In the December 2012 issue of the Proceedings of the National Academy of Sciences (PNAS), a published letter by MIT researchers Ruben Juanez, Howard Herzog and Brad Hagar provides several cogent geophysical counter-arguments to a 2012 PNAS Perspectives piece by Stanford researchers Mark Zoback and Steve Gorelick. The two had questioned the viability of sequestering commercial volumes of captured CO2 due to an attendant potential risk of induced seismicity. Zoback and Gorelick maintained that much of the deep basement rock across North America is at critical stress–a point at which a perturbation, such as commercial CO2 injections, could cause failure and induced seismicity.
However, in their response, Juanez et al. counter that most of the earthquake hypocenters (the focal point of the rock rupture and earthquake in the subsurface) are far deeper that the saline formations that would accept CO2 for storage and that the rock properties in the shallow crust would accommodate stress rather than rupture. Moreover, they point out that in highly seismic areas, such as in southern California, geologic traps have held buoyant CO2 for millions of years. Finally, Juanez et al. point out that the Mountaineer project example that Zoback and Gorelick cite was not representative of the many excellent saline formations that could accept commercial volumes of captured CO2, and that site selection is essential to successful storage. But while Zoback and Gorelick responded with another letter countering Juanez et al.’s views, what’s critical to add here is that there is much more to the debate than rock mechanics (see our blog from June 2012).
So while this healthy scientific debate focuses on the geophysical aspects of injection of commercial volumes of CO2 into saline aquifers, there is a broader set of considerations that must be incorporated into a rational dialogue on the ability of North American geology to accommodate commercial-scale carbon storage. For example, injection of CO2 into depleted petroleum reservoirs, with known capacities, injectivity and infrastructure could accommodate many decades of captured CO2. According to a 2012 National Academy of Science report, there have been no cases of observed humanly perceptible induced seismicity from CO2 injections associated with enhanced oil recovery–which has successfully taken place in Texas and elsewhere for four decades.
Moreover, widely associated with these depleted oil fields are brine formations that offer large volume “stacked storage”–managed CO2 injection and storage in sandstones or carbonate rocks above or below the producing intervals in oil fields. Other storage options include offshore reservoirs in the Gulf Coast, currently being studied by the University of Texas, or management of subsurface reservoir pressures via production of water from the formation such that the reservoirs never reach a critical state of stress. Furthermore, specific EPA geologic sequestration rules require that operators inject CO2 at pressures that would not induce rock failure. All told, while regulators should take care to ensure that significant induced seismicity does not occur, there is a substantial body of evidence, beyond the geophysics, that North America’s ample geologic resources can accommodate many decades of captured CO2.