Page 39 - Air and gas Drilling Field Guide 3rd Edition

30 CHAPTER 2 Air and Gas Versus Mud

Specific Weight (lb/ft 3 )
0 50 100 150
0 0
Injection and Exit
−500
−2000

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

−6000 and Annulus
−2000

−8000
−2500
Bit
−10000 −3000
0 400 800 1200 1600 2000 2400
Density (kg/m 3 )

FIGURE 2-20. Mud drilling specific weight versus depth.

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weight is 75 lb/ft or 10 lb/gal (density of 1.2 kg/liter) at any position in the cir-
culation system. There is some slight expansion of the drilling mud as a conse-
quence of the increase in temperature as the drilling mud flows to the bottom
of the well. This effect is quite small and is neglected in these engineering
calculations.
Figure 2-21 shows the plot of the specific weight of the compressed air in this
example. The compressed air is injected into the top of the drill string at a
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3
specific weight of 1.3 lb/ft (density of 20.8 kg/m ), a pressure of 250 psia
2

(172 N/cm absolute), and a temperature of 60 F(15.6 C). As the air flows down
the drill string the pressure decreases. At the bottom of the inside of the drill
3
3
string the specific weight is 0.84 lb/ft (density of 13.5 kg/m ), at the pressure
2

of 190 psia (131 N/cm absolute), and a temperature of 160 F(71.1 C). The

compressed air exits the drill bit orifices into the bottom of the annulus (bottom
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3
of the well) with a specific weight of 0.77 lb/ft (density of 12.3 kg/m ), at a
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pressure of 180 psia (124 N/cm absolute), and a temperature of 160 F(71.1 C).

As the compressed air flows to the surface through the annulus, it decom-
presses as it flows toward the atmospheric pressure at the exit to the blooey line.
At the surface, the air exits the annulus (via the blooey line) with a specific weight
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3
of 0.0763 lb/ft (density of 1.22 kg/m ). The surface atmosphere for this example
is assumed to be API Mechanical Equipment Standards standard atmospheric condi-
2
tions, dry air, pressure of 14.696 psia (10.1 N/cm absolute), and a temperature