Page 137 -

Equipment in Gas Drilling Systems 115

Gas
Inlet

Connection Pin

Backflow Valve

Control Rod
Piston Case

Control Rod Window

Choke
Piston

Lock Rings

Drive Sub

Flat-Bottom Bit

Figure 5.9 An air hammer with a flat-bottom bit.

can cut the formation rock without the need for a heavy weight on bit.
A few hundred psi of pressure drop across the air hammer is required
under drilling conditions.
A flat-bottom bit is shown in Figure 5.10. Diamond-enhanced inserts
are installed on the flat-bottom face of the bit. The rotational drive spines
allow the bit to rotate while cutting rocks forward. This avoids repeated
cutting actions on the same points of the bottomhole.
In addition to air hammers and flat-bottom bits, float valves are
widely used in gas drilling (Figure 5.11). They are used to prevent back-
flow of gas up the drill string (Lyons et al., 2009).