Page 36 - Air and gas Drilling Field Guide 3rd Edition

2.3 Comparison of Mud and Air Drilling 27

Pressure (psia)
0 50 100 150 200 250 300
0 0
Exit Injection
−500
−2000

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

−6000
−2000

−8000
−2500
Bit
−10000 −3000
0 50 100 150 200
Pressure (N/cm 2 abs)

FIGURE 2-17. Air drilling pressure versus depth.

of the drill string just above the bit orifices. Also shown is a plot of the pressure in
2
the annulus. The pressure is approximately 180 psia (124 N/cm absolute)at
2
the bottom of the annulus just below the bit orifices and 14.7 psia (10.1 N/cm
absolute) at the end of the blooey line at the surface (top of the annulus).
As in the mud drilling example, the pressures in Figure 2-17 reflect the hydro-
static weight of the column of compressed air and the resistance to air flow from
the inside surfaces of the drill string and the surfaces of the annulus. This resis-
tance to flow is due to friction losses. In this example the fluid is compressible.
Considering the flow inside the drill string, the friction losses component domi-
nates the hydrostatic weight component in the column, as the injection pressure
into the inside of the drill string is higher than the pressure at the bottom of the
drill string (inside the drill string just above the bit open orifices). This phenome-
non is a strong function of the inside diameter of the drill string, and the type of
choke or restriction is at the bottom of the inside of the drill string. For example,
if the drill pipe diameter was larger and a smaller choke was present in the
bottom of the drill string, the hydrostatic component would dominate.
Figure 2-18 shows the plots of the temperature in the incompressible drilling
mud as a function of depth. The geothermal gradient for this example is 0.01 F/ft

(0.018 C/m). Subsurface earth is nearly an infinite heat source. The drilling

mud in a mud drilling circulation system is significantly denser than compressed