Continuous mining STRAIGHT AND DEEP

Late in March, a series of four blind holes to a depth of 2,000 ft were drilled at Inco’s Crean Hill mine by Underwater Gas Developers. In a departure from normal practice, Underwater Gas used oil field techniques combined with high pressure air and downhole hammer technology to drill the holes. All four holes — a 15-inch diameter, two 8 3/4 -inch diameter and a 12 1/4 -inch diameter — were to deviate only one degree. That means that at 2,000 ft the target circle would have a 35-ft radius. The 15-inch hole has been gyroscopically surveyed and is reported to have a calculated closure of fewer than 8 ft. The remainder are to be surveyed soon. The contractor used a specially converted Ingersoll-Rand T-5 Long Tower-Trailer Rig. Traditional oil field drilling incorporates a tri-cone bit, but, in this case, a down-hole hammer with a button bit was used for faster penetration and straighter holes. The 70-ft-high rig required nine tandem truck loads of ancillary equipment to keep it running round the clock.

After Inco locates the holes underground, it will drift into them. Three of the holes will be used to deliver backfill and the fourth will deliver roadbed material. According to Richard Brooks, manager of Underwater Gas the significance lies in the narrow deviation.

“Everybody has trouble drilling from surface to a predetermined spot underground. What we’re doing is setting up at a particular spot and coming reasonably close at going straight down.”

All the holes were steel-collared and cemented for the first 20 ft to shut off ground water and to establish verticality. Substantial water was encountered at 200 to 300 ft. This was overcome using an accelerated cement slurry, which was site-mixed and pumped to the bottom and filled the hole completely to surface. This procedure successfully sealed the water and allowed drilling to continue after a 12-hour wait.

The last three holes were drilled in a single pass. The 15-inch hole was initially piloted as a 12 1/4-inch hole, then opened using specially manufactured hammer hole-openers along with string reamers.