Air Rotary
There are actually two types of air drilling: direct rotary air and air hammer. In both, air is used as the drilling fluid, as opposed to a water-based fluid such as in mud rotary drilling. Both are restricted to use in consolidated or semi-consolidated formations.In North Carolina, air drilling is used almost exclusively for drilling in the saprolite and igneous and metamorphic rock of the Piedmont and Mountains. In many cases, hollow-stem augers are first used to drill through the saprolite. With the augers in place to stabilize the borehole, an air drilling rig will drill a socket into competent bedrock. The casing will be set and the augers will be pulled as the annular space around the casing is grouted. After the grout sets up, the air rig will return and the open hole portion of the well will be drilled through the casing.
In direct rotary air drilling, air is pumped down the drill pipe and escapes into the borehole through holes in the bit. As the bit turns, it grinds the cuttings into small pieces, which the compressed air carries to the top of the borehole. Direct rotary air drilling is normally used for softer formation materials, such as limestone, sandstone, and some of the softer metamorphic rocks. It is not as commonly used in North Carolina as air hammer drilling.
In air hammer drilling, compressed air is pumped through the drill pipe to an air hammer bit in the borehole. The pneumatic bit strikes the rock very rapidly. During drilling the pipe string is rotated by the drilling rig to aid in keeping the borehole straight. The compressed air that escapes at the bottom of the air hammer carries the pulverized cuttings to the surface. Air hammer drilling is very rapid in even the hardest materials. It is the most commonly used air drilling method in North Carolina due to its higher efficiency while drilling in our hard metamorphic and igneous rocks.
Air drilling offers many advantages over other drilling methods. While the initial set-up costs for an air rig are fairly expensive, this method is much quicker than other drilling methods for drilling in the hard rock of North Carolina. Additionally, since it does not use water-based drilling fluids, there is less chance of cuttings or drilling fluids clogging the aquifer materials than with mud rotary drilling. Because of its higher drilling efficiency, air drilling is often much less expensive than other methods of drilling.