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Sr and C Isotopic Analysis of Waters Produced from Coal Beds, Atlantic Rim, Wyoming: Predictive Tools for Optimizing Coal Bed Natural Gas Production

McLauglin, Fred 1; Frost, Carol 2; Sharma, Shikha 3
1 Wyoming State Geological Survey, Laramie, WY.
2 Geology and Geophysics, University of Wyoming, Laramie, WY.
3 Environment and Natural Resources, University of Wyoming, Laramie, WY.

Coalbed natural gas (CBNG) production requires extraction of large volumes of water from target formations, which may alter local aquifers and large hydrologic systems. Previously in Wyoming CBNG production outpaced scientifically-defined baseline data of CBNG targeted aquifers. The Atlantic Rim area in south-central Wyoming is a newer CBNG play. In cooperation with the Rawlins BLM, we sampled springs, streams, and CBNG wells across the Atlantic Rim to quantify regional hydrogeology before full-scale CBNG production. Field measurements, chemical analysis, and C and Sr isotopic analysis were used to characterize each water sample. We focused efforts on the Mesaverde Group, the coal-bearing formation target for CBNG production, but also sampled from associated younger and older confining horizons.

87Sr/86Sr ratios identify water/rock interactions. Stream water sourced from the Sierra Madre and spring samples from the Mesaverde Group have the highest 87Sr/86Sr ratios, from 0.7120 to 0.7122, and spring samples from the Lewis and Steele Shales record the lowest 87Sr/86Sr ratios, from 0.7086 to 0.7091.

δ13CDIC ratios distinguish waters of different sources along the Atlantic Rim. Waters from coalbeds associated with microbial production of methane have positive δ13CDIC ratios, up to 22.62‰. By contrast, the δ13CDIC values of other natural waters are negative, from -7.68‰ in streams to -14.95‰ in springs. Measured δ13CDIC values of some samples vary from -2.78‰ to 13.33‰, indicating that the source of water emerging from these springs includes a component from Mesaverde coalbeds.

The δ13CDIC values of CBNG well samples also identified which coalbed aquifers were hydraulically closed, and which coalbeds were hydraulically open. The Sun Dog Pod had the highest δ13CDIC values, with an average of 20.44‰, and the lowest water:gas ratios, both indicating an isolated hydrologic system. Wells sampled from the Brown Cow Pod had the lowest δ13CDIC values, with an average of 2.18‰, and also had the highest water:gas ratios, along with chemical compositions similar to non-coal waters, indicative of an open aquifer system.

Measured δ13CDIC values may identify wells which tap isolated coal beds with optimal water:gas ratios and high gas potential. Ongoing, periodic resampling and isotopic analysis of Atlantic Rim waters co-produced with natural gas will enable us to assess the extent to which isotopic data also allow predictive modeling of lifespan of production.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009