Injecting carbon dioxide into wet, porous rocks deep underground may be a good way to reduce emissions of this major greenhouse gas because the rocks trap the gas better than previously thought, a new study claims.
For many years, scientists have looked into pumping carbon dioxide deep underground, where it could be stored for thousands of years, to reduce levels emitted from power plants. But many environmental groups oppose this process, known as carbon sequestration, because of the possibility that carbon dioxide could leak out.
Earlier this month, scientists who authored the latest Intergovernmental Panel on Climate Change report, which predicted the dire consequences and inevitable global warming, called for governments to take immediate action to curb carbon dioxide and other greenhouse gases that have contributed to climate change.
One of many methods
Scientists have explored numerous methods for reducing greenhouse gas emissions, including injecting sulfur into the air and placing a satellite shield around the Earth. Billionaire Richard Branson announced last Friday that he would give $25 million to any scientist who could come up with technology that would extract greenhouse gases from the atmosphere.
MIT researcher Ruben Juanes, author of the new study, says these methods need not compete with one another.
"Rather than comparing carbon sequestration with other alternatives," Juanes told LiveScience. "What I would say is that all of them at this point should be looked into and should be investigated seriously to really assess their full potential."
Scientists think that carbon dioxide could potentially be stored in three types of underground locations: depleted oil and gas fields, unminable coal seams and briny aquifers. For all these methods, a "geologic cap," usually a layer of largely impermeable rock, above the target area is needed to prevent leaks.
Saltwater aquifers are particularly attractive because they are ubiquitous underground, even far away from oceans, and offer more storage volume, but environmental groups are particularly nervous about leaks of carbon dioxide sequestered in aquifers because the buoyancy of the gas causes it to rise in the water.
These worries are unwarranted, according to the new study published in a recent issue of the journal Water Resources Research.
When carbon dioxide is injected into the porous rock saturated with salt water, it forms a plume and starts to rise through the rock. Because the rock minerals have an affinity for water, the water will coat the crevices and pore walls of the rock in a thin film, just as raindrops might spread out and form a film when they land on your skin.
When injection stops, the water closes up at the bottom of the plume, essentially turning it into a long bubble, Juanes said. The water films in the rock pores swell, eventually touching and closing off the carbon dioxide's escape.
"The plume of CO2 that was sort of a continuous blob is now being chopped of into small blobs," Juanes explained. "And because these blobs are small, they don't have enough buoyancy to continue [their] vertical assent. So these blobs just remain there in the porous medium, trapped."
A viable option?
But whether sequestration is a viable means of curbing plant emissions remains to be seen; cost is an important factor in implementing the strategy because carbon dioxide has to be separated from a power plant's gas stream and compressed before it can be injected into the ground.
"There has to be some economic incentive or some regulatory structure to make that happen," Juanes said.