Recent changes in the Earth's climate are primarily being driven by the burning of fossil fuels—that is, taking carbon from deep in the Earth, and dumping it into the atmosphere at breakneck speed. Wouldn’t it be nice if we could just sort of… put it back?

That’s roughly the idea behind carbon capture and sequestration (CCS). Carbon dioxide is captured from the effluent of a large generator, like a coal power plant, and compressed into a supercritical liquid. That liquid is then transported via pipeline to an injection station where it’s pumped deep underground.

But the technique requires some very specific rock formations if we expect the carbon to stay there. Two new studies have looked at how much CO 2 we could hope to store, and how that storage may be affected by another process that's booming: fracking.

CCS can use the same rock configurations that sometimes host oil and natural gas. Like oil and natural gas, compressed liquid CO 2 is less dense than water, so you need a geological cap that will prevent it from bubbling upward. That cap rock needs to be impermeable to flow (shale often fits the bill). If there’s permeable reservoir rock below that cap—like a sandstone—then you’re in business. Eight hundred meters below the surface, the pressure is sufficient to keep the CO 2 in its compressed liquid state, and the reservoir rock will hold your fluid and the cap rock will keep it from escaping.

Basins of sedimentary rock—where the layers form a bowl shape—typically have all the right characteristics. Eligible reservoirs are typically filled with brine water (and, sometimes, remnant hydrocarbons).

Since the layers aren’t perfectly horizontal, the injected CO 2 will slowly migrate away and upwards along the underside of the cap rock. A couple things happen to it along the way. As the fluid winds its way around the grains of rock, some droplets will get stuck in the spaces between grains and become immobilized. At the same time, CO 2 is constantly dissolving into the brine water in the reservoir. Liquid CO 2 is less dense than brine water, but brine water containing dissolved CO 2 is denser than both. Fingers of that dense solution drop from the bottom of the liquid CO 2 plume and sink downward through the reservoir rock.

Given enough time (a leisurely millennium or so), the CO 2 dissolved in that water will precipitate as carbonate, completing the final stage of sequestration. So rather than resembling an inflated balloon of CO 2 just waiting for a reason to pop, it becomes more and more securely locked away as time goes on.

The big questions for this technology have revolved around safety (will the CO 2 stay down there?), practicality (how much will it cost?), and capacity (can it make a difference?). A new study published in PNAS by researchers at MIT focuses on the last of those—how much realistic capacity there is for carbon capture and sequestration in the United States.

Storage meets extraction

As research into hydraulic fracturing (or "hydrofracking") for shale gas production continues, I can’t help but think of it all as a precursor to the carbon sequestration discussion. Some of the concerns are similar, as are the technical challenges. The recent links between the disposal of spent fracking fluid in deep injection wells and increased earthquake activity are obviously applicable. Such events, caused by the increasing of fluid pressure that can ease the friction locking up a fault, are an important concern for carbon sequestration projects.

You don’t want to do anything that could compromise your cap rock, and concerns over triggering earthquakes large enough to cause damage are not without substance. As Ruben Juanes, one of the researchers involved in the MIT study, told Ars, "I think that the induced seismicity associated with injection wells is a factor to be considered. We, and other groups, are actively looking at some of these issues now. One mitigating factor is that CO 2 injection would take place far away from faults, simply to reduce the risk of leakage. This will very likely reduce the magnitude of the induced seismicity as well, but quantitative assessments are needed."

Indeed, the connection between carbon sequestration and fracking may be even more intimate than these parallels—they may intersect. A recent study published in Environmental Science & Technology pointed out that many US shale gas resources are found in locations that are also attractive for future carbon sequestration. Since fracking, by definition, involves the fracturing of cap rocks, this could diminish the potential for carbon sequestration in that area.

Current capacity

To characterize US capacity, the MIT group looked at eleven locations (shown on the image above) previously identified as suitable. They used simplified models of each location to simulate how much, and how quickly, CO 2 could be injected. While more detailed work will eventually be needed for each location, the new work is a serious improvement on the wide range of previous back-of-the-envelope estimates. At the very least, they’ve moved to the front of a much larger envelope.

"Our estimates are based on models at the geologic basin scale," said Juanes. "They are site specific in the sense that they employ the geometry, depth, and other parameters from each individual basin, including the presence of large faults. However, it is unquestionable that more detailed studies, at a much finer scale, would have to be done to design CO 2 injection at the operational scale. The intent of our study was to understand whether [carbon capture and storage] was viable at the continental scale."

And it’s not just the total available capacity that matters—if you inject too quickly you can inadvertently fracture the cap rock. The sequestration system has to be able to handle CO 2 at the rate it’s generated.

In order to assess whether sequestration systems can operate at a relevant capacity, you need to construct a scenario of future fossil fuel use. For the sake of this estimate, the researchers assumed that fossil fuel use would continue to increase linearly at about the current rate until about 2060, at which point it would instead decrease at that same rate while other energy sources took over.

In this scenario, they found that the US could sequester its captured carbon for at least 100 years. If that potential could be realized, the US could hold the line on CO 2 emissions from electrical generation while transitioning to renewables. Power plants currently account for a little over 40 percent of total emissions in the United States.

To be sure, it’s a long road from here to there. Sites would have to be studied and acquired, and a pipeline network would have to be built to carry captured CO 2 from power plants to the injection facilities. Lots of regulations would be required, from the permitting and operation of injection sites to the carbon pricing that would provide the impetus for generators to participate.

Still, the authors conclude their work "suggests that geologic storage supply will enable [carbon capture and storage] to play a major role within the portfolio of climate-change mitigation options."

PNAS, 2012. DOI: 10.1073/pnas.1115347109 (About DOIs).

Environmental Science & Technology, 2012. DOI: 10.1021/es2040015 (About DOIs).