LOS ANGELES – Flaws in a cement encasement intended to seal BP’s well are most likely the root of last month’s deadly explosion on the Deepwater Horizon drilling rig that sent oil spewing into the Gulf of Mexico, according to interviews, government officials, congressional hearing testimony, drilling reports and other company documents.
The April 20 accident is the subject of multiple investigations that promise to be complex and long. Hearings in the past two weeks offered multiple lines of inquiry into what one engineer calls “a confluence of unfortunate events.”
But at least a dozen experts with intimate knowledge of offshore drilling agreed that, deep in the well, cement, or the “casings,” pipes encased by cement, had to have failed first.
Several specifically finger BP’s design for that cement job that used relatively little cement and relied on an usual configuration that made it harder to test for imperfections, they said.
Cementing is supposed to form an impenetrable seal that can keep the hot, gassy oil from surging up the well. But a single flaw in that seal, perhaps a crack the size of a human hair, can be enough to unleash a volcano of petroleum.
The Mississippi Canyon Block 252 project had encountered problems, including two reported gas kicks, where drillers failed to keep gas from surging up, according to the hearings and company sources. Government and company documents suggest it was over budget. But the Deepwater Horizon had struck oil, something that only happens in about a quarter of exploration wells.
All that was left was to seal the well so another vessel could produce from it later. Cementing involves standard techniques. But “there is an art to it, and every well is a little different,” said Gene Beck, an associate professor of petroleum engineering at Texas A&M who testified at the hearings.
Halliburton, the cementing contractor, worked according to BP’s design.
An exploratory oil well consists of a series of nested pipes, or casings, each plunging deeper into the sea floor. Crews pumped cement down into the well until it crept up the outside of the steel casings, forming a bond between the pipe and the rock deep under the sea floor.
The goal was to let the cement set, add a few temporary cement plugs within the casings for good measure, then haul up the blowout preventer and move off.
But forming a good seal is tricky. Traces of drilling mud, a clay substance used to tamp rising oil as crews work, can pollute wet cement, creating weak spots. Wet cement also can drizzle into the rock or be distributed unevenly, leaving gaps – especially if the casings aren’t properly centered in the well, or “centralized.” The encased pipes may collapse.
Any of these failures can give gas under extreme pressures a chance to creep in and build up in the well bore or between nested casings.
Beck said BP’s encasement design called for only partial coverage of casings deep in the well. Cement did not reach the bottom of next largest casing in high-pressure areas, a decision Beck called “shocking.” He also raised questions about the design’s reliance on a single, central casing instead of several for the 2.5-mile-deep well. For technical reasons, this configuration would make it more difficult later to test the cement job for leaks.
BP declined to comment on this and other issues in this story.
The crew of the Deepwater Horizon conducted pressure tests that assess seals but not the cement.
Hearing testimony suggested the pressure tests revealed some discrepancy. It could have been a confusing, anomalous reading that meant little. Or it might have provided an early hint that gas had begun to leak into the well bore, perhaps creeping up through the mud between the casings, said Tadeusz Patzek, professor of petroleum engineering at the University of Texas in Austin.
Regardless, it appears someone made a decision that the seal was good, so the crew moved to the next step: pumping sea water into the riser and taking mud out.
It was a fatal moment. The switch from mud to relatively lightweight water decreased pressure. Deep in the well, gas surged.
Originating deep in the well, the gas likely blasted through the wellhead sitting on the sea floor, perhaps overwhelming seals in that device. It blasted through a blowout preventer sitting atop the well head and went up the riser, expanding into multiples of its volume in the last stretch of pipe – “an exponential process once it starts,” said Tim Robertson of Nuka Research, an Alaska consultant.
Once the gas hit the rig, an explosion erupted. Eleven died.