ROTARY DRILLING

For rotary drilling, hollow drill rods of steel or aluminium are used. These are thread-connected and transmit torque and feed pressure to the drilling bit or drilling tool which is attached at the end of a column of the drill rods. Rotation of the drill rods is through gearing driven by a prime mover at the surface. As the rods rotate, the drilling tool abrades the rock and the cuttings are cleared by pumping water under pressure or compressed air down the hole through the hollow drill rods. The water or air, along with the cuttings, comes to the surface through the space between the drill rods and the sides of the drill hole. In some drillings, specially those for oil exploration, mud which is not very viscous, is circulated instead of water. The mud which keeps back any water, gas or oil pressure encountered during drilling is known by various trade names such as bentqnite, aquagel, etc. and these muds serve different purposes depending upon their constituents. In the shallow drillings in the mining areas it may be necessary to resort to mud flushing when passing through a fractured or friable zone.

Aluminium rods weigh only half as much as steel rods, but owing to their bigger gauge they possess 90% of the mechanical strength of the latter. The couplings, which are the parts most exposed to wear, are made of chromium-nicked steel. Aluminium rods offer numerous advantages, such as increased machine capacity, easier handling, more rapid and simple recovery of the drill string and faster rotation, all of which contribute to simplifying drilling and reducing costs.

The various methods of rotary drilling are known by the type of drilling tool used but the diamond drilling method is quite common.