DUAL LETTERS REKINDLE INDUCED SEISMICITY DEBATE

Tamar Hallerman
GHG Monitor
12/21/12

Six months after a widely-circulated article from two Stanford geophysicists argued that carbon storage triggers earthquakes and should not be widely deployed, a trio of Massachusetts Institute of Technology professors have published a letter defending the safety record of the technology. In a response published in last week’s issue of Proceedings of the National Academy of Sciences, Ruben Juanes, Howard Herzog and Bradford Hager of MIT acknowledge that CO2 storage, like other processes that require the injection of materials into the subsurface, may induce seismicity, but argued there is “no geologic evidence” that seismic events spurred by CO2 injection cause fault leakages that would render large-scale CCS operations unsuccessful. They added that the starting points for most quakes are five to 10 miles below the surface, far deeper than the formations considered ideal for CO2 storage, which are typically one to two miles down. “[The Stanford paper’s] characterization of seismic activity misrepresented its relevance to CCS,” the trio wrote.

Juanes, Herzog and Hager were responding to a paper published in the ‘perspectives’ section of Proceedings in June. The document sparked a public relations battle with carbon capture and storage proponents after Stanford University’s Mark Zoback and Steven Gorelick argued that CCS is not a viable strategy for widespread emissions reductions because of its “high probability” of triggering small to moderate-sized quakes that could threaten the seal integrity of storage reservoirs and potentially lead to CO2 leakage. “Because even small- to moderate-sized earthquakes threaten the seal integrity of CO2 repositories, in this context, large-scale CCS is a risky, and likely unsuccessful, strategy for significantly reducing greenhouse gas emissions,” the paper argued. A peer-reviewed National Research Council report released the same week, as well as a subsequent hearing where senators expressed concern about the two reports’ findings, caused a scramble among CCS advocates to defend the technology’s safety record.

In their response, the MIT professors question Zoback and Gorelick’s argument that induced seismic activity from injection would compromise the containment of CO2 and lead to leakage. They point to the “large volumes” of gas that have naturally remained sequestered for thousands of years in places like southern California, an area that experiences frequent earthquakes, but has not leaked. Lastly, the trio emphasized that site selection for CO2 storage projects is “key” and that with the proper selection procedures, there are plenty of formations in the U.S. that can accommodate large volumes of CO2 without a real threat of leakage. “A recent study suggests that deep saline aquifers exist throughout the United States that can accommodate the CO2 migration and pressure increases associated with large-scale injection at the century time scale,” they wrote.

Stanford Profs Argue CCS Still Not Ideal for Large-Scale Deployment

Also published in last week’s issue of Proceedings, Zoback and Gorelick argue in a short response to the MIT letter that seismic events have been detected at “several” sites where CO2 has been injected into the subsurface at depths of roughly one to two miles. They cite as an example BP’s In Salah field in Algeria, where, according to the National Research Council, microseismic events have been detected. The pair also notes moderately-sized earthquakes in Texas, Ohio and Colorado due to wastewater injection operations over the last two years as an example of how injecting fluids underground can cause disruptive events. “Seismicity triggered by fluid injection in brittle sedimentary rock at relatively shallow depths is a geophysical fact,” Zoback and Gorelick argue.

The pair disputes Juanes, Herzog and Hager’s argument that the existence of hydrocarbon reservoirs in southern California is evidence that fluids can be safely stored for thousands of years. Instead, they say that is akin to comparing apples with oranges. “Pore pressure and stress evolve together in a hydrocarbon reservoir over long periods of time. When pore pressure increases too rapidly, it is relieved through faulting or hydraulic fracturing as a natural geologic process,” they state in their letter.

Zoback and Gorelick acknowledge that storing “limited volumes” of CO2 in the subsurface is possible in some formations that have the ideal geology, but add that there are not enough suitable storage sites in the world for the widespread deployment of CO2 storage. “We agree that ideal geologic formations can be found for safe storage of limited volumes of CO2 at depth,” the pair concludes. “The purpose of our [original] article is to express reasons for concern about the widely held belief that CCS will be able to function at the extraordinary scale necessary for it to have a major impact on mitigating greenhouse gas emissions. There is ample evidence to suggest that triggered fault slip could render large-scale CO2 storage unsuccessful.”

Hill Says Storage in Oil Reservoirs Should be Folded into Discussion

Meanwhile, in a response to accompany the MIT letter, the Clean Air Task Force’s Bruce Hill argues that the conversation surrounding the geophysical safety of injecting CO2 into saline aquifers should also consider the storage potential of depleted oil reservoirs. Hill says that CO2 storage in depleted oil reservoirs has not generated any cases of felt seismic events in 40 years and cited the additional storage potential of stacked storage formations and offshore oil reservoirs in the Gulf Coast. “All told, while regulators should take care to ensure that significant induced seismicity does not occur, there is ample evidence, beyond the geophysics, that many decades of CO2 can be accommodated by North America’s geologic resources,” Hill says.