ConocoPhillips on Tuesday for the first time described what went wrong to cause an unusual natural gas leak that began in March and continued for weeks at a drilling pad in the Alpine field on Alaska’s North Slope.
The company said the leak, detected on March 4 at the CD1 drilling pad, happened during a drilling operation that put too much pressure on the well. That caused a component of the well to fail about half a mile below the earth’s surface, according to a nine-page technical report.
“Pressure limits were exceeded” during the operation, the company said. The pressure increased during operations to pump diesel fluid into the well to provide freeze protection in an area where permafrost, or frozen ground, can damage pipes.
The company also failed to detect and respond to the rising pressure for three days before the leak was detected, missing an opportunity that would have reduced the gas leak, the report said.
“The pressure increases ... were not recognized and/or addressed and, accordingly, did not lead to investigation or remedial action during that period,” it said.
The gas leak caused the temporary removal of 300 personnel, alarmed residents in the nearby village of Nuiqsut, and halted oil production from the drill site.
It also led to an ongoing investigation by the Alaska Oil and Gas Conservation Commission, and an inquiry by Democrats with the U.S. House Natural Resources Committee.
Rebecca Boys, a spokeswoman with the company, said the incident was unique.
“We’ve never had anything like this happen before,” she said.
The gas was released from a shallow underground area that the company did not expect to contain significant amounts of gas. The gas flowed through different pathways to emerge at the surface of the drill pad in different locations.
The company mitigated the release on March 8 by flowing gas to the surface through the waste disposal well. On April 8, it said it had brought the source of the release under control.
But ConocoPhillips continued to detect trace amounts of gas across the drilling pad for several weeks, as gas trapped beneath underground obstructions slowly made its way to the surface, the company has said.
ConocoPhillips said the portion of the well that failed involved a component called a casing shoe about 2,300 feet under the ground.
The shoe is a structure at the end of a section of pipe, with cement around it that helps to stabilize the pipe in the rock, said Dan Seamount, a longtime commissioner with the oil and gas commission.
Seamount said on Wednesday that the commission continues to investigate the incident and gather information.
“We haven’t come to a conclusion as to what they’re saying is correct or not,” Seamount said.
ConocoPhillips said in its report that it followed requirements to place protective cement around the pipe in certain sections of pipe.
But it did not encase the pipe with cement in the shallow underground zone where the gas originated.
The company has said it was not required to do so because it did not expect that zone to contain significant amounts of gas.
The lack of cement around the pipe in that area, and other factors related to well design, did not cause the gas leak, the company said in its eight-page response letter to the House lawmakers.
But ConocoPhillips told lawmakers that one of the future corrective measures it will take to prevent a similar incident includes taking a closer look at drilling risks and underground zones that require “cement isolation.”
Other corrective measures include developing a standard procedural document for freeze protection operations, improving detection and company communication when pressures are rising in the space outside the pipe, and improving well planning before and during drilling operations.
“We don’t expect this to happen again,” Boys said.
Seamount, with the oil and gas commission, said there’s a good chance the commission will hold a hearing as part of its investigation. He said the commission is working on its own report.
“We’re trying to get it out as soon and accurate as possible,” he said.
ConocoPhillips has said 7.2 million cubic feet of natural gas was released into the atmosphere between March 4 and March 8, when the company was able to begin routing gas to the surface through the disposal well.