Geochemical fingerprinting and statistical variation of 35 elements in produced water and rock material from offshore...

Trace metals and metalloids occur in small quantities in the subsurface water generated from oil wells, called produced water (PW). While these substances are present in low concentrations, PW volumes are sufficiently large that they are still a potential environmental concern. This study has focused on quantifying 71 trace metals and metalloids present in PW from Danish offshore oil production sites. These metals are often a challenge to measure and are globally underreported. By employing optimized sample treatment combined with ICP-OES and ICP-SFMS methods, the full elemental screening of PW samples collected from various offshore platforms has been carried out with high accuracy. Distinct geochemical signatures involving 35 elements have been discovered and they are associated with significant site-specific variations in the concentrations of key trace metals, including W, Ba, Mo, Cu, and Tl. Utilizing Principal Component Analysis (PCA), the study has effectively distinguished between PW samples from different fields, highlighting the relevance of certain trace metals and elemental ratios as potential geochemical markers. Geochemical analysis of the chalk rock material from the same production wells as the fluid samples has shown a correlation of key elements Tl, W, Cu, Mo, Ba, and As in the chalk with the produced water, potentially indicating the origin of the metals. The study has revealed a high compositional variability of PW and found that elements including Zn, Co, Hg, and Cs occur in concentrations of magnitude higher than previous estimates from reports. In addition, there is high variability in concentrations at different sampling times, underlining the need for environmental monitoring and developing more informed management strategies for the main offshore PW stream. The variability in concentrations in space and time leads to large uncertainties in environmental reporting based on a few samples. The detailed sampling campaign reported here for the first time highlights the need for much more frequent sampling, ideally continuous monitoring. The safety of produced water discharge to sea can be significantly underestimated by limited sampling. This paper provides the first field-specific and time varied screening of heavy metals in real produced water and shows the discrepancy in our understanding of the environmental impact of PW.