Pore space: Small spaces for a big problem

One of the misconceptions about sequestration is that there are massive empty spaces underground where supercritical CO₂ will be stored, similar to an underground storage tank.

"There's not some big cave under there," said Lee Spangler, director of the Big Sky Partnership and of MSU's Energy Research Institute. "We're storing CO₂ in the pore space, and there are a lot of mechanisms that will keep it underground."

First among those mechanisms, Spangler explained, is the underground environment itself. Sites are chosen so that an impermeable, nonporous layer of rock is above the storage level. This "cap rock" physically holds the CO₂ underground.

Upon injection, the CO₂ immediately finds its way into the rock's pores, the tiny, porous veins that form naturally in such minerals as basalt and sandstone. If kept at depth long enough, the CO₂ eventually will dissolve into the deep groundwater or "brine," an undrinkable mix of dissolved minerals. That CO₂-laden brine is heavier than the water around it, and so it sinks beneath the normal brine, Spangler said.

Eventually, the CO₂ reacts with the minerals in the brine and turns into a solid form, usually limestone.

How long does it all take? The CO₂ is trapped beneath the cap rock immediately. Decades and centuries will see the CO₂ dissolve and sink. It will take further centuries or millennia to see it permanently trapped in limestone, Spangler said.

Planning for safety

All that means that sequestration doesn't end after the last ton of CO₂ has been injected. The next step is monitoring the site, and many scientists at MSU and elsewhere are working on what Spangler calls "just-in-case" technologies, instruments to scan sequestration sites and watch for any escaping CO₂.

Spangler emphasized that properly chosen sequestration sites don't leak, but even if they did, a CO₂ leak would pose almost no danger to humans or the environment. Leaks would, however, be costly to the companies that spent millions of dollars to put the CO₂ underground in the first place. Letting CO₂ escape would be like allowing an investment in equipment and technology just float away.

Last summer, MSU hosted a month-long series of experiments to test more than 20 different technologies for detecting leaks. The experiments drew researchers from around the world and tested devices ranging from sensors to measure the stress on plants caused by excess CO₂ to advanced laser detection systems.

The experiments took place just west of MSU, where the university has built a one-of-a-kind test site. There, a 240-foot pipe buried in the ground releases small amounts of CO₂ to simulate leaks. The site provides researchers with conditions not yet available in the real world, Spangler said.

"The problem is that when surface detection has been deployed at sequestration sites, those sites have been very well chosen and don't leak, so there's nothing to detect," he said. "This site allows us to test detection technologies to make certain they will work."

Biofilm barriers: Gumming up the works

"The whole business of carbon sequestration needs to have ways to stop leakage before it occurs," said Al Cunningham, a civil engineering professor and founding member of MSU's Center for Biofilm Engineering, who is working on another just-in-case technology.

Cunningham works with biofilms, colonies of bacteria living together in a carbohydrate slime. Biofilms are extremely common and form on most wet surfaces.

It's the slime--known as "extracellular polymer"--that holds one of the keys to sealing the tiny fractures in the rock that sequestered CO₂ might use to escape back to the surface. The polymer is a non-living, slimy, glue-like substance that "basically plugs up the free pore space," Cunningham said.

All that would be needed would be to inject bacteria into the sequestration site and feed them. The bacteria would then do their normal thing, in the process producing enough polymer to form a "biofilm barrier" that seals the site.

Biofilm barriers are not a new idea. For 15 years, scientists at the biofilm center have been working on ways to use biofilms to prevent the spread of groundwater contamination and even to degrade those contaminants.

But it's not all about just sliming the rocks.

"We've discovered a certain kind of bacteria that will precipitate calcium carbonate, given the right conditions," he said.

That means that, under the right conditions, the barrier bacteria will turn a small amount of sequestered CO₂ into limestone, forming new rock to more effectively seal the site.

Cunningham said this "biomineralization" process takes only a few days and can use common, easy-to-grow bacteria. The hope is to someday find bacteria already living beneath sequestration sites that can do the job, which would save having to pump them underground.

Cunningham said the biofilm center could be ready for large-scale tests in a couple of years.

So could this process be used to turn large volumes of CO₂ into limestone? Cunningham didn't rule out the possibility, but he pointed out that even if it were possible to use biomineralization on a massive scale, it would not solve the root problem: too much CO₂ being emitted into the atmosphere.

"The idea behind sequestration is to buy civilization time to convert to energy sources that don't release carbon," he said.

Return to regular view	Text-only              Email this article	Published: 4/27/2009