Scott A. Socolofsky

Multi-phase plumes in uniform and stratified crossflow

Laboratory experiments of multiphase plumes in uniform and stratified crossflows are presented. In uniform crossflow, multi-phase plumes behave as mixed single-phase plumes up to a critical height, h s, where the entrained fluid separates from the dominant dispersed phase. From the experimental results, an empirical relationship for h s was calibrated giving u ∞ /(B/s ) 1/3 =6.3(us/(B/hs )1/3)-2.4, where u ∞ is the crossflow velocity, B is the total kinematic buoyancy flux of the mixed plume, and u s is the slip velocity. Above h s the separated continuous-phase plume behaves like a momentum jet and the bubble column follows the trajectory of the vector sum of u s and u ∞ In stratified crossflow. the trap height in quiescent water, h T, was compared to h s. For h T « h s, the plumes are stratification-dominated and separation occurs at h T = (2.8-0.27 u s/(B/N)1/4)(B/N 3)1/4, where N is the Brunt- Vaisala buoyancy frequency. For h T, » h s. the plumes are crossflow-dominated, and separation occurs at h s. A simple single-phase model was modified to predict the fate of the separated plume above h s.

Evolution of droplets in subsea oil and gas blowouts: development and validation of the numerical model VDROP-J.

The droplet size distribution of dispersed phase (oil and/or gas) in submerged buoyant jets was addressed in this work using a numerical model, VDROP-J. A brief literature review on jets and plumes allows the development of average equations for the change of jet velocity, dilution, and mixing energy as function of distance from the orifice. The model VDROP-J was then calibrated to jets emanating from orifices ranging in diameter, D, from 0.5 mm to 0.12 m, and in cross-section average jet velocity at the orifice ranging from 1.5 m/s to 27 m/s. The d50/D obtained from the model (where d50 is the volume median diameter of droplets) correlated very well with data, with an R(2)=0.99. Finally, the VDROP-J model was used to predict the droplet size distribution from Deepwater Horizon blowouts. The droplet size distribution from the blowout is of great importance to the fate and transport of the spilled oil in marine environment.

Intercomparison of oil spill prediction models for accidental blowout scenarios with and without subsea chemical dispersant injection.

We compare oil spill model predictions for a prototype subsea blowout with and without subsea injection of chemical dispersants in deep and shallow water, for high and low gas-oil ratio, and in weak to strong crossflows. Model results are compared for initial oil droplet size distribution, the nearfield plume, and the farfield Lagrangian particle tracking stage of hydrocarbon transport. For the conditions tested (a blowout with oil flow rate of 20,000 bbl/d, about 1/3 of the Deepwater Horizon), the models predict the volume median droplet diameter at the source to range from 0.3 to 6mm without dispersant and 0.01 to 0.8 mm with dispersant. This reduced droplet size owing to reduced interfacial tension results in a one to two order of magnitude increase in the downstream displacement of the initial oil surfacing zone and may lead to a significant fraction of the spilled oil not reaching the sea surface.

The influence of droplet size and biodegradation on the transport of subsurface oil droplets during the Deepwater Horizon spill: a model sensitivity study

Abetter understanding of oil droplet formation, degradation, and dispersal in deepwaters is needed to enhance prediction of the fate and transport of subsurface oil spills. This research evaluates the influence of initial droplet size and rates of biodegradation on the subsurface transport of oil droplets, specifically those from theDeepwaterHorizon oil spill. A three-dimensional coupledmodel was employedwith components that included analyticalmultiphase plume, hydrodynamic and Lagrangianmodels. Oil droplet biodegradationwas simulated based onfirst order decay rates of alkanes. The initial diameter of droplets (10–300 μm) spanned a range of sizes expected fromdispersant-treated oil. Results indicate thatmodel predictions are sensitive to biodegradation processes, with depth distributions deepening by hundreds ofmeters, horizontal distributions decreasing by hundreds to thousands of kilometers, andmass decreasing by 92–99%when biodegradation is applied compared to simulationswithout biodegradation. In addition, there are twoto four-fold changes in the area of the seafloor contacted by oil droplets among scenarios with different biodegradation rates. The spatial distributions of hydrocarbons predicted by themodel with biodegradation are similar to those observed in the sediment andwater column, although themodel predicts hydrocarbons to the northeast and east of thewell where no observations weremade. This study indicates that improvement in knowledge of droplet sizes and biodegradation processes is important for accurate prediction of subsurface oil spills.

