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Coal mining has always been one of the most dangerous professions. One of the most insidious dangers was gas: Odorless methane could seep into the tunnels and poison the miners as they worked.
In time, a low-tech solution was discovered. Canaries are much more susceptible to methane than people, so the birds were kept deep in the mines. As long as the canaries lived, the danger from methane was minimal.
In recent years, coalbed methane has been seen less as a curse than as an opportunity. Operators have found it profitable to draw the gas from these deep seams for use as fuel. Indeed, coalbed methane accounts for one-twelfth the natural gas production in the United States.
Now a company in Colorado has shown evidence experimentally that microbes are responsible for some of the methane produced in coalbeds. Not only do the microbes digest coal in its native environment, but they also may be capable of being transferred into fresh coal seams and thrive there. It is possible, scientists with the company believe, that those microbes could turn coal layers that are impossible to mine into gas-producing regions.
One of the most productive coalbed methane regions is the Powder River Basin, the semiarid region in Wyoming and Montana that is the single largest source of low-sulfur coal in the United States. It's a landscape that's covered with sagebrush and pocked with surface mines. One estimate puts the total coal resource in the area at some 800 billion tons. Well more than 300 million tons are mined there annually, enough to fill 8,000 railway cars each day.
Although they have very different public profiles—one is a so-called clean fuel, the other is the dictionary definition of dirty industry—methane goes hand-in-hand with coal. The gas permeates the coalbed. Depending on the structure of the formation, a volume of coal can contain six or seven times as much methane as the same volume in a conventional sandstone reservoir.
The gas coats the surface of the mineral, which is riddled with fractures called cleats. But reduce the hydrostatic pressure that's causing this adsorption, and the methane releases from the cleat surfaces and begins to flow through the rock formation.
Coalbed methane production in the Powder River Basin grew out of the inability to recover coal from many seams. But the industry didn't take off until the 1980s, when a provision in a renewable energy law provided incentives for companies to tap the resource. The Clinton administration later encouraged the capture of coalbed methane as a means of mitigating global warming. (Pound for pound, methane is a more worrisome greenhouse gas than is carbon dioxide.) All told, the Powder River Basin's coal seams hold nearly 40 trillion cubic feet of technically recoverable gas, or about 20 percent of the total gas reserves in the United States.
That fits the larger trend of looking for natural gas in unconventional places, such as landfills and deep-ocean ice. After becoming a viable resource only in the 1980s, coalbed methane production has soared from just 100 billion cubic feet in 1989 to more than 1,600 billion cubic feet in 2003. Similarly, the proven reserves of coalbed methane have increased as more and more seams are being examined. The U.S. Energy Information Agency reported that there were fewer than 4 trillion cubic feet of proven reserves in 1989; by 2003, that figure had moved to almost 19 trillion cubic feet. Ultimately, the economically recoverable resource using present technology could top 100 trillion cubic feet.
At present, coalbed methane reserves are thought of in much the same terms as those found in other fields—legacies of millions of years of geologic processes. But scientists at Luca Technologies in Denver have reported evidence that this may not be the entire story. Instead, some of the natural gas found in these seams may be generated today by microbes digesting the coal.
Geologists have suspected for some time that the methane produced in relatively shallow coal seams was "young"—less than 10,000 years old—and the result of some sort of biological activity. But most of the gas, it was thought, was the result of thermal decomposition of the coal: Over time, heat and pressure would shred the carbon molecules found in coal to produce natural gas.
Researchers at Luca Technologies felt this might not be the whole story. Microscopic creatures called Archaea that have been discovered in extreme environments—volcanic hot springs and deep sea thermal vents—produce methane as a by-product. Some researchers have even posited that the natural gas found in marine sediments and methane hydrate deposits were the products of biological processes.
"We put two and two together and came to the conclusion that geologically 'young' might be much, much, much younger than 10,000 years," said Mark Finkelstein, vice president for biosciences at Luca Technologies. In other words, might similar microbes be at work in coal seams today, turning coal into gas?
To find out, Luca obtained 20-foot-long core samples from coal seams in the heart of the Powder River Basin. Segments of the core were kept in oxygen-free containers under controlled conditions, and small samples were crushed to make a slurry. The coal samples were then experimentally treated in a number of ways: nutrients were added to some, oxygen was added to others. Over the course of about five months, the samples were probed for signs of activity.
What the researchers found was astonishing. Methane was being produced at a steady clip in some of the samples in real time. Indeed, at the rate of production found in the lab, the gas found naturally in a typical section of Powder River basin coal could have been produced in less than seven years.
But was this gas the product of microbes or of some non-biological process? Results from other samples are suggestive. Samples exposed to oxygen, which is known to kill methane-producing microbes, generated almost no natural gas, while samples infused with nutrients had accelerated gas production. The best results were equivalent to almost 90 cubic feet of natural gas per ton of coal produced in only five months.
If borne out, these findings could lead to some interesting benefits. First, by matching underground conditions with those most conducive to growing methane-belching microbes, geologists could focus on the coal seams that were the most likely to generate large amounts of gas. Understanding the biology of the underground environment might also enable drillers to preserve the gas-producing microbes, turning coalbed methane into something of a renewable resource. (At present, coalbed methane extraction involves pumping out the groundwater that infuses these seams; by introducing air to these environments, this technique may well kill the microbes that are generating the gas.)
More importantly, once the best methane-producing microbes are isolated, samples could be injected into underground environments that may have the right combination of temperature, pressure, and nutrients, but don't yet produce natural gas. Luca's researchers also have identified depleted oil fields and deep shale strata as potential biogas-producing regions. They plan to do field tests of their idea as early as this summer.
"It's a reasonable transition to go from what you can do in a laboratory bottle to what you can do in the field," Finkelstein said, "but the coal cores came from the ground, the water came from the ground, and the microbes came from the ground. The next step is to go out in the field and give it a try."
Indeed, it could lead to a whole new way of thinking about gas—not as a fossil fuel, but as an almost agricultural product. We could someday grow natural gas in underground farms.
"Instead of having to dig the coal up, we could use coal beds as giant biological reactors," Finkelstein said. "We could get more energy out of the ground than what is being mined today and do it in a more environmentally benign fashion."