The effects of chemical dispersants on buoyant oil droplets

Abstract

Chemical dispersants are designed to disperse and dissolve surface slicks of crude oil and diesel by lowering the interfacial tension between the slicks and seawater. The effects of sub-surface application of dispersant during deep ocean oil spills are not well understood; however, very large quantities of dispersants were applied at the broken wellhead, 1,500 m below the surface, during the 2010 Deepwater Horizon accident. Following the Deepwater Horizon accident, laboratory experiments were conducted that have shown that smaller droplets are produced by the breakup of contaminant jets of oil discharging into seawater if sufficiently high concentrations ( 1%) of dispersants are mixed with the oil. These droplets subsequently form a buoyant contaminant plume that rises toward the surface. Small droplets may detrain from the plume due to ambient density stratification and currents and form subsurface intrusion layers. Droplet size is therefore an important factor in determining the fate of the oil. No data exist on the effect of dispersants on the evolution of droplets in the contaminant plume during their buoyant rise through the oceanic water column. Toward this end, an experimental investigation was conducted. In this experiment, buoyant droplets of Oseberg Blend crude oil, similar to the oil released in the Deepwater Horizon spill, mixed with COREXIT 9500 dispersant at various concentrations, were suspended by a downward flow of synthetic seawater in a vertical water tunnel to simulate free-rise conditions. The droplets were monitored with video cameras over extended periods of time and the hydrocarbon components of the oil that were dispersed or dissolved in the seawater were analyzed with a GCMS. The data indicate that the addition of dispersant to crude oil promotes reduction of droplet size during free-rise and that this evolution is strongly influence by the Dispersant to Oil Ratio (DOR). Video imaging revealed that droplet shrinkage occurs due to "tip streaming," where larger droplets at sufficiently high DORs shed filaments of oil from their edges, or by tearing events. The extent and rate of shrinkage depends on DOR, with significant changes often occurring within the first 10-30 minutes of an experiment. GCMS data were employed to attempt to quantify the phenomena observed in the video records. Seawater samples were extracted after 8 hours and dissolved and entrained hydrocarbons were collected using solid phase extraction. Integration of the chromatograms to estimate total hydrocarbon content, based on reference dilution GCMS injections of crude oil in hexane, indicate that the amount of oil dissolved and dispersed in water is very sensitive to DOR. Tests also demonstrate that most of the oil detected with the GCMS is entrained in the water phase as tiny droplets rather than being dissolved. The results of this investigation suggest that droplets formed by the breakup of escaping oil jets during a undersea spill will shrink during their buoyant rise toward the surface if sufficient dispersant is mixed with the oil. The rate and extent of this reduction in droplet size depends strongly on DOR.