Page 29 - Air and gas Drilling Field Guide 3rd Edition

20 CHAPTER 2 Air and Gas Versus Mud

FIGURE 2-9. Schematic of the internal flow channel of a tricone roller cutter bit designed for
reverse circulation operations (courtesy of Smith International Incorporated).

bit designed for reverse circulation. These drill bits utilize typical roller cutter
cones exactly like those used in direct circulation drill bits (see Figure 2-1). 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.
Most tricone drill bits with a diameter of 5 3/4 inches (146 mm) or less are
designed with the central flow channel as shown in Figure 2-9. Figure 2-1 showed
the typical tricone drill bit for direct circulation operations. These direct circula-
tion drill bits usually have three orifices that can be fitted with nozzles. Tricone
roller cutter drill bits for reverse circulation operations are available in diameters
from 4 1/2 inches (114 mm) to 31 inches (787 mm). 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 4 for details). These skirted drill bits are specifically designed for the
particular drilling operation. These specialized drill bits are usually manufactured
by mining equipment companies.

2.3 COMPARISON OF MUD AND AIR DRILLING
The direct circulation model is used to make some important comparisons
between mud drilling and air and gas drilling operations.