Now that the EPA has set limits on carbon-dioxide emissions from new power plants, there are basically two options left for generating electricity from fossil fuels in the future. First are modern natural-gas plants, which can easily meet the EPA’s standards. Second, in theory, are coal-fired plants that can capture their carbon and store it deep underground.

Carbon capture just got a little harder. (Ralph Wilson/Associated Press)

And the current natural-gas boom makes that task much harder. In a recent study in Environmental Science & Technology, Elliot and Celia find that as much as 80 percent of the suitable basins for carbon storage also overlap with shale-gas and tight-gas fields. The hydraulic fracturing (or “fracking”) techniques that drillers use to extract gas could poke holes in that caprock — potentially rendering them unusable for carbon sequestration. Here’s a map showing the overlap of areas:

(Thomas Elliot and Michael Celia)

And forget making one project work. Carbon-capture would need to scale. Which means there has to be enough space underground to store all that pollution. Power plants in the United States emit roughly 2 billion metric tons of carbon dioxide each year. Converted into a dense liquid form, that’s about 80 million barrels of compressed carbon dioxide that needs to be stored each day. (To get a sense for how enormous that is, the entire country consumes 20 million barrels of oil per day.)

In March, MIT geoscientist Ruben Juanes’s group published a study in the Proceedings of the National Academy of Sciences finding that there appeared to be enough storage capacity in underground saline aquifers to store a century’s worth of carbon-dioxide emissions from U.S. coal-fired power plants.

Elliot and Celia’s study, however, suggests that the fracking boom could make these potential sites more vulnerable to leaks. “As such,” they write, “shale gas production is in direct conflict with the use of shale formations as a caprock barrier to CO2 migration.” They do caution, though, that further geological surveys are needed to determine just how much storage capacity is at risk — that 80 percent overlap is a worst-case scenario.

This does raise another recurring worry about stashing our carbon-dioxide emissions deep underground. That carbon essentially has to be secured for all eternity. (Nuclear waste looks easy by contrast, given that it decays after about 100,000 years or so.) If the carbon escapes, it would start heating the planet. What’s more, a leak near a residential area could be dangerous: While carbon dioxide isn’t directly harmful to humans, it’s heavier than oxygen, which means that a significant release could displace breathable air over a wide area. Shale fracking could, potentially, exacerbate all of these concerns.

Now, it’s still possible that fracking and coal can find ways to coexist. A closer look at these basins may reveal that fracking isn’t as intrusive as thought. What’s more, as Michael Levi of the Council on Foreign Relations has explained, if the United States ever gets serious about tackling global warming, then it might end up leaving a lot of natural gas in the ground anyway. (After all, natural gas may be only half as carbon-intensive as coal, but it’s still a fossil fuel.) It’s conceivable that, in the future, we would drill a little natural gas in some areas and store sequester carbon dioxide underground in others. But right now, these two popular fossil fuels appear to be in conflict.