The Weekly Reflektion 32/2021

The Pryor Trust blowout in 2018 in Oklahoma, USA, killed 5 people. These deaths could and should have been prevented. What can we learn from this Major Accident?

The Pryor Trust fire following the blowout. Photo USCSB Investigation Report

Is your operational team performing as you expect them to? Are your expectations clear?

On January 22nd, 2018, a blowout and fire at a Pryor Trust gas well in Oklahoma, USA, killed 5 rig workers trapped in the driller’s cabin on the rig floor. The chain of events leading to the blowout illustrates issues that we can learn from to improve our operations, and not only in drilling.

The rig was drilling the horizontal leg in a gas reservoir when gas cut mud was noticed in the mud returns. The returns were then routed to a mud-gas separator (MGS) to remove the gas before the mud was circulated down the well again. The associated gas was flared off on site. This was the first signal that the well might be underbalanced, with the mud weight being insufficient to stop the gas entering the wellbore. Despite this, drilling continued with gas being flared off.

Four and a half hours later the decision to change the bit was taken and preparations for pulling out of hole started. The well was circulated for 3 hours to remove any cuttings from the well before the trip, removing any drilled gas that may have come from the formation cuttings. However, gas was still being flared off after the MGS during the circulation.

A new electronic trip sheet was used, which may have caused confusion with respect to mud volumes, and the driller turned off the alarms. Pulling out of the horizontal section the Chemical Safety Board (USCSB) investigation checked the mud volumes and saw a 20-barrel (3m3) pit gain. With the bit in the vertical section, a 45 second flow check was performed, possibly as a result of the difference between theoretical and actual mud volumes, concluding that the well was stable. It was intended to pump a weighted pill, but a lost circulation pill was pumped by mistake, which plugged the drill pipe, making it impossible to circulate and resulting in a wet trip.

Pulling out, a total gain of 88 barrels (14m3) was seen by the investigation team, and the blind rams were closed when the bottom hole assembly (BHA) was out of hole. After rebuilding the BHA, the pressure under the blind rams was checked and zero pressure registered. The blind rams were opened and an immediate gain of 5 barrels was seen in the data analysis after the incident. When testing the BHA, more volume increase was seen, giving a total volume discrepancy of 207 barrels (33 m3). Mud was seen flowing out of the BOP with increasing force, eventually bridging the gap between the top of the BOP and the rig floor. The floor hands evacuated to the drillers cabin as the well blew out with gas reaching the surface, and this ignited quickly. The 2 floor hands joined three other people in the driller’s cabin, but as both exits were blocked by flames, there was no way to escape from the driller’s cabin and the five were killed.

Why were the signals missed? Why did the operational team believe that what they were doing was OK? Why were procedures not followed? Why were they trapped in the driller’s cabin? These issues will be addressed in Reflektions over the next few weeks.

Reflekt AS