Water Wise: Where will the oil spilled in the Gulf go?

Carolyn Gramling (pictured in Clearwater, Fla.) writes Water Wise for EARTH.

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Carolyn Gramling

It’s been about eight weeks since an explosion on the Deepwater Horizon drilling rig damaged a wellhead in the Gulf of Mexico and triggered the onset of the largest offshore oil spill in U.S. history, with oil washing up on shores from Louisiana to northwestern Florida. Last week, a scientific group appointed by the federal government to study the impact of the leak upgraded its previous estimate of how much oil has been flowing from the well. Based on video of the leak, the group estimated that the well has been spewing between 25,000 to 30,000 barrels of oil a day — and between 1.7 million and 2.0 million barrels since the April 20 explosion. That estimate is an increase over the previous estimate, in late May, of 12,000 barrels to 19,000 barrels a day — and an even larger increase over early estimates, in late April, of 5,000 barrels a day.

The new estimate puts the Deepwater Horizon spill so far at four times the size of the Exxon Valdez spill.

BP has spent about $1.25 billion so far on multiple methods to try to stop the leak and contain it, including a wellhead cap installation completed on June 3, which is forcing oil up a pipe to a drillship on the surface of the Gulf. By August, BP says it will have relief wells in place that will stop the flow of oil completely.

Meanwhile, U.S. Geological Survey director Marcia McNutt is leading a group, called the Flow Rate Technical Group, made up of Coast Guard, NOAA, Minerals Management Service, DOE and academics that aims to assess the disaster and BP’s efforts. The Flow Rate Team is actually made up of two teams: a Mass Balance Team that uses airborne imaging technology to calculate the flow rate from the spill on the surface; and a Plume Calculation Team, which is using the video of the ruptured pipe.

The team has already cast doubts on BP’s statement that the containment cap will ultimately capture the vast majority of the oil leaking from the well. One member of the team, marine scientist Ira Leifer of the University of California at Santa Barbara, told McClatchy Newspapers that the installation of the cap may have actually resulted in even more oil leaking to the Gulf, because installing the cap required clearing out the well’s damaged riser pipe, allowing more oil to escape. The team is now studying high-definition video of the leak taken by submersibles after cutting the riser to determine if the flow has increased.

Although there is now a better assessment of the flow rate and how much has already leaked into the Gulf, where the oil is going is still unclear. Several recent experiments offer some ideas about the fate of oil and gas from an underwater point source leak, such as a ruptured well. In 2000, the Minerals Management Service contracted with 23 oil companies to experimentally release crude oil, diesel oil and natural gas off the coast of Norway to simulate a blowout in deep water. Another related project did a similar experiment in a high pressure chamber at the University of Hawaii.

The “Deep Spill” project found that the initial “jets” of oil and gas broke up into tiny, millimeter-sized gas bubbles and oil droplets, forming a buoyant plume. As the plume rose through the water, currents deflected it downstream, while stratification slowed its upward rise. By about 100 meters above the source, these forces would stop the plume’s rise — although some (the larger) oil droplets would rise faster than others, forming thin slicks on the surface of the water. The bulk of the oil and gas in the deeper plume, however, would remain below the surface and spread rapidly, distributing the oil over a wide area at depth.

What might happen to the gas is another question. The lab experiments suggested that in the deep waters of the Gulf, gas hydrates are thermodynamically stable, and could thus form on the surface of the methane bubbles. However, other factors, including the presence of oil, might limit the formation of hydrates on the bubbles. Whether or not hydrates form on the surface of the bubbles matters because they could slow the dissolution of the bubbles, which are otherwise likely to dissolve before reaching the surface — thereby reducing the risk of explosion to any machinery operating above the spill.

Scientists are now starting to put together a clearer picture of the whereabouts of the oil from the current spill — a picture that bears out the idea that the spill may be spreading more at depth than on the surface. University of South Florida scientists embarked on a research cruise in the Gulf in May to look for traces of the spilled oil and study the loop current, a deep ocean current in the Gulf that connects to the Gulf Stream. The loop current, about 200 kilometers to 300 kilometers wide, flows northward at a depth of about 800 meters between Cuba and the Yucatán Peninsula into the Gulf of Mexico, turns briefly east and then flows south again past the Florida Keys to then join the Gulf Stream as it sweeps up the Atlantic coast.

In late May, it seemed as though changes to the current’s flow — specifically, the breaking-off of a loop current eddy, a feature that separates from the main current periodically — might make it less likely to transport the oil swiftly out of the Gulf and into the Atlantic. But on June 8, the scientists warned that new evidence suggests the loop current has reshaped again, so that it is once again likely to carry oil to Florida and the Atlantic. They also found low concentrations of oil off the southern tip of Florida, in the Gulf Stream. A NOAA vessel, meanwhile, found 100-meter-tall clouds of hydrocarbons at depths of more than 1,000 meters.

The USF scientists also reported finding a plume of oil-related chemicals 68 kilometers northeast of the Deepwater Horizon rig and 229 kilometers southeast of the rig, at depths of up to about a kilometer below the surface. Although BP has not provided samples to researchers to chemically “fingerprint” the oil, University of Georgia marine scientist Samantha Joye — who led a separate two-week research cruise in the Gulf to study the plume — announced June 8 that her team had found “strong evidence that the plume does derive from the Deepwater Horizon.” That evidence, she told the Washington Post, consists of broad, diffuse plumes of oil about 1,100 meters to 1,300 meters beneath the surface southwest of the leaking pipe — and the plumes changed direction when BP put on its containment cap.

Where it’s going from there is still uncertain.

Carolyn Gramling
Monday, June 14, 2010 - 07:29