Article by David Hammer and Mark Schleifstein of The Times-Picayune published.
Source: http://www.nola.com/news/gulf-oil-spill/index.ssf/2010/05/gas_surge_shut_well_just_weeks.html
Excerpts from the article:
Powerful puffs of natural gas, called kicks, are a normal occurrence in many deep-ocean drilling operations.
But one intense kick of natural gas caused the Deepwater Horizon drilling rig to be shut down because of the fear of an explosion just weeks before a similar release succeeded in destroying and sinking the platform and sent millions of gallons of oil on a collision course with Louisiana and the rest of the northern coast of the Gulf of Mexico.
Shortly before the accident, engineers argued about whether to remove heavy drilling mud that acted as a last defense against such catastrophic kicks, and the decision to replace the mud with much lighter seawater won out.
“As the job unfolded, … the workers did have intermittent trouble with pockets of natural gas,” said one statement sent to Robert Bea, a University of California at Berkeley engineering professor. “Highly flammable, the gas was forcing its way up the drill pipes.
That same type of frozen natural gas blocked BP’s attempts during the weekend to control the well leak with a huge box lowered 5,000 feet to the sea floor.
Back on April 20, the slush forced its way to the rig, shot 240 feet in the air and heated into a gas that quickly ignited into fireballs.
Bea said the statements he has gathered back up a report last week by The Times-Picayune about the questionable choice made by oil giant BP, rig owner Transocean and others to remove heavy drilling mud that was supposed to help tamp down destructive gas kicks.
The civil courts may provide another vector for understanding the accident, as lawyers continue to file suits against BP, Transocean and other companies connected with the Deepwater Horizon. Among the defendants in those suits: Halliburton, a contractor responsible for installing key cement barriers that were supposed to keep gas out of the well in the first place, and Cameron International, the manufacturer of the blowout preventer valves that were supposed to be a last-ditch way to shut off the well, but failed.
Bea said he has spent more than 200 hours reviewing first-hand reports of the Deepwater Horizon’s operations. A constant theme was that gas kicks were more frequent in this oilfield than others the crew had worked on, and members were concerned.
Deposits of oil are not in underground caverns; they ooze in the pores of a sponge-like layer of rock, along with natural gas in both gaseous and the crystallized hydrate forms. But the hydrates also exist throughout the drilled rock formations, and like the oil below, they exert upward pressure when a drilling operation opens a path to the surface.
A transcript Bea collected from a witness says the companies were confident enough they had a lucrative oil source that they decided to convert from an exploratory well to a more permanent production well, a process that requires them to apply a metal and cement casing to the well hole. They chose casing 7 inches in diameter, Bea said, and that was further sealed with cement pumped in by Halliburton. Bea said his sources reported that Halliburton was using a “new” kind of cement for the seal, something the scientist said made him say, “Uh oh.”
“The cement is infused with chemicals and nitrogen, and those chemicals and nitrogen form a frothy cement that is like shaving soap sprayed from a can,” Bea said. “It was put in there because of the concern about damage or destruction of the seals by methane hydrates.”
The crew on the Deepwater Horizon waited 20 hours for the cement job to cure before opening a key valve at the wellhead so they could place a final cement plug about 5,000 feet down the well.
One of Bea’s witness transcripts describes in detail a heated debate among BP, Halliburton and Transocean officials as they are about to add the final cement plug to the well, 5,000 below the wellhead and 10,000 feet below the rig. They argued about whether to set the plug with drilling mud still in the well and riser, or if they should do it with lighter sea water there instead.
As The Times-Picayune reported last week, Bea’s witness claims the decision was made to displace the heavy mud barrier with water before the final plug was set in order to finish the job more quickly.. The crew was planning to temporarily abandon the well, and before leaving, they would need to remove the riser and the blowout preventer, a massive stack of valves and slicing rams that are supposed to shut off the well in case of an emergency, and some time later another operation would re-tap the well to extract its riches.
The mud in the riser would have to be replaced with salt water before the crew could take the final step of removing the blowout preventer, or else polluting mud and chemicals would spill into the sea, angering environmental regulators.
“The debate comes back that it’s been pressure-tested, the coast is clear, so they will displace the upper 10,000 feet of heavy mud and replace it with salt water,” Bea said. “This is a crucial step, and the reason it’s crucial is if the seal at the bottom is fine, it’s OK, but if it’s not OK, we’re screwed. We don’t have enough pressure (from mud) in the column anymore to fight the reservoir (gas and liquid) pressure.”
“In order for a disaster of this magnitude to happen, more than one thing has to go wrong, or fail. First, a very bad cement job. The wellhead packoff/seal assembly (the equipment directly below the blowout preventer that connects the lower pipe casing to the preventer) while designed to hold the pressure, is just a backup. And finally, the ability to close the well in with the BOP somehow went away,” the witness said.
