Many of the most promising potential sites for reservoirs are saline aquifers about two to three kilometers underground, deep enough that that they are not in contact with the biosphere. There are many such aquifers in ancient geologic formations, 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 they 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 said. "And this is true of many of the places being considered."

Zoback said there are other sites, including some with saline aquifers, where the rock is weaker and would be better able to accept large amounts of gas without spawning seismicity.  And because the locations of most large faults are well mapped, it should be fairly easy to avoid provoking the sort of shaking that would harm people and property directly, so the problems are not necessarily insurmountable. 

"I am not against carbon dioxide sequestration by any means and it certainly has a role," Zoback said.  "What I am asking people to consider is whether or not it should really be one of the key components of a strategy for reduction of greenhouse gas."

Even if earthquakes are induced, he said, it would not be an issue of immediate safety.  It would take a fairly large earthquake to create a rupture that would send carbon dioxide pouring back to the surface and that situation should be fairly easy to avoid. To get big earthquakes, you need big faults and locations such as that would be ruled out during the selection process.

The problem Zoback foresees is that 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 said.  In addition to failing to solve the problem, a lot of time and money would turn out to have been wasted.

Sequestration projects are under way

There are two sequestration projects already under way, in Norway and Algeria, and so far they appear to be working as planned. But Zoback said 3,400 such projects would be needed worldwide by midcentury 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. And while some view the issues associated with injecting gas into the subsurface as mainly a technical challenge, Zoback thinks that challenge may not be an easy one to engineer around.

"My main concern is to get these geological issues out on the table," he said. "I want to get the discussion started."

If sequestration does become a major part of the effort to reduce greenhouse gases, a public education campaign will be needed, Zoback said.

"If we make a massive commitment to injecting carbon dioxide into subsurface reservoirs and then public concern over the earthquakes shuts it all down, then where are we?