T330 Produced Waters: Characterization and Impacts of Subsurface Brine and Formation Water Associated with Hydrocarbon Production

Wednesday, 30 October 2013
8:00 AM - 12:00 PM
Room 406

BORON DESORPTION AND BORON ISOTOPES IN SODIUM BICARBONATE COALBED METHANE WATERS

Paper number: T330-2
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

Coalbed methane (CBM) waters commonly exhibit Na-bicarbonate composition when the methane is of a microbial origin. The evolution of anoxic, Na-bicarbonate groundwater is characterized by Ca-Na cation exchange, a process linked in many aquifers to microbial CO2 production and carbonate dissolution. In general, the development of Na-HCO3- waters can coincide locally with naturally-occurring water quality problems, including sodium, arsenic, fluoride, and/or boron. Boron isotope ratios are connected to mineral surface charge conditions by the fractionation between boron species, uncharged B(OH)3 (enriched in 11B) and B(OH)4- (enriched in 10B). Boron desorption typically mobilizes more 10B from the mineral surface, causing lower 11B/10B ratios in water.

To investigate the behavior of B during the evolution of Na-bicarbonate microbial CBM waters, boron concentrations and isotope ratios (δ11B as ‰ NBS951) were determined in groundwater from the Powder River Basin (PRB) in Montana and Wyoming. Preliminary results from CBM production wells indicate generally low B concentrations (42-178 µg/L) and variable δ11B (24.4-35.2‰). Sodium (358-752 mg/L) and HCO3- (930-2010 mg/L) concentrations negatively correlate with δ11B, implying that δ11B may record the extent of B desorption during the evolution of CBM waters. Given that the higher-TDS Na-HCO3- waters are associated with lower δ11B, we suggest that preferential B leaching from desorption sites is associated with the base-exchange reactions and microbial respiration that characterize CBM waters. The lack of apparent relationship between B concentration and δ11B may imply that the quantities of solid-phase B vary within the aquifer, and that δ11B records the extent to which desorption occurs. These isotope trends indicating boron desorption are consistent with trends documented in Na-bicarbonate groundwater in the Atlantic Coastal Plain exhibiting high concentrations of marine-derived boron. Overall, B isotopes could be a valuable tracer for evaluating the evolution and distribution of CBM waters.