Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Campbell, C. E. Articles by Frost, C. D. Search for Related Content GeoRef GeoRef Citation

Strontium isotopes as indicators of aquifer communication in an area of coal-bed natural gas production, Powder River Basin, Wyoming and Montana

¹ Department of Geology and Geophysics, The University of Wyoming, Dept. 3006, 1000 East University Avenue, Laramie, Wyoming 82071, U.S.A.
² EnCana Oil and Gas (USA) Inc., 370 17^th Street, Suite 1700, Denver, Colorado 80202
³ Crimson Exploration, 717 Texas Avenue, Suite 2900, Houston, Texas 77002

^* Correspondence should be addressed to: frost{at}uwyo.edu.

Development of the coal-bed natural gas resource of the Powder River Basin of Wyoming and Montana has proceeded rapidly, from fewer than 200 wells in 1995 to more than 22,000 wells in 2007. Continued development of this resource will depend on minimization of water production during gas recovery as well as responsible use of the produced water. Ideally, water should be withdrawn only from isolated coal aquifers to prevent any unnecessary water withdrawal from overlying or underlying aquifers. This study uses the ratio of ⁸⁷Sr/⁸⁶Sr of ground water to identify hydraulically isolated coal seams. The ratio of ⁸⁷Sr/⁸⁶Sr of ground water represents a time-integrated record of water–rock interaction, such that water from aquifers composed of different rocks may acquire different Sr isotopic ratios.

Sr isotopic data are presented for 145 samples of ground water co-produced with coal-bed natural gas and 14 water samples from wells completed in sandstone aquifers in the Powder River Basin. The coal zone from which each sample was collected was determined by analysis of gamma logs and correlation with the Wyoming State Geological Survey database.

The Sr isotopic ratios and geochemical compositions of ground waters from coal in the Powder River Basin of Wyoming are influenced by a number of factors, including the coal zone from which ground waters are produced, their residence time, the degree to which coal aquifers are confined, and geographic location. The data indicate that the Upper Wyodak coal-zone aquifer in the Gillette and Schoonover areas in the eastern Powder River Basin appears to be a well-confined, combined sand and coal aquifer unit. In contrast, the Wyodak Rider coal zone aquifer may be only partially confined, allowing interactions between sandstone and possibly other coal aquifers. Wells in this area exhibit highly variable Sr isotope ratios and total dissolved solids, and they also are characterized by greater than average water/gas production ratios, consistent with incomplete isolation of the Wyodak Rider coal zone. Faults in the northeastern part of the Powder River Basin may affect aquifer connectivity, either by acting as seals or conduits. Higher gas production correlates with lower Sr isotopic ratios in this part of the basin. Although a correlation between Sr isotopic ratios of produced water with fracture pattern developed during the well enhancement process might be expected, no strong relationship was observed. Evidently there are many factors in addition to fracture pattern that control interactions between aquifers.

Key Words: aquifer communication • coal-bed natural gas • Powder River Basin • produced water • sodium-adsorption ratio • strontium isotopes • water quality