Methane bubble caused blowout blast in Gulf

Houston Chronicle:

BP crews Friday positioned a 100-ton, four-story containment dome over the major leak site of an out-of-control oil well, an effort that offered the most immediate hope of corralling a spill that already has closed Gulf of Mexico fishing and threatens the environment onshore.

Meanwhile, crews using containment booms, chemical dispersants and controlled burns attacked the massive spill that, for the most part, still gathered off the coast although some oil has made landfall off the Chandeleur barrier islands in Louisiana.

An apparent blowout of the Macondo well in 5,000 feet of water destroyed the Deepwater Horizon drilling rig April 20, killing 11 crew members and triggering the spill flowing at a daily rate estimated at 5,000 barrels, or 210,000 gallons.

In new developments on Friday, the Associated Press reported that workers told BP investigators that the rig blast was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly before exploding.

Some of the best weather since the spill began allowed response teams to burn off as much as 9,000 barrels of oil and vacuum 8,000 barrels of oil-and-water mixture from the surface late this week, the Coast Guard said.

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BP officials believe if the system works as planned, it could capture as much as 85 percent of the crude pouring from the well and significantly help curb the spill's impact.

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Getting the containment system in place is certainly progress. Also the discussion of the methane bubble is the start of understanding what caused the blowout, although the story does not explain how the bubble occurred and what could have been done to deal with it. Perhaps the petroleum engineers will explain that later today.

NPR gives this explanation from an interview with Robert Bea, a University of California Berkeley engineering professor.

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Based on the interviews, Bea believes that the workers set and then tested a cement seal at the bottom of the well. Then they reduced the pressure in the drill column and attempted to set a second seal below the sea floor. A chemical reaction caused by the setting cement created heat and a gas bubble which destroyed the seal.

Deep beneath the seafloor, methane is in a slushy, crystalline form. Deep sea oil drillers often encounter pockets of methane crystals as they dig into the earth.

As the bubble rose up the drill column from the high-pressure environs of the deep to the less pressurized shallows, it intensified and grew, breaking through various safety barriers, Bea said.

"A small bubble becomes a really big bubble," Bea said. "So the expanding bubble becomes like a cannon shooting the gas into your face."

Up on the rig, the first thing workers noticed was the sea water in the drill column suddenly shooting back at them, rocketing 240 feet in the air, he said. Then, gas surfaced. Then oil.

"What we had learned when I worked as a drill rig laborer was swoosh, boom, run," Bea said. "The swoosh is the gas, boom is the explosion and run is what you better be doing."

The gas flooded into an adjoining room with exposed ignition sources, he said.

"That's where the first explosion happened," said Bea, who worked for Shell Oil in the 1960s during the last big northern Gulf of Mexico oil well blowout. "The mud room was next to the quarters where the party was. Then there was a series of explosions that subsequently ignited the oil that was coming from below."

According to one interview transcript, a gas cloud covered the rig, causing giant engines on the drill floor to run too fast and explode. The engines blew off the rig and set "everything on fire," the account said. Another explosion below blew more equipment overboard.

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It will be interesting to see what caused the chemical reaction and how it can be avoided in the future.