Characterization and treatment of Bakken oilfield produced water as a potential source of value-added elements

The oilfield produced water is a major waste stream in places where shale-gas production is growing rapidly. The reuse of produced water merits consideration because this practice helps reduce freshwater demand for fracking and moderates water pollution. Knowledge about the chemistry of produced water is needed to develop sustainable treatment/reuse strategies and set standards for acceptable levels of treatment of produced water. Thus, the author performed the first comprehensive analysis of oilfield produced water collected from the Bakken shale play in the U.S. state of North Dakota that represents the nation's third-largest net increase in proven crude oil reserves. The concentrations of a total of 36 elements in 13 IUPAC groups were determined. Among them, a few metals that are critical to the economy of the United States were detected at elevated concentrations (median, mg/L): K (7,620), Mg (2780), Sr (1610), Li (69), and Mn (33). Heavy metals essential for plants and animals, including Cu, Zn, and Mn, were detected at ppm levels. Measurable concentrations of highly toxic metal ions such as Cd and Pb were not detected. Concentrations of rare earth elements and platinum group metals were below respective detection limits. The produced water samples had very high total dissolved solids (237,680 ± 73,828 mg/L) and total hardness (>31,000 mg/L as CaCO₃) but an extremely low alkalinity (152.4 ± 184.9 mg/L as CaCO₃); therefore, softening by lime and soda was ineffective. Softening by caustic soda removed 99.5% hardness ions (Ca and Mg) under alkaline conditions. This study provides vital insight into the chemistry and treatability of produced water containing various metals.