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UW Researcher Discovers Way to Trace Coal-Bed Methane Water


April 22, 2009 — Little critters in the darkness of coal seams helped lead a University of Wyoming researcher to find a way to trace coal-bed methane (CBM) water and could help energy companies be more efficient and reduce impact footprints.

The research by Shikha Sharma, associate director of the UW Stable Isotope Facility in the College of Agriculture, analyzes the ratio of carbon 13 to carbon 12 isotopes to identify CBM water. A patent on the process is pending.

"Carbon works well because in the coal seams there are lots of methanogens - bacteria - small critters that feed on the coal substrate and produce methane," said Sharma, a research scientist in the Department of Renewable Resources. Through their munching methanogens leave heavier carbon behind in the coal zones. CBM water is highly enriched in carbon 13.

Sharma uses a carbon isotope ratio of dissolved inorganic carbon to help identify the CBM water. The ratio of carbon 13 to carbon 12 in common water is -11 to -15 per milliliter, while the ratio in CBM water is +12 to +22, a very significant difference.

Energy companies must pump water from coal seams to allow methane to escape. Water production from individual wells varies, but on average wells produce 5 to 20 gallons of water per minute, which is about 7,200 to 28,800 gallons of water per day.

"If CBM operators aren't drilling in the right seam, they are pumping water from aquifers under or above the seam," said Sharma. "They would be wasting a lot of groundwater and increasing their drilling costs."

A simple test for carbon 13 can determine if a well is being drilled in the right coal seam

"This could help them in optimizing drilling costs and produce methane with a minimum footprint," Sharma said.

The research is an offshoot of a U.S. Department of Energy grant funded to Carol Frost, a professor in the UW Department of Geology and Geophysics, and Sharma. The aim was to determine how environmental tracers, including Strontium and stable isotopic compositions of oxygen and hydrogen, could be applied to document the sources and causes of salinity in the Powder River.

Using carbon as a tracer has several benefits over others, says Sharma. Oxygen and hydrogen are greatly affected by evaporation and dilution, and significant contribution of strontium from local rock formations overprints the pristine signatures of CBM water.

In case of carbon isotopes, the difference between two end members (methanogenic and non-methanogenic water) is very high. Fingerprinting CBM water using a carbon isotope is possible even when surface waters are inundated with large amounts of snowmelt. Sharma tested three groups of water samples from the Powder River Basin.

First, samples were taken of CBM water from wellheads in different parts of the basin to see if different coal zones could be distinguished based on their carbon isotope signatures. The preliminary data shows there are prominent differences in carbon isotope values of water coming from different coal zones.

Next, surface water was analyzed along the length of the Powder River from its headwaters west of Casper to its confluence with the Yellowstone River in Montana to determine if CBM water discharged into drainages could be traced into major river systems. The water samples show high carbon isotope ratios along the Powder River in areas affected by the most intense CBM development but flatten as the water nears the Yellowstone River.

A third group was taken from a CBM outfall, retention pond, and upstream and downstream monitoring wells near Beaver Creek, a tributary of the Powder River, to establish if carbon isotope signatures could be used to track the seepage of CBM water into shallow ground waters of the area.

The samples tested by Sharma were originally collected by geology and geophysics graduate students Liddi Brinck of Bremerton, Wash., Cat Campbell of Independence, Kan., Shaun Carter of Avon, Conn., and Jason Mailloux of Long Island, N.Y.

The aim was to determine how environmental tracers, including stable isotopic compositions of oxygen and hydrogen, could be applied to document the sources and causes of salinity in the Powder River.

"Our preliminary results show the carbon isotope signatures of the waters can be used to fingerprint contribution of methanogenic waters to groundwaters and surface waters of the region and can therefore help in solving regulatory issues related to discharge of CBM waters," said Sharma.

Results of the study were first published in the March-April 2008 edition of the journal Ground Water. It is available for viewing here.

Photo:
Shikha Sharma measures the carbon isotope composition of water using a mass spectrometer in the UW Stable Isotope Facility.

Posted on Wednesday, April 22, 2009

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