Principle of air rotary drilling

Principle of air rotary drilling

Air rotary drilling was the first method to be developed for use in drilling water wells. Roller type rock bits are used to do the drilling with this method and they consist of either two, three or four roller cones with teeth mounted on a bit body by means of roller or ball bearings.

The bit is then rotated by means of the drill rods being rotated. As the bit rotates, the rollers in turn rotate, thus causing the cone to rotate under the bit. As the teeth on the bit strike the formation they force the formation to break up into smaller pieces.

Air is directed into the drill pipe and goes down through the kelly and through the bit as the bit rotates. As the air comes out at the bottom of the bit it strikes the formation being cut loose and blows it away from the bottom of the bit. This exposes new rock surface for the bit to drill upon. The velocity of the air coming out of the bit is great enough to carry these cuttings away from the bottom of the bit and return them to the surface of the ground.

These cuttings are blown to the surface between the hole that has been drilled and the drill pipe which is turning the bit. Thus, the bit is continuously exposed to new formation and the material that has been cut is continuously being blown to the surface.

This method of drilling is really an extension of hydraulic rotary drilling with just the change from a mud-type medium to remove the cuttings from the hole to the use of an air-type medium to remove cuttings from the hole and, therefore, was not a major step forward.

In drilling with air rotary type equipment, it is generally recommended that rotation speeds in the range of 10 to 20 rpm be maintained on very hard rock, with a very high down-pressure being applied to the bit by means of a down-pressure device on the drilling machine. The proper weight to be applied to a bit varies from bit to bit and depends largely on the design of the bit. The manufacturer's recommendations should be followed when applying down-pressure.

Roller type rock bits are designed with longer teeth for soft formations and shorter teeth for hard formations. Generally, with softer formations, less down-pressure and higher rotation speeds are called for.