Page 634 - Air and Gas Drilling Manual

11-72 Air and Gas Drilling Manual
had a tendency to be damaged and worn by the pipe scale flowing into the sliding
vane cavities of the motor. Due to these problems this early downhole air motor
based on the sliding vane design was abandoned.
In the past decade, several new downhole air motors based on the sliding vane
design have been offered for commercial use. The cross section of these new sliding
vane motors are similar to that shown in Figure 11-16 [1]. The reports on the
performance of these new sliding vane downhole air motors are not available.
Therefore, these motor designs have not been analyzed in examples.
11.3 Conclusions
The specialized surface equipment discussed has general applications to vertical
and directional drilling operations. The specialized downhole air hammer is only
used for straight hole drilling. Therefore, most air hammer drilling is in vertical
boreholes. However, there are some very shallow (near surface) applications where
air hammers are used to drill slant or horizontal boreholes. In these applications
require specialized directional control techniques that usually rely on drill through or
“daylight” drilling (i.e., where the drill bit will exit to the surface).
The specialized downhole rotary motors are nearly always used to drill
directional boreholes. These downhole rotary motors are used in shallow and deep
drilling operations. It is clear that the converted downhole positive displacement
mud motors will require the greatest compressor injection pressures and, therefore,
require the most fuel for the compressors that supply the compressed air. The
downhole pneumatic turbine motors are not pressure driven motors and, therefore, do
not generally require a much greater fuel consumption for the compressors than
conventional air drilling.

Figure 11-16: Typical sliding vane motor cross section.
An important issue for the specialized downhole equipment is that of
lubrication. There are some environmental drilling applications that require no
lubricants be injected into the compressed air flow to the drill string. Also, there are
underbalanced oil and natural gas recovery drilling applications that do not allow