When two Stanford University researchers warned last year that carbon sequestration could lead to earthquakes, Cliff Frohlich wondered, "Where's the proof?"
Then Frohlich found it himself, underneath a Texas oil field.
Underground injection of carbon dioxide to boost oil production "may have contributed to triggering" a series of earthquakes north of Snyder, Texas, several years ago, according to a study released yesterday by Frohlich, senior research scientist at the University of Texas' Institute for Geophysics, and visiting scientist Wei Gan. Their findings are being published in the Proceedings of the National Academy of Sciences (E&ENews PM, Nov. 4).
The results add to the discussion begun last year when two Stanford geophysicists said carbon capture and sequestration (CCS) wouldn't work on a large scale because it would cause earthquakes (ClimateWire, June 19, 2012). The problem with their article, Frohlich thought, was that no one had ever seen earthquakes caused by burying carbon under the earth.
But Frohlich said he didn't set out to find such a quake.
Instead, he'd suggested to his colleague, who is visiting from China University of Geosciences in Beijing, looking at earthquakes that had taken place in the Cogdell oil field near Snyder. A production-boosting technique called "water flooding" had been linked to decades of earthquakes in the area.
The rumbling had started again in 2006 after a 24-year absence. By the end of 2011, there had been 18 earthquakes of magnitude 3 or greater. The largest was magnitude 4.3. That's strong enough to be widely felt but is not very damaging.
Frohlich, who has done extensive research on quakes caused by injection of drilling wastewater, thought they might be able to locate some of the faults that had been activated by water flooding. They could take advantage of a dense array of seismic instruments, called USArray, temporarily installed in the area in 2009 and 2010. The array offered much more precise readings on the earthquakes that occurred while it was in place.
They found something they hadn't expected -- a close correlation between carbon dioxide injections and the new round of quakes.
"When we looked at the injection data, it suggested CO2," Frohlich said.
But he added that it's not definitive proof. "You can't prove it. You'd say gas is the most likely candidate."
Large-scale CCS projects propose injecting much more carbon dioxide than what was put in the Cogdell field. But Frohlich said the study also shouldn't be seen as a death knell for CCS. Instead, it should serve as the basis for additional research into the safety and effectiveness of CCS. And carbon dioxide injection for what the oil business calls "enhanced recovery" could be an interesting line of inquiry.
"I wonder if people have looked very hard at the oil and gas industry," Frohlich said.
But if the Texas study bolsters the finding that carbon dioxide injection can cause earthquakes, it erodes the idea that CCS would always cause earthquakes.
The scientists found that two nearby oil and gas fields experienced no earthquakes during that time, even though they'd had similar amounts of carbon dioxide injection.
"It is possible that in many locations large-volume CO2 injection may not induce earthquakes," the study says.
Something else they found is that the people who live in the area aren't too worried about earthquakes, even those in the realm of magnitude 4 or 5.
"People in Snyder tolerate earthquakes more than people in other places," he said. "If there's a 4 or a 5, people just go on pumping oil."