Page 74 - Air and gas Drilling Field Guide 3rd Edition

4.2 Drill Bits 65

The threaded pin and box connections on most dual wall pipe are shouldered
connections. Most manufacturers of this type of pipe do not use API thread
profiles. The threaded connections for dual wall pipe are discussed in Section 4.4.

4.2 DRILL BITS
There are three basic types of rotary drill bits used in air and gas drilling opera-
tions. These are drag bits, roller cutter bits, and air hammer bits.

4.2.1 Drag Bits
The original drag bits used in the early use of rotary drilling have fixed cutter blades
or elements that are integral with the body of the bit. The earliest drag bits were
simply steel cutter blades attached rigidly to a steel body that is made up to the bot-
tom of the drill string. Later, natural diamonds were used as the cutter elements.
A diamond drill bit has natural diamonds embedded in a tungsten carbide matrix
body that is made up to the bottom of the drill string. The most recent develop-
ment 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 string. Drag bits have no moving parts. Their cut-
ting mechanism is a scrapping or machining action that is best used to drill rock
formations that fail structurally in a plastic mode (e.g., soft, firm, and medium-hard,
nonabrasive rock formations). Although PDC bits have been used extensively in
past decades, they have been confined to drilling operations using water- and
oil-based drilling muds. However, recent field experiments have shown that
PDC bits can be used successfully in air operations using gas, aerated, and foam
drilling fluids.

4.2.2 Roller Cutter Bits
Roller cutter bits use primarily a crushing action to remove rock from the cutting
face and to advance the drill bit. The weight or axial force that is applied to the
drill bit is transferred to the tooth or teeth on the bit. These teeth are pointed
(mill tooth bit) or rounded (insert tooth bit), and the force applied is sufficient
to fail the rock in shear and tension and cause particles of the rock to separate
from the cutting face. The drill bits are designed to remove a layer of rock with
each successive rotation of the bit. Figure 4-5a shows the tooth of a tricone
bit being forced against the rock face. Figure 4-5b shows the rock particles cre-
ated by the failure of the rock face due to the “crushing” action of the tooth.
The circulation fluid entrains these rock cuttings and carries them away from
the rock face.