The U.S. Chemical Safety Board (CSB) released a factual update into its ongoing investigation of the blowout and fire at the Pryor Trust Gas Well on January 22, 2018, in Pittsburg County, Okla., that killed five workers. The CSB determined the incident occurred shortly after drilling crew members removed the drill pipe from the well in a process known as “tripping.”

The investigation has determined the following timeline related to the blowout and fire:

  • January 21, 2018: Crew members from the Patterson-UTI Drilling Company had been drilling a gas well for over a week. Activities were being overseen by the operator of the well, Red Mountain Operating (RMO).
  • At 3:36 p.m., the Patterson crew stopped drilling to remove the drill pipe from the well and change the drill bit.
  • At 6:48 p.m., crew members began the process of removing the drill pipe from the well. The Patterson crew pumped mud into the well during the removal of the drill pipe with the intent to keep the well full of mud. That operation involved closing an isolation valve to prevent mud from flowing out of the well. By 10:30 p.m., the end of the drill pipe reached the top of the curve in the well.
  • January 22, 2018: At 12:35 a.m., the crew pumped fluid also referred to as a “weighted pill” above the top of the curve to prevent gas influx into the lateral portion of the well.
  • At 1:12 a.m., the crew began removing the drill pipe from the vertical section of the well. For this portion of the operation, the Patterson crew performed a “Continuous Fill” tripping method. Mud was continuously circulated in the wellbore using the Trip Tank pumps to keep the well full by replacing the volume of the drill pipe removed with drilling mud. The isolation valve was open for this operation.
  • At the start of this procedure, the drill pipe started pulling wet – meaning the drill pipe being removed had not drained and still contained mud. The Patterson crew was aware that the pipe was plugged. At this point the Patterson crew attempted to pump a volume of mud also referred as a “slug” into the drill pipe to push the mud that remained in the drill pipe out, but this was not successful as the drill pipe was plugged. Therefore, the drill pipe in the vertical section was removed while it still contained mud.
  • By 6:10 a.m., the drill pipe and drill bit were completely removed from the well. At that time, the driller closed the blind rams on the well’s blowout preventer.
  • At 7:57 a.m., the driller opened the blowout preventer blind rams so that a new piece of drilling equipment called a bottom hole assembly could be lowered into the well. At 8:09 a.m., mud was pumped through the bottom hole assembly to test the new equipment.
  • While the rig crew tested the bottom hole assembly equipment, the mud pits gained 107 barrels of mud. Mud pit gains are an indication of a possible gas influx in the well. Data obtained by the CSB indicates that conditions existed that could have allowed a gas influx into the wellbore during the tripping operation.
  • At 8:35 a.m., with testing complete, the bottom hole assembly was lifted out of the wellbore. At 8:36 a.m., mud blew upwards out of the well. The mud and gas from the well subsequently ignited causing a large fire.

In addition to the factual report, the CSB released an animated timeline of the events leading up to the incident. The animation can be found at the CSB website.