Page 45 - Air and Gas Drilling Manual

1-22 Air and Gas Drilling Manual
Figure 1-21 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 specific
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weight of 1.3 lb/ft (at a pressure of 260 psia and temperature of 60˚F). As the air
flows down the drill string the pressure remains approximately the same. At the
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bottom of the drill string the specific weight is 1.2 lb/ft (at a pressure of 270 psia
and a temperature of 160˚F). The compressed air exits the drill bit orifices into the
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bottom of the annulus (bottom of the well) with a specific weight of 1.1 lb/ft (at a
pressure of 260 psia and a temperature of 160˚F).

Figure 1-21: Air drilling specific weight versus depth.
As the compressed air flows to the surface through the annulus it decompresses
as it flows to the low atmospheric pressure at the surface. At the surface the air exits