Simulation of scenarios of oil droplet formation from the Deepwater Horizon blowout.

Knowledge of the droplet size distribution (DSD) from the Deepwater Horizon (DWH) blowout is an important step in predicting the fate and transport of the released oil. Due to the absence of measurements of the DSD from the DWH incident, we considered herein hypothetical scenarios of releases that explore the realistic parameter space using a thoroughly calibrated DSD model, VDROP-J, and we attempted to provide bounds on the range of droplet sizes from the DWH blowout within 200 m of the wellhead. The scenarios include conditions without and with the presence of dispersants, different dispersant treatment efficiencies, live oil and dead oil properties, and varying oil flow rate, gas flow rate, and orifice diameter. The results, especially for dispersant-treated oil, are very different from recent modeling studies in the literature.

Enhanced diffusion from a continuous point source in shallow free-surface flow with grid turbulence

Shallow flows are ubiquitous in nature and are prone to instabilities that result in the formation of large-scale, two-dimensional coherent structures that are expected to significantly enhance eddy diffusivities compared to the stable turbulent base flow. We present the results of an experimental study in free-surface flow to determine the mixing coefficient for a passive tracer plume in two-dimensional grid turbulence. The grid consists of a row of cylinders 2.5 times the water depth and spaced to achieve a porosity of 50%. Depth-averaged dye concentrations are measured using a light absorption planar concentration analysis method; turbulence statistics are calculated from measurements of horizontal flow velocity using a two-component laser Doppler velocimeter. A base case without grid turbulence (having only bottom-generated turbulence) and three cases with grid turbulence are presented. For experiments with grid turbulence, injections were made at 40 water depths downstream of the grid, at the cylinde...

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

Significance Environmental risks posed by deep-sea petroleum releases are difficult to predict and assess. We developed a physical model of the buoyant jet of petroleum liquid droplets and gas bubbles gushing into the deep sea, coupled with simulated liquid–gas equilibria and aqueous dissolution kinetics of petroleum compounds, for the 2010 Deepwater Horizon disaster. Simulation results are validated by comparisons with extensive observation data collected in the sea and atmosphere near the release site. Simulations predict that chemical dispersant, injected at the wellhead to mitigate environmental harm, increased the entrapment of volatile compounds in the deep sea and thereby improved air quality at the sea surface. Subsea dispersant injection thus lowered human health risks and accelerated response during the intervention. During the Deepwater Horizon disaster, a substantial fraction of the 600,000–900,000 tons of released petroleum liquid and natural gas became entrapped below the sea surface, but the quantity entrapped and the sequestration mechanisms have remained unclear. We modeled the buoyant jet of petroleum liquid droplets, gas bubbles, and entrained seawater, using 279 simulated chemical components, for a representative day (June 8, 2010) of the period after the sunken platform’s riser pipe was pared at the wellhead (June 4–July 15). The model predicts that 27% of the released mass of petroleum fluids dissolved into the sea during ascent from the pared wellhead (1,505 m depth) to the sea surface, thereby matching observed volatile organic compound (VOC) emissions to the atmosphere. Based on combined results from model simulation and water column measurements, 24% of released petroleum fluid mass became channeled into a stable deep-water intrusion at 900- to 1,300-m depth, as aqueously dissolved compounds (∼23%) and suspended petroleum liquid microdroplets (∼0.8%). Dispersant injection at the wellhead decreased the median initial diameters of simulated petroleum liquid droplets and gas bubbles by 3.2-fold and 3.4-fold, respectively, which increased dissolution of ascending petroleum fluids by 25%. Faster dissolution increased the simulated flows of water-soluble compounds into biologically sparse deep water by 55%, while decreasing the flows of several harmful compounds into biologically rich surface water. Dispersant injection also decreased the simulated emissions of VOCs to the atmosphere by 28%, including a 2,000-fold decrease in emissions of benzene, which lowered health risks for response workers.

