Non-Target Screening of Organic Compounds in Offshore Produced Water by GC×GC-MS

Abstract

Produced water is the largest by-product of oil and gas production. At off-shore installations, the produced water is typically reinjected or discharged into the sea. The water contains a complex mixture of dispersed and dissolved oil, solids and inorganic ions. A better understanding of its composition is fundamental to 1) improve environmental impact assessment tools and 2) develop more efficient water treatment technologies. The objective of the study was to screen produced water sampled from a producing field in the Danish region of the North Sea to identify any containing organic compounds. The samples were taken at a test separator and represent an unfiltered picture of the composition before cleaning procedures. The analytes were isolated by liquid-liquid extraction and derivatized using a silylation reagent to increase the volatility of oxygenated compounds. The final extracts were analyzed by comprehensive multi-dimensional gas chromatography coupled to a high-resolution mass spectrometer. A non-target processing workflow was implemented to extract features and quantify the confidence of library matches by correlation to retention indices and the presence of molecular ions. Approximately 120 unique compounds were identified across nine samples. Of those, 15 were present in all samples. The main types of compounds are aliphatic and aromatic carboxylic acids with a small fraction of hydrocarbons. The findings have implications for developing improved environmental impact assessment tools and water remediation technologies.

Critical review of the OSPAR risk‐based approach for offshore‐produced water discharges

“…A study used a nontarget screening of chemicals in the aqueous phase of PW and detected around 1500 organic compounds in each sample (Bergfors et al, 2020). Only a small fraction could be identified, leaving more than 1000 compounds unknown (Bergfors et al, 2020), not including those of the particulate phase of PW. The knowledge gap on PW composition seriously questions the accuracy and general usability of the SB approach, as it cannot include the UCM in the hazard assessment, and chemicals present at only a fraction of their toxic level can still contribute to combined toxicity (Deneer et al, 1988).…”

“…In practice, it is not achievable to include all PW constituents in the SB approach, as PW samples often contain >1000 different compounds (Bergfors et al, 2020). Instead, PW is represented by a shorter list of compounds, containing those assumed to be the major contributors to its ecotoxicity.…”

“…A fraction of PW composition remains unknown, sometimes referred to as an "unresolved complex mixture (UCM)" (Sørensen et al, 2019). A study used a nontarget screening of chemicals in the aqueous phase of PW and detected around 1500 organic compounds in each sample (Bergfors et al, 2020). Only a small fraction could be identified, leaving more than 1000 compounds unknown (Bergfors et al, 2020), not including those of the particulate phase of PW.…”