DRILLING TECHNIQUE The dual-tube flooded reverse-circulation drilling technique makes it possible for the bit to penetrate loss circulation zones without the use of a more traditional raised bore rig
Integrated drilling services and equipment provider Boart Longyear last month completed another unique project using its dual-tube flooded reverse-circulation technique.
The technique, used to drill 60-inch shafts through loss circulation zones, was used to create a ventilation shaft for an underground longwall coal mine in south-west Wyoming, in the US, says Boart Longyear US/Mexico territory contract manager Jason Lamb.
“The task entailed guiding a 60-inch drill bit to a depth of 550 m through a challenging geological formation that included two aquifers – all in just over four weeks,” he says.
Boart Longyear has also completed projects using this drilling method in mining projects in Africa.
Lamb says the dual-tube flooded reverse-circulation drilling technique is effective, as it enables the drillbit to penetrate loss circulation zones without having to use a more traditional raised bore rig.
“The technique pumps air through the outer tube and forces mud and cuttings upward through the inner tube, preventing them from plugging porous rock formations.”
He notes that Boart Longyear’s drilling team worked around the clock by rotating three-member crews on the US project, with the company using an LM200 top head drive rig fitted with stabilisers and the bit.
To minimise the risk of a mine-entry collapse, the borehole was drilled off to the side of the mine tunnel and a 54-inch casing with a 1/2-inch wall thickness was installed and cemented in place, Lamb explains, adding that underground mine crews then mined over and punched through the concrete to open up the shaft.
“Drilling such a large-diameter hole through loss circulation formations in a single pass – and in less than a month and a half – is something few companies are capable of performing,” says Lamb.