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

Abstract

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 Ideally, water should be withdrawn only from isolated coal aquifers to prevent any unnecessary water withdrawal from overlying or underlying aquifers.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 87 Sr/ 86 Sr of ground water to identify hydraulically isolated coal seams. The ratio of 87 Sr/ 86 Sr 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 pro duced, 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 connectiv ity, 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.