Observations of bubbles in natural seep flares at MC 118 and GC 600 using in situ quantitative imaging

This paper reports the results of quantitative imaging using a stereoscopic, high-speed camera system at two natural gas seep sites in the northern Gulf of Mexico during the Gulf Integrated Spill Research G07 cruise in July 2014. The cruise was conducted on the E/V Nautilus using the ROV Hercules for in situ observation of the seeps as surrogates for the behavior of hydrocarbon bubbles in subsea blowouts. The seeps originated between 890 and 1190 m depth in Mississippi Canyon block 118 and Green Canyon block 600. The imaging system provided qualitative assessment of bubble behavior (e.g., breakup and coalescence) and verified the formation of clathrate hydrate skins on all bubbles above 1.3 m altitude. Quantitative image analysis yielded the bubble size distributions, rise velocity, total gas flux, and void fraction, with most measurements conducted from the seafloor to an altitude of 200 m. Bubble size distributions fit well to lognormal distributions, with median bubble sizes between 3 and 4.5 mm. Measurements of rise velocity fluctuated between two ranges: fast-rising bubbles following helical-type trajectories and bubbles rising about 40% slower following a zig-zag pattern. Rise speed was uncorrelated with hydrate formation, and bubbles following both speeds were observed at both sites. Ship-mounted multibeam sonar provided the flare rise heights, which corresponded closely with the boundary of the hydrate stability zone for the measured gas compositions. The evolution of bubble size with height agreed well with mass transfer rates predicted by equations for dirty bubbles.

Simulating Gas–Liquid−Water Partitioning and Fluid Properties of Petroleum under Pressure: Implications for Deep-Sea Blowouts

With the expansion of offshore petroleum extraction, validated models are needed to simulate the behaviors of petroleum compounds released in deep (>100 m) waters. We present a thermodynamic model of the densities, viscosities, and gas-liquid-water partitioning of petroleum mixtures with varying pressure, temperature, and composition based on the Peng-Robinson equation-of-state and the modified Henrys law (Krychevsky-Kasarnovsky equation). The model is applied to Macondo reservoir fluid released during the Deepwater Horizon disaster, represented with 279-280 pseudocomponents, including 131-132 individual compounds. We define >n-C8 pseudocomponents based on comprehensive two-dimensional gas chromatography (GC × GC) measurements, which enable the modeling of aqueous partitioning for n-C8 to n-C26 fractions not quantified individually. Thermodynamic model predictions are tested against available laboratory data on petroleum liquid densities, gas/liquid volume fractions, and liquid viscosities. We find that the emitted petroleum mixture was ∼29-44% gas and ∼56-71% liquid, after cooling to local conditions near the broken Macondo riser stub (∼153 atm and 4.3 °C). High pressure conditions dramatically favor the aqueous dissolution of C1-C4 hydrocarbons and also influence the buoyancies of bubbles and droplets. Additionally, the simulated densities of emitted petroleum fluids affect previous estimates of the volumetric flow rate of dead oil from the emission source.

Comment on “Evolution of the Macondo Well Blowout: Simulating the Effects of the Circulation and Synthetic Dispersants on the Subsea Oil Transport”

the Effects of the Circulation and Synthetic Dispersants on the Subsea Oil Transport” P et al. use regional circulation and transport models to simulate the transport of oil from the Macondo well blowout, and conclude that subsurface dispersants may not have been particularly helpful in keeping oil submerged. They reach this conclusion because, without treatment, their assumed droplet sizes were already sufficiently small that much of the oil would have stayed submerged anyway. However this conclusion is based on a model of initial droplet sizes that we do not believe is appropriate. Their characteristic droplet size d (eq S3 in the Supporting Information) comes from Boxall et al. who studied water droplets stirred into oil in a reactor. For droplets larger than the Kolmogorov length scale, Boxall et al. found that standard Weber number scaling applied, or