SR ISOTOPIC TRACING OF EFFECTS OF COAL BED METHANE DEVELOPMENT ON SURFACE AND SHALLOW GROUND WATER SYSTEMS, POWDER RIVER BASIN, WYOMING

The Powder River Basin of eastern Wyoming and Montana is the largest coalbed-methane producing basin in the United States. Coalbed methane production is accompanied by the extraction of large volumes of water, as coal seams must be depressurized by removal of water to release methane either bound to coal or in solution. In April 2003, 47 million barrels of water were discharged from 11,243 coalbed methane wells in the Wyoming portion of the Powder River Basin. The purpose of this study is to use Sr isotopes to fingerprint water produced from coal aquifers and to trace its movements in surface and shallow ground water systems.

Our two study areas include the Beaver Creek watershed in the eastern part of the basin and the Coal Creek drainage in the western part. At Beaver Creek the shallow aquifers yield ground waters with 87Sr/86Sr of 0.7128, in contrast with ground water from the methane-producing Paleocene Wyodak-Anderson and Big George coals, which have 87Sr/86Sr of 0.7140. The water extracted from the coal seams is discharged into 2 reservoirs within the Beaver Creek drainage. The Sr isotopic ratios of Beaver Creek surface water samples are intermediate between the ambient alluvial and deep coal aquifer waters (0.7134 to 0.7138), reflecting the input of coalbed water. Within one month of commencement of coalbed water production, alluvial well waters at 25 ft depth downstream of the upper reservoir also showed a shift in Sr isotope ratio towards more radiogenic ratios (87Sr/86Sr=0.7134).

At the Coal Creek site, the surface and shallow alluvial waters are more radiogenic than at Beaver Creek (87Sr/86Sr=0.7136 to 0.7141), probably reflecting the more radiogenic Sr detritus shed into the western side of Powder River basin from the Archean-cored Bighorn Mountains. The coals in this portion of the basin include the Eocene Felix coal, which yields ground waters with the lowest Sr isotopic ratio of any coal aquifer waters in the basin (87Sr/86Sr=0.7115 to 0.7119). Because of the difference in Sr isotopic ratios between shallow and coal aquifer waters at this site, it is expected that this fingerprint will be useful in tracing the movement of coal aquifer water when production begins in August 2003.