Page 85 - Air and gas Drilling Field Guide 3rd Edition

76 CHAPTER 4 Downhole Equipment

The third digit feature numbers for roller cutter bits are 1 to 7 and refer,
respectively, to standard roller bearing, standard roller bearing for air applica-
tions, standard roller bearing with gauge protection, sealed roller bearing, sealed
roller bearing with gauge protection, sealed journal bearing, and sealed journal
bearing with gauge protection.
The third digit is the feature number. These numbers are interpreted differ-
ently for drag bits and roller cutter bits. For diamond and PDC drag bits, the fea-
ture numbers are 1 to 8 and refer, respectively, to step type, long taper, short
taper, nontaper, downhole motor, side-track, oil base, and core ejector.
Table 4-1 also shows an insert rock bit comparison chart for four manufacturers
(second through fifth columns). Feature 2 on all the charts of the bit manufac-
turers shows the insert bits designed for air drilling operations. Although the com-
parison chart shows insert tricone drill bits, the sealed roller bearing and sealed
journal bearing bits are also often selected for air and gas drilling operations.
It should be noted that single cone drill bits and air hammer bits are not pres-
ently classified in accordance to the IADC code system.

4.3 BOTTOM HOLE ASSEMBLY
Figure 4-16 shows a typical bottom hole assembly (BHA) for a direct circulation
rotary drilling operation. The BHA is the section of the drill string below the drill
pipe (see Figure 4-1). This section of the drill string is the most rigid length of the
string. It is this section of the drill string that determines how much weight can
be placed on the drill bit and how “straight” a vertical borehole will be drilled
with the drill string.
The assembly in Figure 4-16 is composed of a drill bit at the bottom, drill collar
tubulars, a near bit stabilizer directly above the bit, a stabilizer at the middle of the
assembly, and a stabilizer at the top of the assembly. The addition of stabilizers to
the drill collar string generally improves the straight drilling capability of the drill
string. Highly stabilized drill strings are necessary when drilling in “crooked hole
country.” Crooked hole country usually refers to rock formations that tend to
deflect the bit and, thus, the drill string as the drill bit is advanced.
Hard-to-medium rock formations that are tilted to a high angle from horizon-
tal are one of the main causes of severe borehole deviations from vertical. All
deep rotary drilled boreholes will tend to have some deviation and tend to have
a corkscrew three-dimensional shape (usually to the right). The deviation from
vertical can usually be kept below 3 to 5 with good drilling practices. In gen-

eral, air drilled boreholes can have more deviation than mud drilled boreholes
(assuming same rock formations). However, most of the increased deviation
from vertical is due to the fact that air drilling penetration rates are significantly
higher than a mud drilling operation and drillers tend to take advantage of that
increased drilling rate and let the deviation get away from them. To correct this
tendency, it is good practice to utilize a more stabilized BHA when drilling
an air drilled borehole than would be used in a comparable mud drilled