Hydromechanical Drilling into a Burning Coal Seam, Richland County, Montana

Abstract

Subsurface coal fires in the Northern Great Plains region render the overlying land useless by sterilizing the soil and by causing highly irregular subsidence. Hot gases and glowing particles emitted from the subsurface fires cause a significant danger of range fires in that arid region. A powerful waterjet drill has been used in Richland County, Montana to drill into a burning coal seam. Fifteen vertical holes were drilled through 20 ft (6.5 m) of Tertiary Fort Union Formation to a burning zone and nine horizontal holes were drilled into the burning zone from the outcrop on the Yellowstone River. The drilling rates were measured at 0.8 to 18 in./sec (2 cm/sec to 0.5 m/sec) and are a function of the lithology under attack. The waterjets extinguished the fire but it later re-ignited in coal underlying fractured non-combustible overburden because atmospheric oxygen could not be excluded from the hot mass. The fire is observed to advance underground in an irregular front along fractures in the coal rather than in a simple plane. Thermocouples measured temperatures up 1,000°C and thermal gradients up to l,600°C/m were determined. Carbon monoxide concentrations in gas emitted from this fire were generally in the range of 20 to 70 ppm, but concentrations of more than 1,000 ppm were measured, thereby presenting a significant danger to humans and beasts.