Page 75 - Air and gas Drilling Field Guide 3rd Edition

66 CHAPTER 4 Downhole Equipment

FORCE ON BIT

BIT TOOTH ROCK
BOTTOM OF CUTTINGS
BOREHOLE

a) b)

FIGURE 4-5. 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.

Surface

Rock Body
with
In Situ Stresses

Tension
in
Upper Surface
of
Rock Face

FIGURE 4-6. Schematic of tension rock face at bottom of air drilled borehole.

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 by the teeth in conjunction with the flow of the circulation
fluid allows 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 dril-
ling mud, the hydrostatic pressure due to the fluid column compresses the rock
face and places the rock face and the rock material in the immediate vicinity of