STABLE BROMINE ISOTOPIC COMPOSITION OF COAL BED METHANE (CBM) PRODUCED WATER

As a result of their generally conservative behavior, bromide and chloride compositions in natural waters have long been used as environmental tracers. While the distribution of stable bromine isotopes (⁸¹Br/⁷⁹Br) in natural waters remains a rudimentary snapshot; it is already clear that the stable bromine and chlorine (³⁷Cl/³⁵Cl) isotopic compositions in natural waters do not always correlate well. The range of stable bromine isotopic values is often much larger than theoretically expected, indicating fractionating processes and factors are still unknown. To help identify possible processes, this study will characterize the stable bromine and chlorine isotopic composition in a high-organic matter subsurface environment with active methanogenic bacterial populations. Produced water and gas from coal bed methane (CBM) wells perforated within the coal and shale units of the Cherokee Basin in southeastern Kansas, USA, were analyzed for δ³⁷Cl, δ⁸¹Br, and ⁸⁷Sr/⁸⁶Sr isotopic compositions at the University of Waterloo Environmental Isotope Lab. These data were combined with existing water and gas chemistry and isotopic composition data to characterize halogen isotopic composition within the CBM play.

Compared with seawater evaporation, a slight excess of bromide is evident in 3 of 15 samples. The range of δ³⁷Cl values was smaller than δ⁸¹Br values, with δ³⁷Cl between -0.81 and +0.68 ‰ (SMOC), and δ⁸¹Br values between -0.01 and +2.71 ‰ (SMOB). Sr isotopic data cluster in two groups, and correlate with fluid temperatures, providing an effective measure of cross-formational flow, yet trends with depth and Cl, Br, and Sr isotopes are not evident. A significant number of analysis at this site (60%) compared with the worldwide groundwater database (4%) sampled water with δ⁸¹Br values above 2.00‰. It is suspected that interactions of dissolved bromide with bromine in organic-rich units and/or gas phase are affecting the isotopic composition.