The most promising storage containers are called deep saline aquifers—rock formations one to three kilometers underground, below the depth of freshwater aquifers, and beneath a layer of impermeable rock called the caprock, which acts as a seal. A modeling study by MIT researchers, published earlier this year in PNAS, estimated that deep saline aquifers in the United States could hold at least a century’s worth of the carbon dioxide produced by the nation’s coal-fired power plants.
But this hypothesis has not been tested on a large scale, and Zoback doubts that some of the target storage sites could safely hold as much carbon dioxide as assumed. Even small quakes could damage caprocks, say Zoback and Gorelick. That would threaten the integrity of carbon dioxide repositories, potentially allowing the greenhouse gas to escape into the atmosphere.
Ruben Juanes, a geoscientist and professor of energy studies at MIT, who coauthored the previous PNAS study on carbon dioxide storage capacity in the United States, says the data are too sparse to support Zoback and Gorelick’s sweeping conclusion.
“Currently, there are no models that can forecast the occurrence or magnitude of seismic events caused by fluid injection into the subsurface,” Juanes says. And there is “little field experience to inform the risk” of CCS, since no current projects operate at the scale needed to significantly offset carbon dioxide emissions.
Further, says Juanes, damage to a caprock wouldn’t allow carbon dioxide to escape in all cases. For example, he notes that the Mount Simon Sandstone, a candidate repository in the Illinois Basin, lies beneath at least two caprocks besides the primary one.
Zoback and Gorelick agree that the Mount Simon Sandstone could be a viable carbon dioxide container. But they say there could be dangerous seismological consequences if the region were to rely too heavily on this one location.
“You can find circumstances in which CCS can be done,” Zoback says. “It’s just the enormous scale that is challenged.”