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136 Part II Gas Drilling Systems

Table 6.1 Atmospheric Pressures and Temperatures at Different Elevations
o
Elevation (ft) Pressure (psia) Temperature ( F)
0 14.696 59.00
2,000 13.662 51.87
4,000 12.685 44.74
6,000 11.769 37.60
8,000 10.911 30.47
10,000 10.108 23.36

temperature of the atmosphere decrease as elevation increases. The decrease
in atmospheric pressure reduces the mass flow rate of gas at the suction end
of the compressor, while the drop in temperature increases the mass flow
rate of the gas.
Table 6.1 gives the average atmospheric pressure and temperature for
latitudes from 30°Nto60°N. The temperature data in the table should
be used with caution because onsite temperatures vary significantly with
seasons.
The minimum required volumetric flow rate of site air should be
determined based on Q go and the site atmospheric pressure and tempera-
ture using the ideal gas law:
Q a = 0:0283T a Q go (6.47)
p a
where

p a = actual atmospheric pressure at the drilling site, psia or kPa
T a = actual atmospheric temperature at the drilling site, °Ror °K
Corrections for site humidity should also be made. The density of
water vapor is less than the density of air under the same pressure and
temperature. Consequently, the density of humid air is less than the den-
sity of dry air, and humid air contains less mass than dry air. More
importantly, water vapor in the air is usually removed in the after-cooling
system of compressors to reduce the detrimental effects of freshwater on
borehole conditions. Even though its removal is incomplete, the remain-
ing water vapor can be liquefied at the bit. When small bit orifices
are used, the temperature at the bit can be lower than the dew point
and even the ice point of water due to the Joule-Thomson effect
(discussed in Chapter 7).