Shutting down the Nabors' drilling rig involved in last week's North Slope well blowout has proven a difficult task, according to the latest update from the Alaska Department of Environmental Conservation, issued Monday afternoon.

The rig, drilling an exploration well for the Spanish oil company Repsol on the Colville River delta, underwent an emergency shutdown Wednesday morning when an unexpected blast of pressurized natural gas erupted from the shallow hole along with 42,000 gallons of drilling mud.

No one was hurt and the gas passed uneventfully into the atmosphere, contributing to the greenhouse gas load but so dispersed it was no longer a threat to explode. The well stopped leaking on its own last weekend, officials said.

Repsol had hoped a well control crew from Texas would quickly restore control of the well, perhaps as early as Friday. But that hope has proved elusive. The rig and its load of drilling mud are frozen solid. Well control efforts will require the rig to operate.

The DEC said the well-control workers were forced to set up boilers outside the rig to generate steam for thawing mud, plumbing and access doors. Night temperatures have been in the negative mid-teens, with daytime highs around 5 below zero.

"Frozen material in the drill cellar is several feet thick, and at this time crews are not able to estimate how long it will take to complete the necessary thawing operations," the DEC said in its 2 p.m. update.

Cleanup crews scraped about 200 cubic yards of drilling mud and contaminated snow from the upwind side of the rig, the DEC said. But cleanup operations on the rig itself and downwind will have to await word that the well is controlled, in case it begins leaking gas again.

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