The geochemical evolution of water coproduced with coalbed natural gas in the Powder River Basin, Wyoming

Water produced with coalbed natural gas (CBNG) attains its characteristic sodium-bicarbonate composition through a series of processes, including dissolution of salts, precipitation of salts, pyrite oxidation, ion exchange, sulfate reduction, and methanogenesis. After CBNG-produced water is discharged to the surface, interaction with the atmosphere will initiate the precipitation of calcite, iron hydroxide, and barite among other minerals. The interaction of CBNG-produced waters with semiarid Powder River Basin soils can mobilize accumulated salts,which, through infiltration, can then reach the water table, potentially affecting the quality of the groundwater. Themobilization of the soil-based salts may render the composition of the water recharging the near-surface groundwater very different from the initial chemical composition of theCBNG-producedwater. Additionally, prolonged exposure toCBNG-producedwater can cause the salinization and sodification of soils surroundingCBNG-produced water ponds and streams carrying CBNG-produced water. This can impact the quantity of biomass and the species composition of the vegetation in proximity to CBNG-produced water discharge locations. The high sodium to calcium and magnesium ratio in CBNGproducedwater requires carefulmanagement to prevent sodification of irrigated soils when it is used as an irrigation source. In many instances, irrigation with CBNG-produced water requires the addition of soil amendments such as gypsum and sulfur to maintain the fertility and physical qualities of the soil. An understanding of the geochemical evolution of CBNG-producedwater is necessary to anticipate and address these potential environmental issues associated with production of CBNG.