Strontium Isotopes as Tracers in Groundwater Systems Related to Coalbed Methane Production, Powder River Basin, Wyoming

B. N. Pearson and C. D. Frost
University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY

The Powder River Basin of northeastern Wyoming and southern Montana contains a large portion of the nation’s coal reserves in the Tongue River Member of the Paleogene Fort Union Formation.These coal horizons also contain significant reserves of biogenic methane in the water of these coals. Conservative estimates place these reserves at 37 trillion cubic feet of gas, an amount equal to the gas reserves in the Gulf of Mexico.

Coalbed methane production involves dewatering the target horizon to reduce the pressure and allow methane to return to a free state. A typical well produces 11,000 gallons of water per day that is discharged to the surface. The rapid development in the Powder River Basin (152 wells in 1995 to 4800 in late 2000) has outpaced the monitoring of the effects on the geology, hydrology, and environment.

This study seeks to use the strontium isotopic composition of produced groundwater to characterize coal and sandstone aquifers and monitor the impact of coalbed methane development on adjacent aquifers. Groundwater acquires the strontium-87 to strontium-86 ratio by dissolution of geologic material and ion exchange along the flow path. Thus, the ⁸⁷Sr/⁸⁶Sr ratio provides a record of water rock interaction during recharge and transport. Waters produced from coal horizons generally have a higher and more variable ratio than sand aquifer waters.With further study and establishment of baseline conditions in developing fields, this tracer could provide a powerful tool for the study of the effects of methane production on aquifer communication and recharge patterns.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid