Page 38 - Air and gas Drilling Field Guide 3rd Edition
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2.3 Comparison of Mud and Air Drilling 29
Temperature ( F)
40 60 80 100 120 140 160 180 200
0 0
Injection and Exit
−500
−2000
−1000
Inside Drill String
−4000 and Annulus
Depth (ft) −1500 Depth (m)
−6000
−2000
−8000
−2500
Bit
−10000 −3000
0 20 40 60 80
Temperature ( C)
FIGURE 2-19. Air drilling temperature versus depth.
than drilling mud. Thus, compressed air has poor heat storage qualities relative to
drilling mud. Also, compressed air flowing in the drilling circulation system is
flowing rapidly and, therefore, the flow is turbulent inside the drill string and
in the annulus. Turbulent flow is very efficient in transferring heat from the sur-
face of the borehole to the flowing air in the annulus and in the inside of the drill
string. It is assumed that the compressed air entering the top of the drill string at
60 F(15.6 C) will heat up rapidly and transfer heat in the rock formations to and
from the steel piping and casing in the well. Under these conditions the com-
pressed air exiting the annulus has approximately the same temperature as the
air entering the top of the drill string.
Figure 2-19 shows that the temperature of the compressed air at any position in
the borehole is approximately the same as the geothermal temperature at that
depth. Thus, the temperature of the flowing air at the bottom of the hole is the bot-
tom hole temperature of 160 F(71.1 C). There is some local cooling of the air as it
exits the open orifices of the drill bit at the bottom of the hole. This cooling effect is
more pronounced if nozzles are used in the drill bit (when using a down hole
motor). This cooling effect is known as the Joule–Thomson effect and can be esti-
mated [8]. However, it is assumed that this effect is small and that the air flow
returns very quickly to the bottom hole geothermal temperature.
Figure 2-20 shows the plot of the specific weight of drilling mud for this exam-
ple calculation. The drilling mud is incompressible and, therefore, the specific
