Page 227 - Air and Gas Drilling Manual

The actual atmospheric temperature of the air at the drilling location, T at (that will be
used by the compressor), is
t at = 80˚F Chapter 5: Shallow Well Drilling Applications 5-69
T = t + 459 67
.
at at
o
.
T at = 539 67 R
Thus, P g and T g become
2
P = P at = 1 967 lb/ft abs
,
g
o
T = T at = 539 67 R
.
g
Using Equation 4-11, the specific weight of the gas entering the compressor is
(1 967, ) (1 0. )
γ =
g
(53 36. ) (539 67. )
γ g = 0 0683 lb/ft 3
.

Note the above specific weight value can also be obtained from Figure D-4 in
Appendix D.
The approximate minimum volumetric flow rate for this example adjusted for
the actual atmospheric conditions at the drilling location can be determined by
equating the weight rate of flow of the air at the two atmospheric conditions. This
reduces to
 0 0763. 
q g = 550
 0 0683. 
q g = 614 acfm

The above volumetric flow rate is the adjusted minimum value for the atmospheric
conditions at this drilling location (2,000 ft above sea level).
Illustrative Example 5.6a Determine the approximate reverse circulation
minimum volumetric flow rate of air for a well with a 5 7/8 inch openhole borehole
(5 7/8 inch drill bit diameter) and a drill string composed of 4.50 inch concealed
inner-tube dual wall pipe (see Table 3-6). The anticipated drilling rate is assumed to
be 40 ft/hr and the maximum depth of the well is 1,200 ft. The formation to be