Page 83 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-11
There are special reverse circulation tri-cone drill bits. These are fabricated using
the same mill tooth and insert tooth cone designs as the direct circulation drill bits
discussed above. Figure 1-9 shows the schematic of the inside flow channel of a
reverse circulation tri-cone 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. These reverse circulation drill
bits are manufactured by geotechnical and mining equipment companies.

Figure 3-9: Cutaway of the interior of a tri-cone drill bit (courtesy of Reed Rock Bit
Company).
Single Cone Bits
There are single cone or “monocone” drill bits. Unlike tri-cone drill bits that
drill by a crushing action, the single cone bits drill by a scraping action. Thus, the
single cone drill bits utilize wear resistant tungsten carbide inserts in the cutting
structure. These drill bits are most effective in smaller diameters (~ 2 3/4 inch to 6
1/8 inch) and, with the appropriate cutting structure, are suitable for drilling soft as
well as medium and hard rock formations.
The principal advantage of the 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
diameters. Small diameter tri-cone 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 3-10 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. But like the tri-cone drill bits used for air and gas drilling operations, it is
standard practice to use single cone drill bits with open orifices (i.e., no nozzles).