Page 82 - Air and gas Drilling Field Guide 3rd Edition

4.2 Drill Bits 73

two large orifices in the bit face that allow the return air flow with entrained rock
cuttings to flow to the inside of the drill string and then to the surface. Reverse
circulation air hammers are available in larger outside housing diameters, 6 in
(152 mm)to24in(610 mm). These air hammers drill boreholes with diameters
7
from 7 8 = in (200 mm)to33in(838 mm).
There are five air hammer bit cutting face designs. Figure 4-12a shows the pro-
file of the drop center bit, and Figure 4-12b shows the profile of the concave bit.
Figure 4-13a shows the profile of the step gauge bit, and Figure 4-13b shows the
profile of the double gauge bit. Figure 4-14 shows the profile of the flat face bit.
In the past, air hammer manufacturers have provided the air hammer bits for
their specific air hammers. This practice ensured compatibility of bit with ham-
mer housing. Increased air hammer use in drilling deep oil and natural gas recov-
ery wells has attracted traditional oil field drill bit manufacturers to fabricate air
hammer bits. Although the air hammer bit faces are somewhat uniform in design,
the shafts are different for each air hammer manufacturer. The air hammer face
and shafts are integral to the bit; manufacturing air hammer bits is complicated.
Fortunately, the air hammer has proven in the past decade to be very effective
in drilling deep boreholes. This has given rise to competition among traditional

a) b)

FIGURE 4-12. Air hammer bit face profile designs: (a) drop center bit and (b) concave bit
(courtesy of AB Sandvik Rock Tools).

a) b)

FIGURE 4-13. Air hammer bit face profile designs: (a) step gauge bit and (b) double gauge bit
(courtesy of AB Sandvik Rock Tools).

FIGURE 4-14. Air hammer bit face profile design, flat face bit (courtesy of AB Sandvik Rock
Tools).