How Much Oil?

Experts still don’t agree on the amount of oil that gushed into the Gulf—but everyone knows it was a lot.

April 10, 2015

Photo: Georgia DNR

How big? It’s probably the most basic question you can ask about an oil spill. When an oil well is leaking into the bottom of the sea, though, the calculation is anything but straightforward. Nearly five years after the Macondo well was capped, putting an end to an 87-day-long oil and gas gush, experts still don’t agree on how much oil came out.

The numbers that emerged in the first days of the spill were almost unbelievably divergent. BP initially estimated a flow rate of 1,000 barrels per day. When scientists questioned that figure, the company responded that “there’s just no way to measure” the spill. (Then how did they come up with that number?) Soon after, a group called SkyTruth suggested the flow rate was 5,000 barrels per day based on satellite imagery. The government then adopted the same figure, using data collected by the National Oceanic and Atmospheric Administration. NOAA eventually revised its estimated upward to between 12,000 and 18,000 barrels per day, then to 24,000 to 30,000 barrels, and later to 35,000 and 60,000 barrels. At one point, BP admitted that the flow rate could be as high as 100,000 barrels per day—100 times the company’s initial estimate.

The problem was that no one could agree on an appropriate method. The government scientists’ first strategy was called the Bonn Convention protocol—they used the color of the oil on the water’s surface to judge its thickness and performed a mathematical calculation. They later turned to measuring the velocity of individual oil particles coming out of the well using video images. Concluding that this technique still produced an underestimate, the scientists looked at the structure of the “billows” of oil to calculate the flow rate. Finally, just days before the well was capped, scientists were able to place a device at the wellhead to measure the pressure of the escaping oil. They then settled on an “official” flow rate of approximately 53,000 barrels per day.

Once the well was finally sealed, experts attempted to calculate the total spill volume based on the pressure conditions that existed inside the well. The word “microsips,” a measure of compressibility in the rocks, entered the Gulf-area lexicon. Using this approach, the administration estimated that 4.9 million barrels, or about 210 million gallons, flowed into the water. Recovery teams collected 17 percent of that volume, leaving 171 million gallons of oil in the Gulf.

The estimate became a major economic issue, because BP’s fines would largely be based on the volume of the spill. In a 2013 trial, the company’s experts clashed with government scientists over those microsip calculations. The judge eventually concluded that 168 million gallons leaked and 34 million were recovered, leaving 134 million gallons in the water.

The judge’s view, however, doesn’t appear to have convinced the government. Many administration websites continue to state that 210 million gallons were spilled in the Gulf disaster.

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

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