Page 198 - Air and Gas Drilling Manual

5-40 Air and Gas Drilling Manual
This will give a volumetric flow rate to the drilling operation of 1,400 acfm (factor
of safety of 1.51).
A comparison must also be made between the injection pressure required to drill
this borehole and the pressure capability of this compressor. The injection pressure
is obtained by a sequence of calculation steps that determine the bottomhole pressure
in the annulus, the pressure at the bottom of the inside of the drill string, and then
the injection pressure (upstream through the circulation system). In these
calculations, the actual volumetric flow rate through each of the two compressors is
700 acfm. Also, in these calculations, the actual atmospheric conditions at the
drilling location will be used (i.e., 12.685 psia and 40˚F) (see Table 4.2 or
Appendix D). The atmospheric pressure of the air, P at, entering the compressor is
.
p at = 12 685 psia
P at = p 144
at
2
P at = 1 827 lb/ft abs
,
The atmospheric temperature of the air, T at, entering the compressor is
o
t at = 40 F
.
T at = t at + 459 67
o
.
T at = 499 67 R
Thus, P g and T g become

P = P at = 1 827 lb/ft abs
2
,
g
T = T at = 499 67 R
o
.
g
Using Equation 4-11, the specific weight of the atmospheric air entering the
compressor is
(1 827, ) (1 0. )
γ =
g
(53 36. ) (499 67. )
γ g = 0 0690 lb/ft 3
.

The volumetric flow rate of air entering the top of the drill string (and entering the
two compressors), Q g, is