Page 239 - Air and Gas Drilling Manual

Chapter 5: Shallow Well Drilling Applications 5-81
psig at API standard conditions. When the drilling rig is moved to the 6,000 ft
surface location, the volumetric flow rate from the compressor is 840 acfm. The
compressor volumetric flow rate is greater than the required adjusted minimum
volumetric flow rate of 280 acfm (factor of safety of 3.00). Therefore, this
compressor unit is capable of producing the volumetric flow rate to drill the
borehole.
A comparison must also be made between the injection pressure required to drill
this borehole and the pressure capability of this compressor system. The injection
pressure is obtained by calculating through 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 of the
compressor is used, the 840 acfm. Also, in these calculations, the actual
atmospheric conditions at the drilling location will be used (i.e., 11.769 psia and
40˚F). The atmospheric pressure of the air, P at, entering the compressor is
.
p at = 11 769 psia
P = p 144
at at
2
P at = 1 695 lb/ft abs
,
The atmospheric temperature of the air, T at, entering the compressor is
o
t at = 40 F
T = t + 459 67
.
at at
o
.
T at = 499 67 R
Thus, P g and T g become

P = P at = 1 695 lb/ft abs
2
,
g
T = T at = 499 67 R
o
.
g
Using Equation 4-11, the specific weight of the gas entering the compressor is
(1 695, ) (1 0. )
γ =
g
(53 36. ) (499 67. )
γ = 0 0636 lb/ft 3
.
g