News Feature: The perplexing physics of oil dispersants
M. Mitchell Waldrop
Massive amounts of oil, gas, and dispersant streamed into the Gulf of Mexico during the Deepwater Horizon disaster. Understanding the chemistry and physics of this mix as it churned through the salt water turns out to be an exceedingly complex problem with plenty of unknowns.
On April 30, 2010, 10 days after a blowout destroyed the offshore drilling platform Deepwater Horizon off the coast of Louisiana and triggered what was fast becoming the worst oil spill in US history, the well’s owner, British Petroleum, sent a remotely piloted submarine 1,500 meters down to the floor of the Gulf of Mexico. Once the vehicle arrived at the broken wellhead, which was still spewing more than 6,000 liters of oil per minute, it stuck the end of a kilometers-long hose into the erupting plume and started pumping in dispersants: detergent-like chemicals designed to fragment the hydrocarbons into tiny droplets. It was the start of a campaign that would ultimately inject the plume with almost 3 million liters of the chemicals.
Using dispersant at that depth was a roll of the dice; the chemicals had been used before on surface oil slicks with varying degrees of success but never in such cold, deep waters. No one could be sure what effect they would ultimately have on the ocean ecosystem, on coastal fisheries, or even on the oil itself. The responders could only hope that the injection would work as intended and that the resulting oil droplets would be consumed by the Gulf’s many petroleum-eating bacteria without ever making it to the surface.
So did it work? That depends on whom you ask.
The oil companies certainly think it did, says Tamay Özgökmen, a mechanical engineer at the University of Miami in Coral Gables, FL, who has spent much of the past eight years studying the Deepwater Horizon incident and its aftermath. The companies point to plummeting concentrations of toxic vapors over the oil slick — cleanup crews could finally work without respirators — and to aerial photographs suggesting that less oil was reaching the surface. So from the companies’ perspective, says Özgökmen, deep-sea dispersants have gone from being a desperation move to being standard operating procedure. “They’re preparing for it in future oil spills,” he says.
The National Academy of Sciences’ Ocean Science Board tends to agree: In a draft consensus report released on April 5, the Board’s panel on oil-spill-dispersant use concluded that yes, the deep-sea injection had generally been effective at dispersing the oil, making the hydrocarbons easier for bacteria to digest, preventing surface oil from fouling nearby shores, and enhancing worker safety by mitigating exposure to hazardous oil-related chemicals (1).
But the report — and the numerous researchers studying dispersants’ effects — also emphasized the many remaining uncertainties. The benefits of massive deep-sea dispersants are still a matter of intense debate among scientists. And the risks are even murkier. Dispersants by themselves don’t pose much of a near-term risk. They produce little more than burning eyes and coughing in humans, and except in the immediate vicinity of the Deepwater Horizon plume, the National Academy of Sciences report concluded, they never got close to acute toxicity thresholds for sea life living at the water’s surface. But biologists are still trying to figure out the long-term threat to human and ecosystem health posed by millions of liters of the stuff combined with unknown quantities of crude oil laced with its own brew of toxins and carcinogens.
Underlying it all is a mystery: where did the oil actually go, and how did the dispersants affect its movements? Before the wellhead was finally capped on July 15, 2010, it had released an estimated 760 million liters of oil — and as much as 25% of it remains unaccounted for.
Although definitive answers are hard to come by, major clues have emerged in the years since the accident as researchers have studied the real-world physics of oil, water, and dispersants. They have analyzed and reanalyzed the data recorded during the disaster, studied oil-droplet formation in the laboratory (with and without dispersants), tracked currents in the Gulf with fleets of high-tech buoys, and constructed innumerable computer simulations. Researchers know vastly more than they once did about what happened to the oil in the deep sea plume as it rose from the wellhead; how the oil interacted with sunlight, wind, and waves as it spread across the surface; and exactly what role the dispersants played.
