Page 79 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-7
string. Drag bits have no moving parts. Their cutting mechanism is a scrapping
action that is best used to drill rock formations that fail structurally in a plastic
mode (e.g., soft, firm and medium-hard, non-abrasive rock formations). The modern
drag bits require incompressible liquid circulation fluids to keep the diamonds from
being damaged by excessive heat. Thus, these modern drag bits have very limited
applications in air and gas drilling operations.
3.2.2 Roller Cutter Bits
Roller cutter bits use a crushing action to remove rock from the cutting face and
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 3-5a shows the tooth of a tri-cone bit being forced against the rock face.
Figure 3-5b shows the rock particles created 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.

a)

b)
Figure 3-5: The rock crushing action of a tooth of a roller cutter drill bit, a) prior to
failure of rock, and b) after failure of rock.
The roller cutter element(s) of a drill bit has a series of teeth that are designed to
crush rock over the entire rock face after a single rotation of the drill bit. The
repeated crushing action of the teeth in conjunction with the circulation fluid allow
rock particles at the rock face to be continuously removed and the drill bit advanced.
When this crushing action takes place at the bottom of a well filled with drilling
mud, the hydrostatic pressure due to the fluid column compresses the rock face and