Earthshaking possibilities may limit underground storage of carbon dioxide

Dec 13, 2010

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-sized 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."

Carbon dioxide is a major cause of a . In many countries, including the U.S., 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 for sequestration to make a significant contribution to reducing , the volume of gas injected into reservoirs annually would have to be almost the same as the amount of fluid now being produced by the oil and gas industry each year. This would likely require thousands of injection sites around the world.

"Think about how many wells and pipelines and how much infrastructure has been developed to exploit oil and gas resources over the last hundred years," he said. "You need something of comparable scale and volume for ."

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. Those tectonic forces exist even in the interior of the country, far from California and the San Andreas.

The interior may seem quite stable viewed on a human timescale, but that is only because the rate at which the intraplate faults are failing is quite slow, Zoback said. Although there are not many earthquakes in any one area, or many large earthquakes, the faults are nonetheless failing.

"So, in that context, when we start perturbing the system by changing fluid pressure [as we inject massive amounts of carbon dioxide into the subsurface], we have the potential for activating faults," he said.

Zoback emphasized that 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 out on 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 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.

There are two sequestration projects already underway around the world, in Norway and Algeria, and so far they appear to be working as planned. But Zoback said 3400 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 massive 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?"

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A_Paradox
not rated yet Dec 13, 2010
I am still waiting for people to start taking seriously my suggestion of carbon dioxide sequestration/mitigation through growing kelp around the equator. Infrastructure, living space, market gardens, processing plants for use of the kelp in manufacturing, and entertainment parks, would all be built on [insulated] ice rafts scores of kilometres across. The ice rafts could be shaped so as to allow thousands of kilometres length of cable to be suspended horizontally, parallel to the surface at optimum depths for the nurturing of kelp, sea-grasses and other types of seaweed.

In addition the equatorial regions can be used for fish farming, up to about 5 degrees north or south of the equator; this is the typhoon-free zone.
Raveon
5 / 5 (1) Dec 13, 2010
CO2 is already sequestered, all we have to do is stop unsequestering it.
Vorsana
not rated yet Dec 14, 2010
This is important because earthquakes would also increase the risk of the CO2 contaminating ground water, which is a serious concern.

The amounts of CO2 involved are actually even greater than suggested here. To use 2007 as an example, the US oil industry used about 1.049 billion metric tons of oil. In contrast, the CO2 emissions that year (including from burning natural gas) were 5.984 billion metric tons. Even if the CO2 is liquified to aid transport and injection underground, it's weight doesn't change. So we're talking about a massive amount of dedicated infrastructure, many times the size of the entire oil industry, to move the stuff around. The need for climate change action is real, but we shouldn't ignore what CCS actually will mean.
thermodynamics
5 / 5 (1) Dec 14, 2010
This is a good article. However, it focuses on "non-reactive" sequestration - even though it does not point that out. There are two different methods being looked at. In non-reactive storage the CO2 is pumped into deep saline aquifers, exhausted oil or gas fields, or unmineable coal seams. In those approaches the CO2 does not react and stays in the formation as a supercritical fluid with the potential to reach the surface again or contaminate ground water.

However, there is another approach where the CO2 is pumped into layers of basalt (found throughout the world) and reacts with the minerals in the basalt to form carbonates (which is similar to weathering that takes place slowly at the surface). The result is the CO2 turning into rock to fill the spaces in the basalt formations. I am sure the author knows of this approach but it was probably not in their area of study.

http://www.eureka...0305.php

See the above link for details.
A_Paradox
not rated yet Dec 25, 2010
@thermodynamics

The mineralisation option looks promising, at least up to a point. As Vorsana points out though, the enormous amounts of CO2 being produced and the widely spread burning sites, make the pressurised sequestration process difficult in terms of the amount of infrastructure needed and also expensive in terms of the energy needed to re-compress the CO2 gas.

I remember reading, years ago in Scientific American magazine of a proposal to permeate new concrete with liquefied CO2. It was proposed that CO2 would be well and truly liquid at room temperatures if subject to about 60 atmospheres pressure. The beneficial result was to be that the concrete quickly turned into limestone. It was said this normally will take thousands of years.
A_Paradox
not rated yet Dec 25, 2010
...cont:

Maybe a cheaper method would be to put pre-fab concrete structures into huge CO2 filled bell enclosures with the concrete floating on the internal water surface, tow the whole thing out beyond the continental shelf, use rubble/sand/ballast from barges to weigh the bell down and sink it. After the requisite time the extra weight could be removed by remote robots the bell brought back up and the newly transformed limestone structure brought back to shore.

Just a thought :-)

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