Page 613 - Air and Gas Drilling Manual

particularly the case when drilling at shallow depth with high volumetric flow rates
of air or gas. At shallow depth the bottomhole annulus pressure is low and the
turbine must be choked in order to limit the torque and power in the gear box and at
the drill bit. Chapter 11: Specialized Drilling Equipment 11-51
Illustrative Example 11.3 Determine the injection pressure and resulting
compressor fuel consumption when a Rift Pneumatics Model 650, 6 1/2 inch
outside diameter, downhole pneumatic turbine motor is utilized to drill the openhole
interval from 7,000 ft to 10,000 ft of the Illustrative Example 8.3 series in Chapter
8. The gear box for this motor has a 112.5 to 1 reduction ratio. The positive
displacement motor is to use a 7 7/8 inch tri-cone roller cutter drill bit to drill at
10,000 ft. It is required that the downhole motor develop approximately 28
horsepower when drilling under load. In the Illustrative Example 8.3 series two
semi-trailer mounted Dresser Clark Model CFB-4, four-stage, reciprocating piston
compressors where selected. Each of these compressors where driven by a Caterpillar
Model D398 diesel fueled prime mover. These two compressor units provide a total
of 2,400 acfm to the borehole. The drilling location is at 4,000 ft above sea level.
The annulus solution given in Illustrative Example 8.3b will be used as the starting
point for this example.
Figure 11-14 shows the power performance plot of the Model 650, 6 1/2 inch
outside diameter, downhole pneumatic turbine motor actuated with a flow rate of
2,400 acfm (at a surface location of 4,000 ft, i.e., 2.635 lb/sec). In order to obtain a
28 horsepower output at the drive shaft, the governor is set to give a drive shaft
speed of about 40 rpm. This would limit the turbine rotor speed to 4,500 rpm. It
should be noted that a higher weight rate of flow of air or gas will result in a larger
power parabola than that shown in Figure 11-14. The larger power parabola will
have a higher peak power (that occurs at a higher drill bit speed) and a higher
theoretical runaway speed (where there is no load on the motor). Likewise, a lower
weight rate of flow of air or gas will result in a smaller power parabola.
In Illustrative Example 8.3b the bottomhole annulus pressure was determined.
This annulus bottomhole pressure was found to be

p a5 204 5 psia
.
P a5 p a5 144

2
,
P 29 448 lb/ft abs
a5
Using the downhole pneumatic turbine motor to drill the borehole does not alter
the annulus solution obtained in Illustrative Example 8.3b. However, it will alter
the pressure calculations from the bottom of the annulus to the top of the inside of
the drill string (the injection pressure).
Drill Bit Open Orifices
The downhole pneumatic turbine motor is being used to drill a deep borehole.
Therefore, the bottomhole pressure is high and drill bit nozzles are not needed to
choke the motor. The inside diameter of the open orifices, D o, is