Air rotary drilling is a method used to drill deep boreholes in rock formations. Borehole advancement is achieved by the rapid rotation of a drill bit which is mounted at the end of the drill pipe. The drill bit "cuts" the formation into small pieces, called cuttings. This method utilizes air as a circulating medium to cool the drill bit, bring drill cuttings to the surface, and maintain borehole integrity. Once the air and cuttings return to the surface they are captured in a cyclone where the cuttings drop out the bottom. Standard split-barrel and thin-wall sampling are not utilized with this method. A broad range of coring equipment is supported for consolidated rock.
There are several variations in air rotary techniques including direct air rotary casing hammer (ARCH), downhole hammer, and under reaming.
In ARCH, often called "top drive", both rotational downforce and impact force are provided by an above-hole impact hammer and a rotating drill head. Drill cuttings are removed from the borehole by the injection of high pressure compressed air, down the middle of the drill rod, exiting out of the annular space between the drill rod and borehole. Samples of the substrate from the direct air method are often thought to be less correlative to the depth of the bit face.
Under reaming is a variation of the top drive casing advancer method that uses a rotating cutting bit in front of the advancing casing. The bit cuts a slightly larger diameter hole than the outside diameter of the casing. The larger diameter hole allows the casing to be more easily advanced in tight formations and in deeper drilling applications where the longer drill stem encounters significant sidewall friction.
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