Influence of High Total Dissolved Solids Concentration and Ionic Composition on γ Spectroscopy Radium Measurements of Oil and Gas-Produced Water
journal contributionposted on 19.08.2019, 21:43 by Moses A. Ajemigbitse, Travis L. Tasker, Fred S. Cannon, Nathaniel R. Warner
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.