Page 34 - Air and Gas Drilling Manual

Chapter 1: Introduction 1-11
Reverse circulation drilling operations require specially fabricated drill bits.
Figure 1-9 shows a schematic of the interior flow channel of a tri-cone rotary drill bit
designed for reverse circulation. These drill bits utilize typical roller cutter cones
exactly like those used in direct circulation drill bits (see Figure 1-1). These bits,
however, have a large central channel opening that allows the circulation fluid flow
with entrained rock cuttings to flow from the bottom of the borehole to the inside of
the drill string and then to the surface.

Figure 1-9: Schematic of the internal flow channel of a tri-cone roller cutter bit designed
for reverse circulation operations (courtesy of Smith International Incorporated).
Most tri-cone drill bits with a diameter of 5 3/4 inches or less are designed with
the central flow channel as shown in Figure 1-9 above. Figure 1-1 showed the
typical tri-cone drill bit for direct circulation operations. These direct circulation
drill bits usually have three orifices that can be fitted with nozzles. Tri-cone roller
cutter drill bits for reverse circulation operations are available in diameters from 4
1/2 inches to 31 inches. The larger diameter bits for reverse circulation operations
are usually custom designed and fabricated. Dual wall pipe reverse circulation
operations require special skirted drill bits (see Chapter 3 for details). These skirted
drill bits are also custom designed for the particular drilling operation. Most reverse
circulation tri-cone bits are manufactured by companies that specialize in
geotechnical and mining drilling equipment.