Cable-Tool DrillingThe cable-tool method was the earliest use; now it is employed only where the oil sands are not very deep or when there isn't much hard-rock drilling. It is cheaper than rotary drilling, because the hole doesn't need to be lined all the way down with expensive steel pipe as is the case with the rotary-drill hole. In cable-tool drilling, a bit on the end of a heavy drill stem suspended from a wire cable is raised and allowed to fall so that bit pounds and crushes the rock at the bottom of the well. Each time it falls it grinds its way down deeper. The up-and-down motion of the drill stem is achieved by hooking the wire cable from which it hangs to a walking beam. A crank on a wheel rocks the beam up and down. From time to time the tools are raised from the hole by a cable wound around a bull wheel, for sharpening. While the tools are out, the well is flushed out, and a bailer is dropped down to remove the drilling chips and sludge. If the well passes through beds of soft sand or mud is likely to cave in, sections of steel pipe or casing, are lowered into it as a lining to keep the hole from flooding or caving.
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