STANFORD UNIVERSITY — Storing massive amounts of carbon dioxide underground in an effort to combat global warming may not be easy to do because of the potential for triggering small to moderate earthquakes, according to Stanford geophysicist Mark Zoback.
While those earthquakes are unlikely to be big enough to hurt people or 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,” 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.” There may be ways around the problem says the researcher, but the issue is serious enough he urged that storing C02 underground not be relied on as a key to reducing greenhouse gases.
Carbon dioxide is a major cause of a global warming. In many countries, including the United States, China, and India, the majority of carbon dioxide is produced by coal-burning power plants and refineries. Keeping some of that carbon dioxide from entering the atmosphere by storing it underground could reduce the amount of warming.
The other complication, Zoback said, is that to make a significant contribution to reducing carbon dioxide emissions, the volume of gas injected into reservoirs annually would be huge, almost the same as the amount of fluid now being removed from underground by the oil and gas industry each year. This would likely require thousands of injection sites around the world. 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.
In the United States, the problem with potential earthquakes arises, Zoback said, because the interior of the continent is crisscrossed by ancient faults that are often poised to fail – what he calls “a state of failure equilibrium” – because of the immense tectonic forces acting on them.
Those forces drive the huge tectonic plates across the surface of the globe and trigger the occasional violent upheavals on faults along the plate margins, such as the San Andreas fault zone in California.
Zoback said any earthquake triggered by injecting gas would have happened anyway, because the fault was going to fail eventually. “You are just advancing the time at which the earthquake occurs,” he said. But the quakes would still be potential hazards to the reservoirs.
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.
If sequestration does become a major part of the effort to reduce greenhouse gases, a public education campaign will be needed, Zoback said.
BY LOUIS BERGERON
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