And in June 2018, researchers embarked on the largest experimental simulation of the Deepwater Horizon spill to date at a huge saltwater tank in New Jersey. In the two-phase experiment, which will conclude with a second series of experiments in July 2019, the scientists will gather a trove of data in hopes of pinning down some of the last remaining uncertainties stemming from a disaster whose scale and speed took everyone by surprise.
On the Surface
The real-world chemistry and physics of the air-sea interface are about as complicated as it gets. As soon as oil from any spill hits the surface, for example, it starts baking in the sun, boiling off volatile compounds and losing almost half its volume as it turns into a tarry gunk that resists dispersant action (2). The fumes were bad news for the Deepwater Horizon cleanup crews; not only were the gases a fire hazard but also they included some 40 times the allowed exposure levels for benzene, a known carcinogen. As much as 25% of the oil in that incident seems to have evaporated in this way.
In addition, explains Eric D’Asaro, an oceanographer at the University of Washington in Seattle, the surface of the ocean isn’t like a flat puddle of rainwater. It moves, surges, and heaves. Breaking waves and ocean currents are constantly shattering the oil slicks back into droplets and dragging them under again, he says, “until there’s an equilibrium between things that are carried up and carried down.” The finest droplets go deepest, says D’Asaro, who’s a member of the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE). This means that the so-called oil slick is actually a thick layer of oil droplets extending down as much as 10 meters.
Dispersants add another level of complexity (see Fig. 1), says Joseph Katz, a mechanical engineer at Johns Hopkins University in Baltimore, MD, who studies the effects of these chemicals with funding from a consortium funded by the Gulf of Mexico Research Initiative, which separately funds CARTHE. He works with a laboratory wave tank that allows him to introduce oil slicks and then watch through a system of lasers and microscopes as the breakers smash the slicks into an underwater cloud of oil droplets.
“Without dispersants,” says Katz, “I found the size distribution to be understandable.” That is, the droplets showed a range of sizes down to about 100 micrometers, or about as small as a turbulent eddy can get before it’s dissipated by fluid viscosity. “But with dispersants, I couldn’t predict the distribution,” he says. Instead of a cutoff at 100 micrometers, he saw droplets as small as 1 micrometer (3).
A closer look showed what was happening, says Katz: in the presence of dispersants, which lower the surface tension between oil and water, the droplets were developing all sorts of threads and tails. “They look like sperm cells,” he says. In fact, the dispersants were concentrating in the tails, which would grow longer and longer until they broke up to produce the microdroplets.
Above the surface, Katz found that dispersants cause a 100-fold increase in the concentration of ultra-fine oil droplets floating in the air (4). It’s less clear how these floating droplets form — the popping of bubbles, maybe? — but their presence raises new health concerns: what happens when people breathe in infinitesimal droplets that are filled with toxins and carcinogens from the oil and are so small that they can penetrate deep into the lungs? “Go to the literature, and you find we don’t know much,” says Katz.
Adding still more complexity are the currents that stir the Gulf on every size scale, from local riptides at the beach to the giant Loop Current: a powerful flow that rises between the Yucatan Peninsula and Cuba, wanders around the Gulf in an erratic and hard-to-predict path, and finally exits between Cuba and Florida to become the Gulf Stream. One nightmare scenario during the Deepwater Horizon incident was that the Loop Current would capture the spreading oil slick and end up fouling a good chunk of Florida or conceivably even the East Coast. That this didn’t happen was purely a stroke of luck: the Loop Current was flowing south of the Deepwater Horizon site at the time of the accident and was in the process of spinning off a giant eddy that kept the oil relatively close to shore.
However, that simply meant that the fate of the Deepwater Horizon oil was subject to a host of poorly understood, smaller-scale currents. In August 2012, CARTHE members sought to map those flows in unprecedented detail with the Grand LAgrangian Deployment (GLAD) — an experiment that seeded the blowout region with 317 custom-made floats designed to drift with the currents the way oil would, and then tracked them via GPS for 10 days (5). GLAD was the largest experiment of its kind ever conducted until early 2016, when the consortium followed up with more than 1,000 drifters in the Lagrangian submesoscale experiment (LASER) (6).
