Karen Frantz
GHG Monitor
11/08/13
A new study published in an online edition of the Proceedings of the National Academy of Sciences this week suggests that injections of supercritical CO2 and other gasses for enhanced recovery into the Cogdell oil field near Snyder, Texas, may have triggered earthquakes nearby. Some CCS experts, though, are downplaying the study’s results. Although the study’s authors suggest that “significant risks accompany large-scale carbon capture and storage as a strategy for managing climate change,” Bruce Hill, senior scientist-geologist at the Global CCS Institute said there is no indication that large earthquakes could occur. “People kind of get hyped up about seismicity, but this is a very, very small risk,” he said.
The study, which was written Wei Gan and Cliff Frohlich at the University of Texas at Austin’s Institute for Geophysics, looked at seismic data from the EarthScope USArray Program and the National Earthquake Information Center and determined that a number of earthquakes that occurred in and near Cogdell field between 2006 and 2011 was correlated with the use of CO2 for enhanced recovery, which has been used in Cogdell field since 2001—and with “nearly constant injection volumes” since 2004. They found that 93 earthquakes occurred between 2009 and 2010, and of those, three were of magnitude 3 or greater. A 4.4 magnitude earthquake also occurred in 2011. “If [EOR] triggered the 2006-2011 seismicity, this represents an instance where gas injection has triggered earthquakes having magnitudes 3 and larger,” the study says. “Understanding when gas injection triggers earthquakes will help evaluate risks associated with large-scale carbon capture and storage as a strategy for managing climate change.”
Lack of Causation
But Brian McPherson, an associate professor of Civil and Environmental Engineering at the University of Utah, said the study “never makes any deterministic connection” between the earthquakes and the CO2 injections at the site. “The only coincidence is the timing,” he said. He also said the study notes that the CO2 injection zone is at 2 kilometers below the ground’s surface, whereas the earthquake hypocenter is listed at 5 kilometers below ground—a difference of about 9,000 feet. “If they’re in the same horizon, if they’re in the same reservoir, then stress propagation is something to think about and something to base a model on,” he said. “But these guys didn’t do any modeling, they just looked at the seismic data, looked at the USArray data, and then they looked at the production injection records and the timing of those … and said, oh, well, they occurred about the same time, within the same year or two.”
He also criticized the study for placing information about the CO2 injection zone and the hypocenter at different pages. “They never explicitly state that the injections caused the events, but they forced the readers to connect the dots,” he said. “One could infer that they made an attempt to separate those two facts.” He said he was “surprised” that the study appeared in PNAS, but noted that it comes on the heels of last year’s report from the committee that sparked major criticism among industry experts.
Further Study Needed
Gan and Frohlich said in the study that there are still questions that need to be addressed, and appeared to suggest that concerns from Stanford University scientists Mark Zoback and Steven Gorelick that there was a “high probability” that large-scale CO2 injections could trigger earthquakes may be overstated. “If the recent Cogdell earthquakes are triggered, it is still puzzling why there are no earthquakes in similar nearby fields, such as the Kelly-Snyder field and the Salt Creek field,” the study says, referring to other petroleum fields in Texas. “Like Cogdell, both fields have experienced a combination of years of sustained injection/extraction of water/oil, followed by recent increases in gas injections … This observation, and the fact that no other gas injection sites have reported earthquakes with magnitudes as large as 3, suggests that despite Zoback and Gorelick’s concerns, it is possible that in many locations large-volume CO2 injection may not induce earth-quakes. What is different about Cogdell that allows earthquakes to occur there?” Gen and Frohlich wrote, calling for further study.
Charles McConnell, executive director of the Energy & Environment Initiative at Rice University and former Assistant Secretary for Fossil Energy at the Department of Energy, also said that the study underlines the need for the continuation of research and development efforts for CCS so that advancements in technology can be accomplished. “I’m concerned that any data point that gets collected can be the determining factor for how people would view what’s going on,” he said. “And absent the inclusion of it in a coordinated, systematic methodical research program, it becomes another random data point that may or may not e significant, may or may not be peer reviewed and completely integrated, and may have a number of other factors associated with it that could make the determinations more or less accurate.”
He said the DOE’s Regional Carbon Sequestration Partnerships foster collaborative efforts between industry, government, universities and others, but that the concern is that “over time the funding for those partnerships has been cut back severely by the federal government and it leads to the potential for more random individual studies that are not coordinated as part of a comprehensive national program.”
No ‘Public Hazard’ Associated With CO2 Injections
Hill noted that although there is a potential for earthquakes with CO2 injections, the potential is vanishingly small, particularly for earthquakes that could be felt by people. He also noted that operators can manage the pressure at the subsurface. “You’re putting some in but you’re taking some out,” he said. “So you have the ability to manage that pressure.” He added, “We should be mindful that injecting fluids could activate faults and the process of site selection, risk analysis, monitoring and surveillance, and management of injection is an important part of ensuring that carbon storage doesn’t pose any threat of harm.” He also said “there’s really not public hazard associated with this.”
Susan Hovorka, a senior research scientist at the University of Texas at Austin’s Bureau of Economic Geology, also said that when considered properly, the study’s results are “positive” for CCS. “Seismicity occurred [in Cogdell] in the 1980’s, and was published in 1989; however, neither occurrence of events nor reporting caused a stop in injection,” she said. “The field is still under CO2 flood for EOR, in fact the operator has expanded since the initial seismicity. They modified the injection and withdrawal patterns to better manage pressure differences in the sensitive parts of the field. This kind of adaptive management of the subsurface is a good analog for how CCS would manage such issues. Both the new and the 1989 papers on Cogdell document that felt seismicity is unusual near injection, that it is not a big worry, and show how injection can be modified if unexpectedly large microseismicity was encountered.”
