Page 78 - Air and gas Drilling Field Guide 3rd Edition

4.2 Drill Bits 69

Air passageways
Air screens

FIGURE 4-9. Cutaway of the interior of a tricone drill bit (courtesy of Reed Rock Bit Company).

gauge protection allows air bits to drill long abrasive sections without appreciable
loss of gauge. However, it should be noted that some gauge loss will always occur
in hard abrasive formations. It is good practice to design the well profile (i.e.,
borehole diameters and associated casing diameters) in such a manner that long
sections having hard abrasive formations can be drilled with the drill bit diameter
1
1
1
sequence of 6 in (152 mm), 6 8 = in (156 mm), 6 4 = in (159 mm), 6 2 = in
1
3
3
(165 mm), and 6 4 = in (171 mm), the sequence of 7 8 = in (181 mm), 7 8 =
5
3
7
in (187 mm), 7 8 = in (194 mm), and 7 8 = in (200 mm), or the sequence of 8 8 =
1
3
in (213 mm), 8 2 = in (216 mm), 8 4 = in (222 mm), and 9 in (229 mm). Using
one of these drill bit diameter sequences allows anticipation of loss of gauge.
1
The top of a long hard abrasive section can be drilled with a 6 2 = -in (165 mm)-
1
diameter drill bit and, when there is a bit change, followed by a 6 4 = -in
(159 mm)-diameter drill bit, and then near the bottom of the section followed
1
by a 6 8 = -in (156 mm)-diameter drill bit for the last bit change.
Most air or natural gas drilling operations use insert tricone drill bits. Even
though previous drilling operations with mud may have shown that a mill tooth
bit had been successful in drilling a particular rock formation, the mechanics of
the rock cuttings creation process at the bottom of the air borehole require that
an insert bit be used in order to generate smaller rock cuttings. The smaller the
rock cuttings generated by the drill bit, the more efficient the rock cutting crea-
tion and transport of cuttings particles from the bottom of the borehole.
Nearly all tricone drill bits are equipped to accept nozzle inserts in three open
orifice flow channels in the drill bit body. Nozzles of various sizes (in 32nds of an
inch) are used extensively in mud drilling operations. Standard practice for