Modeling the influence of deep water application of dispersants on the surface expression of oil: A sensitivity study

Abstract

Although the effects of chemical dispersants on oil droplet sizes and ascent speeds are well-known, the fate and transport of dispersed oil droplets of different sizes under varying hydrodynamic conditions can be difficult to assess with observations alone. We used a particle tracking model to evaluate the effect of changes in droplet sizes due to dispersant application on the short-term transport and surface expression of oil released under conditions similar to those following the 3 June 2010 riser cutting during the Deepwater Horizon event. We used simulated injections of oil droplets of varying size and number under conditions associated with no dispersant application and with dispersant application at 50% and 100% efficiency. Due to larger droplet sizes in the no-dispersant scenario, all of the simulated oil reached the surface within 7 h, while only 61% and 28% of the oil reached the surface after 12 h in the 50% and 100% dispersant efficiency cases, respectively. The length of the surface slick after 6 h was 2 km in the no-dispersant case whereas there was no surface slick after 6 h in the 100% dispersant case, because the smaller oil droplets which resulted from dispersant application had not yet reached the surface. Model results suggest that the application of dispersants at the well head had the following effects: (1) less oil reached the surface in the 6-12 h after application, (2) oil had a longer residence time in the water-column, and (3) oil was more highly influenced by subsurface transport.

The choice of droplet size probability distribution function for oil spill modeling is not trivial

“…The estimated rising velocities of the different quantiles are provided in the Supplementary Information (Table S1). The surfacing time of the lognormal and Rosin-Rammler DSDs from case 1 with a d50 of 900 µm (minimum surfacing time of 4 h, 8.75 h for 50% and 51.55 h for 99% of the oil) matches the estimates without subsea dispersant injection (SSDI) at the Macondo wellhead from Testa et al (2016) and Gros et al (2017) that suggest that all the oil reached the surface within 7 h for the largest droplets. In our idealized conditions with the same d50, the lower tail of the VDROP-J DSD requires an extra 125.25 h to surfacing as compared to the Rosin-Rammler DSD and 167.5 h compared to the lognormal one (Figure 1b).…”

Oil Droplet Size Distributions in Deep-Sea Blowouts: Influence of Pressure and Dissolved Gases

“…For 87 days, about 5 million barrels of oil and 170 000 t of gas (C 1 –C 5 ) spilled into the ocean in a depth of approximately 1500 m. , Because of the deep-sea nature of the spill, up to 50% of this oil stayed below the sea surface as a result of density stratification and dispersion. To numerically model the mass distribution, propagation, and biodegradation of the oil in the ocean for such a deep-sea blowout, the initial oil droplet size distribution (DSD) is one of the key parameters. − …”

Petroleum dynamics in the sea and influence of subsea dispersant injection during Deepwater Horizon

“…These events have not been documented for any previous oil spill. Some efforts (7,8) to model these phenomena neglected relevant physical or chemical processes, and more complete attempts (9, 10) still do not fully couple the evolving states, compositions, and properties of fluid particles under depth-varying pressure and temperature conditions. However, the pressure-dependent, temperature-dependent, and composition-dependent states and properties of petroleum fluids influence their behaviors in the deep sea (11), and only indirect observations of these phenomena are available (2,4,12,13).…”