TRACE ELEMENT DETERMINATION IN PRODUCED WATERS FROM CONTIGUOUS UNITED STATES HYDROCARBON RESERVOIRS WITH ICP-OES: THE ADVANTAGES OF A HIGH SALINITY TOLERANCE

The rapid and efficient development of hydrocarbon production from continuous reservoirs during the past decade has resulted in significant interest in the associated produced waters. Produced waters are generated during hydrocarbon extraction and constitute a major waste stream due to the high volumes, estimated at >500 billion gallons/year across the United States (US), and the high salinities, often ranging from 35 g/L to greater than 350 g/L, depending on the reservoir. Data on the trace element composition of produced waters is required for understanding the potential environmental impacts from spills, the presence of mineral commodities, and for basic geochemical interpretation of reservoir fluids. Measuring trace elements in high salinity brines is challenging. In general, there are a dearth of methods that can provide direct chemical analysis of these complex, high salinity samples. Typical approaches for trace element determination in produced water use analytical methods, such as inductively coupled plasma – mass spectrometry (ICP-MS), developed and optimized for low salinity drinking water samples that require significant dilution, often up to 1000 times. As an alternative, an inductively coupled plasma – optical emission spectrometer (ICP-OES) equipped with a high salinity sample introduction system can be used to directly measure trace elements within high salinity samples. We present ICP-OES results demonstrating the detection for 17 trace elements (As, Al, Ba, Be, Cd, Cr, Co, Cu, Hg, Mo, Ni, Pb, Rb, Sb, U, V, and Zn) from 15 produced waters ranging in salinity from 17 to 370 g/L from the Bakken/Three Forks, Eagle Ford, Marcellus, Wolfcamp/”Cline”, and Utica reservoirs. The produced waters data from the ICP-OES were compared with ICP-MS data in order to understand the differences in the limits of detection, limits of quantitation, recovery from matrix spikes, and sensitivity to variation in major ion composition of the sample matrix. While ICP-MS has higher sensitivity than ICP-OES, many trace elements were undetected using this method. In contrast, the ICP-OES enabled the direct detection and quantification of a majority of the 17 trace elements. This work demonstrates the potential of the ICP-OES approach for conducting robust chemical analysis of complex, highly saline produced waters.