Partitioning of semi-soluble organic compounds between the water phase and oil droplets in produced water

When selecting produced water treatment technologies, one should focus on reducing the major contributors to the total environmental impact. These are dispersed oil and semi-soluble hydrocarbons, alkylated phenols, and added chemicals. Experiments with produced water have been performed offshore on the Statoil operated platforms Gullfaks C and Statfjord B. These experiments were designed to find how much of the environmentally relevant compounds were dissolved in the water phase and not associated to the dispersed oil in the produced water. Results show that the distribution between the dispersed oil and the water phase varies highly for the different components groups. For example the concentration of PAHs and the C6-C9 alkylated phenols is strongly correlated to the content of dispersed oil. Therefore, the technologies enhancing the removal of dispersed oil have a higher potential for reducing the environmental impact of the produced water than previously considered.     

Chemical composition and acute toxicity in the water after in situ burning--a laboratory experiment

The chemical composition and toxicity of a water soluble fraction (WSF) of oil versus the underlying water after in situ burning (ISB), has been studied in a laboratory experiment. A system for allowing water sampling after ISB was developed. Seawater samples and oil were collected prior to and immediately after ISB, and chemical analysis was conducted. The chemical characterization of the water showed that the disappearance of water soluble oil components during ISB was insignificant. Acute toxicity tests with the marine copepod Calanus finmarchicus and Microtox® bioassay was performed to establish LC₅₀/EC₅₀ values of the water. The results were compared with regular WAF systems with unburned weathered oil, and indicated no increase in toxicity in the underlying water after ISB.     

Large-scale oil-in-ice experiment in the Barents Sea: monitoring of oil in water and MetOcean interactions

A large-scale field experiment took place in the marginal ice zone in the Barents Sea in May 2009. Fresh oil (7000 L) was released uncontained between the ice floes to study oil weathering and spreading in ice and surface water. A detailed monitoring of oil-in-water and ice interactions was performed throughout the six-day experiment. In addition, meteorological and oceanographic data were recorded for monitoring of the wind speed and direction, air temperature, currents and ice floe movements. The monitoring showed low concentrations of dissolved hydrocarbons and the predicted acute toxicity indicated that the acute toxicity was low. The ice field drifted nearly 80 km during the experimental period, and although the oil drifted with the ice, it remained contained between the ice floes.     

Composition of the water accommodated fractions as a function of exposure times and temperatures

The water accommodated fractions (WAFs) of nine oils in seawater have been studied. The oils range from light condensate to heavy crude, and include one highly biodegraded oil and one very wax rich oil. This study has identified large variations in the chemical composition of WAFs, depending on oil type, temperature, and mixing time. Experiments at different temperatures (2-13 °C) showed that it takes longer time to reach equilibrium at the lowest temperatures, and that this varies for the different oil types. Oils with higher pour point (wax rich oils) need a longer time to establish WAF in equilibrium than oils with lower pour points (naphthenic oils). At 13 °C a mixing time of 48 h, as recommended in standard procedures, seems to be sufficient for asphalthenic and paraffinic oils. The results demonstrated that for WAF prepared from an unknown oil, or at lower temperatures, different mixing times should be tested. Since the WAF often is used in toxicity testing, the toxicity might be underestimated if the mixing time is too short.