Study Estimates Subsea Dispersant Marginally Reduced Surface Oil

Scientists at the University of Miami and the University of Western Australia measured oil droplet size and simulated oil dispersion under conditions similar to those at the Deepwater Horizon wellhead.

MGP-UWA-EMI-1397 Researchers used this autoclave apparatus with an external camera that visually captured oil droplets in water. (Photo provided by Zachary Aman)

The authors reported that, based on modeling, dispersant applied at the Macondo wellhead slightly reduced oil droplet size, contributing a 1-3 percent reduction in the oil that reached the surface. The team published their results in the February 2015 issue of Chemical Engineering Science: High-pressure visual experimental studies of oil-in-water dispersion droplet size.

The effectiveness of dispersing oil in water is a determining factor in how oil moves through the water column, with larger droplets more likely to rise than smaller droplets. The pressure difference between an oil reservoir and the surrounding water results in the release of hydrocarbons through a turbulent environment that atomizes much of the oil and gas. Past research on these processes (Paris, et al., 2012) at the Macondo site suggests that small oil droplets may naturally stratify at 1,000 meters depth. The purpose of this study was to improve quantification of oil droplet size distributions during the blowout and use that data in model scenarios to improve understanding about the fate of this entrained oil with and without dispersants.

The team used a high-pressure chamber to simulate different pressures and turbulence speeds and a high-speed, high-resolution camera to measure oil droplet size and distribution. “This is the first time that we’ve been able to visually monitor how droplets break up and coalesce at up to 120 times atmospheric pressure, which is roughly equivalent to the pressure underneath 1.2 kilometers of water,” said Zachary Aman, associate professor of mechanical and chemical engineering at the University of Western Australia. “When paired with the high pressures and flow rates of Macondo, the results suggest a natural mechanism by which oil is dispersed into small droplets.” The experiments indicated that dispersants did reduce mean oil droplet diameter from about 80 to 45 micrometers.

Using these data, researchers predicted oil droplet size and dispersion for a scenario depicting the Deepwater Horizonblowout, incorporating data about the vertical terminal velocity of droplets, ocean currents, diffusion, temperature and salinity, and potential biodegradation rates. The terminal velocity is a physical parameter that informs how quickly droplets may rise through the water column. They conducted a 100-day (April to July 2010) field-scale Macondo blowout simulation at 300 meters above the wellhead, the depth of deep water plume observed.

Simulations yielded evidence that without dispersant there was about 15-20% less oil trapped below 1,000 meters yet little change in the amount of oil reaching the surface. “These results support our initial modeling work that the use of toxic dispersants at depth should not be a systematic oil spill response,” said Claire Paris, Associate Professor of Ocean Sciences at the UM Rosenstiel School. “It could very well be unnecessary in some cases.”

The authors suggest that additional studies incorporating a range of oil types are needed to establish a more general understanding of the oil breakup model. Future work using computational fluid dynamics could help improve blowout plume turbulence estimates for various scenarios.

The study’s authors are Zachary M. Aman, Claire B. Paris, Eric F. May, Michael L. Johns, and David Lindo-Atichati.  Both Paris and Lindo-Atichati are members of the C-IMAGE consortium of the Gulf of Mexico Research Initiative (GoMRI).