Page 41 - Air and gas Drilling Field Guide 3rd Edition

32 CHAPTER 2 Air and Gas Versus Mud

Velocity (ft/sec)
0 20 40 60 80 100 120 140
0 0
Exit
Exit
−500
−2000

Mud Drilling −1000
−4000
Depth (ft) Air Drilling −1500 Depth (m)

−6000
−2000

−8000
−2500
Bottom
Bottom
−10000 −3000
0 5 10 15 20 25 30 35 40
Velocity (m/sec)

FIGURE 2-22. Annulus velocities for drilling mud and compressed air.

(6.4 m/sec). Here again, this is the critical velocity for the transportation of bit
rock cuttings. In the air example, however, the air velocity increases up the annu-
lus to approximately 126 ft/sec (38.4 m/sec) at the exit to the annulus.
It is instructive to compare the power (per unit volume) of these example
flows at the positions in the annulus where the power is likely the lowest. For
both of these examples the lowest power is just above the drill collars in the
annulus around the bottom of the drill pipe. The kinetic energy per unit volume,
KE, is [1, 10]
1
KE ¼ rV 2 (2-1)
2
3
3
where KE is the kinetic energy per unit volume (lb-ft/ft , N-m/m ), r is the den-
3
3
2
4
sity of the fluid (lb-sec /ft ¼ slug/ft , kg/m ), and V is the average velocity of
the fluid (ft/sec, m/sec).
The density of the fluid, r,is
g
r ¼ (2-2)
g
3
3
where g is the specific weight of the fluid (lb/ft , N/m ) and g is the acceleration
2
2
of gravity (32.2 ft/sec , 9.81 m/sec ).