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Kessler, J. D., et al. (2011), A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico, Science, 331(6015), 312-315. Ryerson, T. B., et al. (2011), Chemical data quantify Deepwater Horizon hydrocarbon flow rate and environmental distribution, PNAS, doi: 10.1073/pnas.1110564109. Socolofsky, S. A., Adams, E. E., and Sherwood, C. R. (2011) Formation dy- namics of subsurface hydrocarbon intrusions following the Deepwater Horizon blowout, Geophys. Res. Lett., 38, L09602. Socolofsky, S. A., Bhaumik, T., and Seol, D.-G. (2008) Double-plume inte- gral models for near-field mixing in multiphase plumes, J. Hydraul. Eng., 134(6), 772-783. Socolofsky, S. A., and Adams, E. E. (2005) Role of slip velocity in the be- havior of stratified multiphase plumes, J. Hydraul. Eng., 131(4), 273-282. Socolofsky, S. A., and Adams, E. E. (2002) Multi-phase plumes in uniform and stratified crossflow, J. Hydraul. Res., 40(6), 661-672. Selected Literature Cited Acknowledgements For Further Information This poster is based on work supported in part by the National Science Foundation (NSF) under RAPID grants CBET-1045831, CBET-1046890, and OCE-1048976, NSF grant CTS-0348572, by the U.S. Geological Survey (USGS), Coastal Marine Geology Program, and by grants from the BP/ Gulf of Mexico Research Initiative (GoMRI) in support of the GISR and C-IMAGE consortia. Any opinions, findings and conclusions, or recommendations in this material are those of the authors and do not necessarily reflect the views of NSF, the USGS, or GoMRI. The poster design is based on a template by Purrington, C.B. “Design- ing conference posters.” Retrieved February 15, 2012, from http://colinpurrington.com/tips/acadmic/posterdesign and on format rules in the LaTeX beamerposter style class (see e.g., www.ctan.org). Please contact [email protected]. More information on this and related projects can be obtained at http://ceprofs.civil.tamu.edu/ssocolofsky/ An online PDF of this poster and an animation of Fig. 4 is available at http://ceprofs.civil.tamu.edu/ssocolofsky/ Near-field dynamics of the Deepwater Horizon accidental blowout: Chemical partitioning, intrusion dynamics, and dispersant effectiveness Scott A. Socolofsky, E. Eric Adams, Steven F. DiMarco, Thorsten Stoesser, and Christopher R. Sherwood Author Affiliations Scott A. Socolofsky, Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas, USA. E. Eric Adams, Ralph M. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. Steven F. DiMarco, Department of Oceanography, Texas A&M Univer- sity, College Station, Texas, USA. Thorsten Stoesser, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA. Christopher R. Sherwood, U.S. Geological Survey, Woods Hole, Massa- chusetts, USA. Introduction The accidental blowout of the Deepwater Horizon (DH) MC 252 well resulted in formation of large, subsurface in- trusions of spilled oil, dissolved hydrocarbons, and re- sponse byproducts (e.g., dispersant, drilling mud, etc.). The dominant deepwater intrusion was observed around 1200 m depth and was documented by numerous CTD casts during and shortly following the event (Kessler et al. 2011). These intrusions are believed to have contained all of the released methane and significant fractions of the dissolved, lighter hydrocarbons (Ryerson et al. 2011). In this poster we apply an integral numerical model to elucidate the mechanisms responsible for formation of the subsurface intrusions and to predict some of the structure and composition of these submerged layers. The results demonstrate that the Deepwater Horizon blowout plume was controlled by ambient density stratification. This work further validates and extends the results reported using empirical equations in Socolofsky et al. (2011). Model Results Methods Rising plumes resulting from deep- water spills of oil and gas differ from single-phase plumes because the sources of the buoyancy (e.g., dis- persed oil droplets and gas bubbles) can follow separate trajectories from the entrained ambient seawater and from each other. The plume schemat- ics at right demonstrate the two ex- tremes of this separating behavior for a strong crossflow and for pure den- sity stratification. When stratification and crossflow are present, the two separation mechansism are compet- ing, and Socolofsky et al. (2011) found using empirical relations (Socolofsky and Adams 2002, 2005) that the DH blowout was controlled by stratification, with the current only deflecting the plume down- stream. In the work presented here, we apply an integral numerical model developed for stratification-dominate plumes (Socolofsky et al. 2008). Fig. 1 presents a schematic of the integral plume model, which consists of an inner, rising plume of entrained water, oil and gas, a detrainment algorithm to generate the peel when the plume fluid becomes too heavy, and a down-draught, ring plume of water and fine oil droplets that can exchange fluid with both the ambient reservoir and the inner plume. The E’s in the figure de- note the entrainment fluxes. The model also tracks the heat transfer from the warm hydro- carbons, the dissolution of the gas, and the concentration of dis- solved gases in the plume. Conclusions A double plume integral model designed for stratifica- tion-dominated plumes successfully predicts the eleva- tion and light hydrocarbon composition of the subsea in- trusions that formed following the DH accident. The model predicts complete trapping of methane below 800 m depth and minimal methane hydrate formation. The agreement between the model and the measurements demonstrates that the mechanism resonsible for creating the observed deep hydrocarbon layers is stratification- dominated peeling and subsequent intrusion formation. Existing blowout response models lack some of the plume physics identified here. Although these models decelerate by entrainment of stratified water and allow for separation of the gas from the plume, they do not have a peeling mechanism or downdraught plume, they cannot predict multiple intrusions, and they do not allow for subsequent inner plume structures that can result in rapid transport of large oil droplets to the surface near the location of the blowout source. Discussion 4 6 8 10 12 14 16 18 10 −4 10 −3 10 −2 10 −1 10 0 Temperature [deg C] Concentration [mol/L] 0.3 m (Height above well head) 1.2 m 4.6 m 14 m 36 m Plume Centerline Plume Edge Stable Hydrate Region Unstable Hydrate Region Methane Hydrate Stability Diagram Fig. 6. Plume water temperature and dissolved methane concentra- tion as a function of height above the diffuser. 