The process is required to ensure Buchanan's atmosphere is inert.

"Although current sampling of the underground environment at the Buchanan Mine indicates safe and stable conditions, safety agencies have required that the company seal all mine openings - the agencies' traditional method of creating an inert environment underground - prior to approval of re-entry into the mine," the producer said in a statement.

As part of the review of the operation, more than 100 monitoring stations for sampling and approximately 50 boreholes were drilled for pumping of water and inert gas.

"A small mined-out area near the current location of mining has been the focus of intensive monitoring efforts," it confirmed, adding that water and inert gasses were also injected into that area.

"It is believed that these actions have eliminated any possible combustion."

The mine has been idled since July 9, when a series of roof falls in mined-out sections damaged a portion of the operation's ventilation controls.

The company said in its third-quarter report in October that the idling of the mine had cost it $US84 million since July in atmospheric management and monitoring expenses, as well as lost income from sales after stockpiles at the operation emptied.

Allowing time for an additional waiting period following the sealing, Consol engineers said the mine could reopen in early January.