Fracking technology has already made it practical to exploit previously inaccessible natural gas and oil in the United States (see “Natural Gas Changes the Energy Map”). Now several companies are demonstrating a way to use microörganisms that eat coal and excrete methane—the main ingredient in natural gas—as a possible means of extracting fuel from coal resources that had been too expensive to mine.

Many coal beds contain large amounts of methane that can be harvested by drilling wells. In recent decades, researchers have demonstrated that a large fraction of the natural gas found in the coal beds is produced by naturally occurring microörganisms that feed on coal, and they have found ways to stimulate the microbes to produce more methane. Luca Technologies, based in Golden, Colorado, is using this approach to increase production from coal beds with existing methane wells. Another company, Next Fuel, based in Sheridan, Wyoming, recently showed that it could use similar technology to produce methane from coal beds that didn’t already have methane in them, raising the possibility that vast amounts of coal that’s currently too expensive to mine could be converted into natural gas.

Though the idea of microbial production is not new, says Julio Friedmann, chief energy technologist at the Lawrence Livermore National Laboratory, the technology has taken great strides in recent years, in large part because researchers know more about the different microörganisms that work together to digest coal and produce methane. “I know a handful of companies working on those technologies that seem to have pretty good recovery of natural gas at pretty good cost,” he says. That, he adds, creates a potentially significant market opportunity, and “I wouldn’t have guessed that a couple of years ago.”

One of the challenges has been understanding exactly which microörganisms live in a particular coal bed and what nutrients to pump into it to encourage the growth of the methane producers while discouraging the growth of others that compete with them. Bob Cavnar, CEO of Luca Technologies, says improved DNA sequencing has been essential. It allows the company to quickly sample a coal bed, determine which organisms are present, and tailor its mix of nutrients. “It used to take three months between sampling and enhancing,” he says. “Now within three days I know that a particular site has microbes. Within 14 days, I can figure out how to feed them.”

Cavnar says that because his company enhances existing wells rather than drilling new ones, its costs are low enough to make the approach profitable in the United States even with today’s very low natural-gas prices. But it could be even more promising in Asian countries such as China, where prices for natural gas are far higher and where natural gas could provide a far cleaner alternative to the coal that dominates electricity production. Next Fuel, for one, is focusing its efforts in Asia—the first large-scale demonstrations of its technology were in Indonesia and China.

At this point, it’s too early to know how much methane these techniques could produce. Cavnar says that if the company can convert even one-third as much coal into natural gas as lab tests have demonstrated, it would create reserves in the Powder River Basin in Wyoming and Montana that are almost as large as key shale gas reserves opened up by fracking.

But Friedmann says there are too many variables to make accurate guesses about microbial methane production from coal. “Those reserves are essentially unaccessed,” he says. “China has the world’s third-largest coal reserve. How much of that you could turn into something else by some modern technology, nobody knows.”