0 0.5 1 1.5 2 0 50 100 150 200 250 Bubble Diameter [cm] Height [m] 0 0.005 0.01 0.015 0.02 0 50 100 150 200 250 Mass Transfer Coefficient [cm/s] 10 0 10 1 10 2 10 3 0 50 100 150 200 250 Bubble Mole Flux [mol/s] Fig. 5. Variation of several bubble properties as a function of height above the well head. a.) gas bubble effective diameter, b.) bubble mass transfer coefficient, and c.) total mole flux of gases in bubbles. Dynamics of Gas Bubbles Rising above the Well Head Methane Ethane Propane Gas Stripping Nitrogen Oxygen Fully Dissolved by 300 m above Source a.) b.) c.) −2000 −1000 0 1000 0 500 1000 1500 Volume Flux [m 3 /s] 0 0.005 0.01 0 500 1000 1500 Dissolved Gas Concentration [mol/L] 0 20 40 0 500 1000 1500 Fluorescence, Wetlab [mg/m 3 ] Height [m] Fig. 3. Comparison of select model output with measured Wetlab fluorescence at Station B54 from the R/V Brooks McCall on May 30, 2010. a.) measured fluorescence data, b.) modeled plume fluid flow rates, c.) concentration of dissolved gases in the inner plume. Select Model Data Compared to Measured Hydrocabon Profile a.) b.) c.) Data from R/V Brooks McCall, Station B54 “Benchmark” on May 30, 2010. Lowest Intrusion Region Inner Plume Outer Plume Lowest Intrusion Second Intrusion Second Intrusion Methane All Others −50 0 50 100 150 200 250 300 350 −150 −100 −50 0 50 100 0 500 1000 1500 Oil Droplets > 0.3 mm Oil Droplets < 0.3 mm Dissolving Gas Bubbles Fig. 4. Visualization of the dispersed phases in the blowout plume in a 5 cm/s crossflow. Each symbol is representative of the trajectory of many individual bubbles or droplets. Downstream Distance [m] Cross-stream Distance [m] Height [m] Visualization of the Simulation Results in a Weak Current Fig. 2. CTD stations measured by R/V Brooks McCall using CDOM fluo- rescence probe. Map adapted from image stored at: http://www.epa.gov/bpspill/dispersants/bp-map-may30.jpg. CTD Measurements (R/V Brooks McCall) 0 2 4 Kilometers B54 Intrusion Detected Intrusion Not Detected Well Head 88 o 20’ W 88 o 26’ W 88 o 24’ W 88 o 22’ W 28 o 46’ N 28 o 44’ N 28 o 42’ N Northing [deg Lat] Westing [deg Lon] Strong Crossflow Oil Plume Separation Pure Density Stratification Peeling (Detrain- ment) Intrusion Fig. 1. Schematic of the double plume integral model by Socolofsky et al. (2008). Mass Transfer via the Ranz-Marshall Equation Non-dimensional Correlations in Clift et al. (1979) Gas Mixture of Hydrocarbons and Stripped Gases Size and Density from the Peng-Robinson Equation of State and Modified Henry’s Law Fig. 7. Schematic of the Discrete Bubble Model and an outline of the relevant equations. Discrete Bubble Model Figs. 2 and 3 summarize the model validation. CTD profile B54 by the R/V Brooks McCall on May 30, 2010 is compared in Fig. 3 to the intrusion structure predicted by the model. The model matches the elevation of the dominant intrusion at 1200 m depth. The intrusion flux is predicted to exceed 1000 m 3 /s and to contain most of the methane (Fig. 3c). Fig. 4 shows a visual- ization of the distribution of oil droplets and gas bubbles predicted by the model for a weak current of 5 cm/s. Fig. 5 illus- trates the fate of the bubbles. The model is initialized with large gas bubbles as a conservative estimate of their maximum life span. The model predicts complete dissolution within 300 m above the well head and modest gas stripping. Fig. 6 tracks the temperature and dissolved methane concentration in the plume and compares these to the stable hydrate-forming region at the ambient conditions of the DH release. Integral Plume Model Peeling Region Comparison of the model-predicted intrusion struc- tures in Fig. 3b. with the measured data in Fig. 3a. vali- dates the model and also helps interpret the elevated CDOM measured around 800 m depth, likely resulting from a second intrusion layer. Although the gas bubbles fully dissolve below this layer (see Fig. 5), Fig. 3c. shows that some of the dissolved methane escapes the lowest in- trusion and continues upward in the plume to the second intrusion. Fig. 4 also shows the effectiveness of disper- sants to trap small oil droplets in the intrusions. Dissolution of the gas bubbles is accomplished using the Discrete Bubble Model (DBM) approach, depicted in Fig. 7. In this model, the mass transfer and drag for a single bubble is calculated at each model grid elevation and then these results are broadcast to all bubbles of the same size at that level. This approach is typical of other blowout models, and the predicted dissolution appears to match the observations (e.g., Kessler et al. 2011). Model results for temperature and dissolved gases near the well head predict no whole-sale methane gas hydrate formation in the plume (see Fig. 6). Although the gas bubbles and plume fluid rapidly cool to ambient tempera- ture, entrainment dwarfs the gas dissolution so that dis- solved methane concentrations in the plume water are below that required for stable hydrate formation. This was also observed (though at lesser depth) for the Deep- Spill experment in 2000. Initial bubble diameters of the order of 0.1 mm are required to push the plume predic- tions into the hydrate region.
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