When the Deepwater Horizon drilling platform in the Gulf of Mexico exploded on April 20, 2010, 11 workers tragically lost their lives and the Macondo well began gushing crude oil—and it would continue to do so for nearly three months. A parade of attempts to shut down the flow taught the public just how difficult it was to tame a wild well 5,000 feet beneath the surface of the sea.

The various techniques had oddly colloquial names considering the engineering feats they represented— names like “top hat," “top kill," and “junk shot." One by one, they failed to do the job. Then, on July 15, a new cap was placed over the top of the damaged wellhead under the direction of the US government—and it held. That was the end of the 24/7 camera footage showing black billows of oil rocketing upward.

While that description may make it sound like a simple operation, it was anything but. A mad scramble and some gutsy decisions were going on behind the scenes. The story of the science and engineering that drove that success was published this week in the Proceedings of the National Academy of Sciences.

An all-star team

In June, the government pulled together a “Well Integrity Team”—an all-star group of experts tasked with helping bring the marathon oil spill to an end. Both BP and the government scientists wanted to cap the well in some way, even as BP worked on drilling the precision “relief well” that would intercept the Macondo well deep below the surface and permanently close it off. Capping involved some big risks, though.

The only thing worse than oil spewing out of a hole 5,000 feet underwater would be oil spewing out of lots of holes 5,000 feet underwater—especially given that those extra holes wouldn’t be tidy pipes with only one outlet. That was the dreaded subsurface blowout scenario, where the last vestige of control would be lost.

The fear went like this: Beneath the surface, the Macondo well contained a number of what were essentially emergency pressure relief valves. Those valves prevent catastrophic failure of the well casing in case the pressure inside or outside got too high. When the “top kill” (pumping heavy drilling mud into well in the hopes it would act as a stopper) failed, BP became concerned that those pressure relief valves had opened, allowing the mud to escape out through casing instead of blocking up the well. (Later, it became apparent that they had simply underestimated the flow rate of the oil.)

If the well were capped, pressure inside would increase, and oil would escape through those openings. The pressure would eventually reach the point where it would actually fracture the sediment around the well, creating a myriad of uncontrollable flow paths to the surface—a subsurface blowout.

The Well Integrity Team’s job was to monitor the well after it was capped for the first sign of trouble and remove the cap (a much larger version of the “top hat”) before a subsurface blowout occurred. Armed with model simulations of how the well would behave in various scenarios, they carefully watched the pressure inside the well slowly build after the cap was put on. If it climbed to 7,500psi within six hours, they knew the well was intact and would hold. If the pressured stayed below 6,000psi, they knew the well was leaking and they would need to abort to avoid a blowout.

A veil of uncertainty

Of course, the measurements came up smack in between those two scenarios—a no-man’s land of uncertainty. The only thing they knew is that they had 24 hours to figure out what was going on before the risk of a blowout was too great. They needed to work with the data they had gathered and do some more modeling. Quickly.

The paper describes that work very matter-of-factly. “This additional analysis was carried out overnight from July 15 to July 16 in the form of independent reservoir modeling by the Well Integrity Team to see if capping stack pressures measured immediately after shut in could be explained without invoking leakage below the seafloor.” Hidden in that sentence is a remarkable story.

That additional analysis was largely carried out by Paul Hsieh, a United States Geological Survey hydrogeologist who wasn’t on site at the time. BP’s instruments weren’t set up to allow the pressure measurements to be downloaded and sent to Hsieh. Instead, Hsieh received a smartphone photo of the data on a monitor and instructions to supply an answer by morning.

Tweaking equations developed for groundwater flow to accommodate crude oil, and navigating a labyrinth of archaic unit conversions, Hsieh worked on model simulations throughout the night. His quick and dirty simulation, which matched the data admirably, showed there was a good chance the well’s behavior did not signify a leak or impending subsurface blowout. Despite reservations among some of the other advisors, Energy Secretary Steven Chu decided to keep the cap closed and continue the test past the 24-hour decision point.

In addition to a lot more analysis and modeling of the ongoing well pressure measurements, a swarm of BP ships performed periodic seismic imaging (which works like sonar) of the subsurface around the well, looking for any signs of leaking oil.

In the end, no blowout occurred and the Macondo well remained intact until the relief well intercepted it. The leaking well was cemented shut on August 3. Television news coverage moved on, the work in the Gulf turned to assessing the fate and ecological impact of the spilled oil, and the fight over fines and responsibility for the accident intensified.

The Well Integrity Team describes a number of factors that led to their success. Most relate to “an unprecedented level of collaboration and coordination among scientists, engineers, and emergency response officials from public and private sectors," including access to BP’s data and experienced personnel. They also highlight the terrific and timely analysis supplied to decision makers by the scientists who were brought in—scientists like Paul Hsieh, whose contributions throughout the project earned him recognition as the 2011 Federal Employee of the Year.

PNAS, 2012. DOI: 10.1073/pnas.1115847109 (Open Access) (About DOIs).