might easily be missed in geologic surveys, which worry him and Gorelick. If one of these were to slip, it could create cracks in the overlying cap rock that give the CO2 a pathway back to the surface.
And unlike wastewater, said Zoback, “supercritical CO2 [the liquid form of CO2 that would be injected underground] is relatively light, and wants to rise.”
While plenty of places would be unsuitable for CCS according to this analysis – there are two situations where Zoback and Gorelick suggest it wouldn’t be a problem. One is in an oil or gas field where the fossil fuels have largely been extracted already. “Since you’re lowering the pressure during extraction,” said Zoback, “there’s little danger in raising it again by pumping in CO2, as long as you don’t go above the initial pressure.” The other is in places where the underground rock (but not the cap rock) is friable, or easily cracked. That’s the case under the North Sea, where CO2 captured in natural-gas extraction has been injected into the Sleipner Field since 1996, without incident.
Unfortunately, safe locations like these aren’t distributed evenly around the globe. For the U.S., the best storage locations by these criteria are most likely in the Gulf of Mexico, Zoback said. If you’ve got a refinery on the Gulf Coast, he said, and if you can separate the CO2 and inject it underground locally, there’s no doubt that it’s a viable idea.
But much of the CO2 generated in the U.S. comes from other parts of the country. “Can you ship all of that south to the Gulf?” he asked. “It raises substantial questions.”
Again, Zoback stressed, neither he nor Gorelick is arguing that CCS is necessarily a bad idea – only that it might be a bad idea on the grand scale it would take to make a major dent in global warming. “We want to start a dialogue about this. As climate modelers go forward, this is something they need to think about.”