Page 79 - Air and gas Drilling Field Guide 3rd Edition

70 CHAPTER 4 Downhole Equipment

vertical direct circulation air or natural gas rotary drilling operations is to use tri-
cone drill bits with open orifices (i.e., no nozzles). Thus, for well planning calcu-
lations (discussed in Chapters 8 to 12) it is important to ascertain from the drill
bit manufacturer the open orifice minimum inside diameters for the drill bits used
in the operation.
There are special reverse circulation tricone drill bits. These are fabricated
using the same mill tooth and insert tooth cone designs as the direct circulation
drill bits discussed previously. Figure 2-9 shows the schematic of the inside flow
channel of a reverse circulation tricone drill bit. This large orifice allows the
return flow of drilling fluid and entrained rock cuttings to flow from the annulus
through the large orifice in the bit body to the inside of the drill string.

Single Cone Bits
There are single cone or “monocone” drill bits. Unlike tricone drill bits that drill
by a crushing action, single cone bits drill by a scraping action. Thus, single cone
drill bits utilize wear-resistant tungsten-carbide inserts in the cutting structure.
1
These drill bits are most effective in smaller diameters: 2 4 = in (70 mm)to6 8 =
3
in (156 mm). These drill bits, with the appropriate cutting structure, are suitable
for drilling soft, as well as medium-and-hard, rock formations.
The principal advantage of single cone drill bits is the large size of the support
bearing of the cone and the tungsten-carbide inserts in the small drill bit dia-
meters. Small diameter tricone drill bits are very fragile and subject to pinching
and bearing damage if forced into an out-of-gauge borehole or used to ream an
out-of-gauge borehole. These single cone drill bits are not subject to pinching
and other damage when used to ream out-of-gauge boreholes. It is therefore
good drilling practice to use single cone drill bits to drill small diameter sections
in deep wells.
Figure 4-10 shows a typical single cone drill bit. Single cone drill bits are also
equipped to accept nozzle inserts in three open orifice flow channels in the drill
bit body. However, like the tricone drill bits used for air and gas drilling opera-
tions, it is standard practice to use single cone drill bits with open orifices (i.e.,
no nozzles). Thus, for well planning calculations (discussed in Chapters 8 to
10) it is important to ascertain from the drill bit manufacturer the open orifice
minimum inside diameters for the drill bits to be used in the operation.

4.2.3 Air Hammer Bits
Percussion air hammers have been used for decades in shallow air drilling opera-
tions. These shallow operations have been directed at the drilling of water wells,
monitoring wells, geotechnical boreholes, and mining boreholes. In the past
decade, however, percussion air hammers have seen increasing use in drilling
deep oil and natural gas wells. Percussion air hammers have a distinct advantage
over roller cutter bits in drilling abrasive, hard rock formations.