Wednesday, February 13, 2008

Getting Stuck in the Hole

This is the way it happens sometimes with drilling: a drill rig - core or rotary - might get stuck in the hole, sometimes at a relatively shallow depth, sometimes deeper. If the rig gets stuck at a shallow depth, it's easy and not too expensive to just move the rig a few to several feet and start the hole over again. If the rig gets stuck near the target depth, sometimes that will be the end - E.O.H. - end of hole. When a drill rig gets stuck somewhere between shallow and deep, one has to carefully consider whether to re-drill the hole, or whether to call it "Deep Enough." That call depends on many factors, including geology, cost of drilling, and assay results from nearby holes.

One time, a core rig got stuck at about half the target depth. The drillers then had to try to get two sets of rods out of the hole, the original HQ-diameter steel with which they started the hole, and the reduced-to, narrower NQ with which they had drilled the lower part of the hole. The drillers had gotten stuck in bad ground after drilling through an old mine working, which drilled as several runs of nothing (a void) and then some cruddy, sandy material that contained wood from old, possibly rotten mine timbers. They discovered an unknown underground mine working - tunnel, raise, or stope - and that was not what we were hoping to discover (an ore deposit is more like what we were looking for). For the next couple hundred feet, they continued to hit voids and possible workings. Recovery picked up after a while - and then - the rods got stuck, possibly in a fault zone.

Sometimes if a hole is lost due to getting stuck, the down-hole survey can't be completed, other times it can be. The survey will determine the actual direction of the drill hole under the ground, whether it shot off in some unplanned direction, whether it spiraled around, or whether it went relatively straight.

To get unstuck, core drillers can go back into the hole with even narrower rods - BQ for the above example (the diameter of the rods used to get unstuck will depend on the diameter of the rods already in the ground). A cutting tool will be attached to the narrower rods; it will spin around in the hole to cut the rods at the lowest possible place, usually just above the core barrel. With luck, everything but the drill bit, core barrel, and shell can be recovered without any extra loss of drill steel. Sometimes, though, the drillers will end up having to cut the rods higher in the hole, leaving some expensive steel behind. Sometimes, the drill hole is blasted in order to break the rods at a certain point, if cutting or twisting and turning doesn't work. Getting stuck and unstuck can be an interesting, though often expensive and sometimes frustrating, experience.

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