Carbon Capture and Storage (CCS) underground could trigger earthquakes, a Stanford geophysicist warns.
While they probably wouldn't be big enough to hurt people or damage property, they could still cause serious problems for the reservoirs containing the gas.
"It is not the shaking an earthquake causes at the surface that creates the hazard in this instance, it is what it does at depth," Mark Zoback said. "It may not take a very big earthquake to damage the seal of an underground reservoir that has been pumped full of carbon dioxide."
The danger of earthquakes arises, Zoback says, because the interior of the North American continent is crisscrossed by ancient faults that are often close to failure because of the immense tectonic forces acting on them.
"So, in that context, when we start perturbing the system by changing fluid pressure, we have the potential for activating faults," he said.
Many of the most promising potential sites for reservoirs are saline aquifers about two or three kilometers underground. There are plenty of these about, especially in the upper Midwest, Zoback said. And since the water in them is too salty for consumption or irrigation, they are good candidates.
But those formations also tend to be dense, well-cemented sedimentary rock, with low permeability, and may not be able to accept large amounts of fluid before becoming stressed to the point of failure.
"These are the settings most likely to induce seismicity," Zoback says. "And this is true of many of the places being considered."
The seismicity could create small pathways through the rock by which carbon dioxide would gradually seep back into the air.
"If the carbon dioxide permeates back out of the reservoir, the effort to keep it out of the atmosphere will have been futile," he says.
Only two sequestration projects are already underway, in Norway and Algeria, and so far they appear to be working as planned. But Zoback says 3,400 such projects would be needed worldwide by the middle of the century to deal with the volume of carbon dioxide that we will be generating.
"Finding that many ideal sites around the globe is not impossible, but it is going to be a tremendous challenge," he said.
It's the latest in a series of reports casting doubt on CCS as a means of reducing carbon in the atmosphere. Last month, a Duke University study suggested that it could contaminate drinking water, while a Danish scientist compared the dangers earlier this summer to those of the storage of nuclear waste.