Page 78 - Air and Gas Drilling Manual

3-6 Air and Gas Drilling Manual
provides torque and the rotation of the drill string while allowing the air and
entrained rock cuttings to return to the surface.

Figure 3-4: Side inlet sub for dual wall pipe reverse circulation operations (courtesy of
Holte Manufacturing Company).
The threaded pin and box connections on most dual wall pipe are shouldered
connections. Most manufactures of this type of pipe do not use API thread profiles.
The threaded connections for dual wall pipe will be discussed in Section 3.4.
3.1 Drill Bits
There are three basic types of rotary drill bits. These are drag bits, roller cutter
bits, and air hammer bits.
3.2.1 Drag Bits
Drag bits have fixed cutter blades or elements that are integral with the body of
the bit. The earliest drag bits were simply steel cutter blades rigidly attached to a
steel body that is made up to the bottom of the drill string. Later natural diamonds
were used as the cutter elements. A diamond drill bit has natural diamonds that are
embedded in a tungsten carbide matrix body that is made up to the bottom of the
drill string. The most recent development in drag bit technology is the
polycrystalline diamond compact (PDC) bit. These drill bits have specially
designed diamond cutter elements bonded to small tungsten carbide studs. These
studs in turn are embedded in a steel body that is made up to the bottom of the drill