Drilling Engineer’s squad of drilling rigs—including three CME-55s, three CME-75s and a Geoprobe 7822DT—stands ready for rotary drilling applications. Field crews have tri-cone and Kenclaw drill bits available to adapt the drill stem to the particular requirements of each job. The drilling rigs also have down-the-hole drilling capabilities for jobs that require faster borehole advancement.
Rotary drilling is well suited to drilling in semi-consolidated and consolidated surfaces. It offers good penetration and quick removal of cuttings. The technology is useful for well-bore drilling and mineral exploration, among other applications.
As in auger drilling, the drilling rig uses the weight and rotation of the drill stem to push into the subsurface. In rotary drilling, however, the drill stem is relatively smooth, lacking the helical flights of auger drills. A sharp drill bit cuts into the earth at the bottom of the bore hole.
The drilling rig injects a flushing media into the drill pipe. The pressure of the injection forces the flushing media out of small holes in the drill bit. This cools and lubricates the drill bit. The media then pushes cutting to the surface by rising through the annular space—the space between the drill stem and the borehole.
Air and mud are both options for the flushing media; the choice of the material depends on the lithology of the site. If the drill plan calls for air as the flushing media, the drill crew may add a small amount of water to the air. This minimizes dust and helps cool the borehole.
If the drill plan calls for mud as the flushing media, the drill crew carefully monitors the viscosity of the mud to ensure that it is thick enough to lift cuttings to the surface. The drill crew carefully monitors the cuttings as they arrive on the surface in order to create a well log to provide a detailed history of the borehole.
Down-the-hole drilling is a variation on rotary drilling that adds the percussive cutting power of a pneumatic drill situated at the end of the drill pipe. The down-the-hole drill hammer rapidly strikes the subsurface as the drill rig rotates the drill stem. The compressed air that the drilling rig uses to drive the drill removes the cuttings as the drill produces them. The cuttings that down-the-hole drilling produces are large and defined.
Dual-tube (or dual-wall) reverse-circulation is another technique that can be used in rotary drilling. The technique involves advances a double-walled drill pipe into the subsurface. The drilling rig injects high-pressure air into the central tube as it advances. Because of the way the drill stem is set up, there is very little anular space around the drill stem and cuttings take the path of least resistance to the surface in the outer tube.
In unconsolidated sediments, drill crews can advance the dual-tube drill pipe with a percussive hammer system. In semi-consolidated and consolidated formations, the drill crew can use a top-head rotary drill to advance the pipe.
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