In both cases, says D’Asaro, the drifter paths showed the currents very clearly. But strikingly, he says, “we found that sometimes there were places where the drifters gathered together” — typically at a junction between waters of different density.
One recurring example is at the mouth of the Mississippi River, he says. “There is a fan of fresh water coming out, making a rather sharp boundary with the saltwater in the ocean.” Salt water is heavier, so it dives underneath and creates a “front” that can collect floating things.
During an oil spill, says D’Asaro, that can be good news or bad news: “If there is oil on the salty side, it will be prevented from going on shore. But if the front intersects the shore, it will become a conduit for the oil.” Either way, he adds, modelers need to learn how to predict these fronts so that clean-up crews in future oil spills will know the best places to pick the stuff up.
In the Plume
Meanwhile, another group of CARTHE investigators was finding a whole new set of complexities in the plume of oil and gas rising from the Gulf seafloor.
“Think of it like a volcanic eruption, where the heat and force of explosion [send] the rock and hot gases high into the atmosphere,” says Scott Socolofsky, a civil engineer at Texas A&M University in College Station. The heat and force were there in plenty, says Socolofsky. From a combination of observation and experiment, as well as a detailed computer model of
“Think of it like a volcanic eruption, where the heat and force of explosion [send] the rock and hot gases high into the atmosphere.”
— Scott Socolofsky
the plume (7) that incorporated factors such as fluid dynamics, the buoyancy of oil and gas, and their solubility in seawater, Socolofsky and other researchers know that what came roaring up the broken drill pipe was a 100 °C, high-pressure mix of oil and natural gas that abruptly decompressed as it slammed into the frigid, 4 °C bottom waters of the Gulf. The gas reacted like the fizz from a shaken soda can, flashing into a mass of bubbles that helped break the oil into a cloud of fine droplets. And from there, says Socolofsky, “as the gas bubbles and oil droplets started to rise, their lightness created a plume that entrained the ambient seawater and carried it along with them.”
But bubbles and droplets have only a limited capacity to lift the dense bottom water, says Socolofsky. So at a certain point, he says, “they started getting off this upward rising train: ‘This is as high as I can go.’” In Deepwater Horizon, this happened at a depth of roughly 1,100 meters, or about 400 meters above the seafloor. The smaller drops and dissolved compounds spread out into an intrusion — a kind of underwater mushroom cloud that was very dilute and hard to see directly but that was detected from chemical traces.
Meanwhile, says Socolofsky, the larger droplets and bubbles kept rising. But they didn’t make it to the surface, either, because the gas inside them steadily dissolved into the surrounding seawater as they rose. So did everything soluble in the oil droplets: his plume model estimates that 27% of the original mass of the oil disappeared in this way.
The model also suggests that the dispersants injected at the wellhead enhanced this effect by shrinking the droplet and bubble sizes by about a third, which increased the surface-to-volume ratio and made it easier for volatiles such as benzene to dissolve on the way up. That didn’t appreciably cut down the total amount of oil reaching the surface, says Socolofsky, but it definitely improved the air quality for the cleanup crews. “The workers’ respirator alarms quit going off,” he says.
In short, says Socolofsky, researchers now have a good general understanding of what the plume looked like. Unfortunately, he adds, “that doesn’t answer the question of where the oil went.” For that, he says, you’d need to calibrate the models with the actual size distribution of the oil droplets coming out of the wellhead, with and without dispersants. “That’s a challenging measurement,” he says, “and it was not done on Deepwater Horizon.”
