Influence of High Total Dissolved Solids Concentration and Ionic Composition on γ Spectroscopy Radium Measurements of Oil and Gas-Produced Water

Abstract

Radium measurements in high total dissolved solids (TDS) fluids from oil and gas extraction can have unfavorable precision and accuracy, in part because these high-level impurities incur attenuation. γ spectroscopy is often recommended for determining radium activities in these fluids, but even this method can produce a range of reported activities for the same sample. To reduce measurement duration and to maintain or improve accuracy, we propose a method to rapidly assess both ²²⁶Ra and ²²⁸Ra and to account for the self-attenuation of γ rays in high-TDS oil and gas fluids when they are monitored by a well detector. In this work, comparisons between a NaCl-only and a multi-cation-chloride synthetic brine spiked with known amounts of ²²⁶Ra and ²²⁸Ra indicated that both the TDS concentration and the type of TDS (i.e., Na only vs Na-Mg-Ba-Ca-Sr) influenced self-attenuation in well-detector γ spectroscopy, thus highlighting the need to correct for this TDS-influenced self-attenuation. Radium activities can be underestimated if the correction is not applied. For instance, ²²⁶Ra activities could be ∼40% lower in a sample when measured directly at the 186 keV energy level if the attenuation of the high TDS of the fluid is not considered. We also showed that using a NaCl-only brine to match the matrix of high-TDS oil and gas brines is inadequate to produce accurate measurements, rather, the full set of cations should be included.