Page 40 - Air and gas Drilling Field Guide 3rd Edition

2.3 Comparison of Mud and Air Drilling 31

Specific Weight (lb/ft 3 )
0 0.5 1 1.5
0 0
Exit Injection
−500
−2000

Inside Drill String −1000
−4000
Depth (ft) Annulus −1500 Depth (m)

−6000
−2000

−8000 Bit
−2500

−10000 −3000
0 4 8 12 18 20 24
Density (kg/m 3 )

FIGURE 2-21. Air drilling specific weight versus depth.

of 60 F(15.6 C) [9]. Figure 2-21 shows a typical friction resistance-dominated drill
string flow (as opposed to hydrostatic column weight dominated). This type of
flow has a drill string injection pressure at the top that is higher than the pressure
above the drill bit inside the drill string at the bottom of the string. Friction-domi-
nated flow usually results when the drill bit is run with no nozzles.

2.3.3 Borehole Cleaning
Figure 2-22 is the concluding plot of these comparison calculations and shows a
side-by-side comparison of the annulus velocities of the drilling mud and the com-
pressed air as they flow to the surface. It is the power of these return flows up
the annulus that keeps the rock cuttings entrained and moving to the surface at
a rate that allows the drill bit to be safely advanced.
The drilling mud flows in the annulus around the drill collars with an average
velocity of approximately 5.0 ft/sec (1.5 m/sec). The drilling mud slows to an
average velocity of approximately 2.0 ft/sec (0.61 m/sec) in the annulus around
the drill pipe (this is the critical velocity for the transportation of bit rock
cuttings).
For the air drilling case, the lowest velocity of the compressed air flow in the
annulus is just above the drill collars with a velocity of approximately 21.1 ft/sec