Frustratingly, it’s also a measurement that’s almost impossible to make in the laboratory. Droplet formation depends on the surface tension between the oil and water, which doesn’t scale. So to reproduce the full range of droplets coming out of the 50-centimeter Deepwater Horizon pipe, an experimenter would need a model pipe at least twice the size of the largest stable oil droplets, which are about 12 millimeters across. (Anything bigger will quickly break up from unstable oscillations.) That works out to a minimum pipe size of roughly 25 millimeters. But a nozzle that big would fill up any lab-sized tank in minutes, turning it an impenetrable black. Most laboratory experiments use nozzles with diameters of 1 or 2 millimeters.
This uncertainty in the droplet size leaves plenty of room for interpretation. For example, University of Miami oceanographer Claire Paris and her collaborators have created their own model of the plume (8). It incorporates much of the same chemistry and physics as the model developed by Socolofsky and his coworkers, including factors such as solubility and buoyancy. But it uses different experimental data, suggesting that the violence of the eruption from the wellhead smashed the oil into droplets so small that the dispersants couldn’t have made them much smaller. And if that was the case, says Paris, “the injection of dispersants did not significantly change the amount of oil that reaches the surface. Maybe 3%.”
Complicating things still further is the presence of all that gas in the outflow, mainly methane, ethane, and propane. CARTHE member Michel Boufadel, an environmental engineer at the New Jersey Institute of Technology in Newark, recently worked with Özgökmen and several other colleagues to reanalyze the Deepwater Horizon data (9) and concluded that there had been a lot more gas in the jet than people had assumed. “It was not just big blobs of gas, but a very violent tumbling and churning,” says Boufadel, who was a member of the panel that prepared the National Academy of Sciences’ recent consensus report on dispersants. So who knows what really happened when dispersants were injected into this maelstrom. “Dispersants like to stay at the interfaces,” he says. So maybe they were reacting with the gas all along and not the oil. “There are not many experiments, or even models for this kind of churn flow,” he says.
To sort all this out, says Boufadel, “we need a full-scale experiment.”
What’s Left in the Tank
That’s where the huge tank in New Jersey comes in. By US law, says Özgökmen, you can’t put oil in the ocean even for an experiment. So the researchers have turned to the Ohmsett facility, an above-ground saltwater tank operated by the US Interior Department in Leonardo, NJ. Roughly the size of four Olympic-length swimming pools placed end to end, the facility is designed for testing oil cleanup methods. But a CARTHE team led by Boufadel took over the tank June 18–29, 2018 in the first phase of their effort to recreate the Deepwater Horizon disaster at something approaching full scale.
The focus in this initial phase was to nail down the dynamics of droplet formation without dispersants. For each run, Boufadel and his colleagues injected some 10 tons of oil through a pipe that was being towed along the bottom of the tank to simulate current flow. The pipe was 25 millimeters across, big enough to generate the full range of droplet sizes, which the researchers measured with a camera that was developed for the task at the University of Miami and that could image even very small droplets over a wide range of distances.
The results are still being prepared for publication, says Boufadel. But the data taken so far cover the full gamut of conditions, including churn flow with 50%/50% oil and gas, bubbly flow with 5–10% gas, and smooth flow with no gas. In the experiment’s second run in July 2019, the group will measure how droplet formation is affected under each condition by different levels of dispersants.
Hopefully, the results will clear up some of the uncertainties about where the oil went after Deepwater Horizon. But it will definitely be a culmination of CARTHE’s work on the accident, says Boufadel. Like all the other Gulf of Mexico Research Initiative consortia, the group is now moving into a data consolidation phase that’s geared toward integrating the immense amount of science that’s been done on the Gulf of Mexico — and improving the computer models that response teams will use in the next oil spill.
That’s when, not if, says Boufadel. Future blowouts may or may not be as inaccessible as Deepwater Horizon, he says — although with oil companies drilling in deeper and deeper waters around the world, that’s always a possibility. “But there are a lot of pipes underwater,” he says. “And if you have an oil release, it doesn’t have to be a mile below the surface.”
Since Deepwater Horizon, says Boufadel, “it’s good to be ready.”
Published under the PNAS license.
