Page 178 - Air and Gas Drilling Manual

5-20 Air and Gas Drilling Manual
It is only necessary to obtain the approximate minimum volumetric flow rate for
a drilling operation. The borehole is never drilled with only the minimum
volumetric flow rate. Primary compressor units have fixed volumetric flow rate
outputs and it is operationally inefficient to vent excess compressed air to the
atmosphere. Erosion of the drill string or the rock formations is usually not a
consideration. Therefore, compressor units are selected to give a total volumetric
flow rate that exceeds the required minimum volumetric flow rate.
Figure 5-2 gives the minimum volumetric flow rates of air for a 4 3/4 inch
openhole borehole with a 2 3/8 inch drill pipe. The flow rate is determined for the
maximum depth of the well which is 1,200 ft. The annulus around the drill collars
at the bottom of the drill string will have higher air velocities than in the annulus
around the drill pipe. But this is a small effect. Thus, ignoring the drill collars in
the drill string when determining the minimum volumetric flow rate for the well
geometry yields a value that is conservative (i.e., a value that is slightly higher than
the actual minimum required). Therefore, Figure 5-2 can be used to determine the
approximate minimum volumetric flow rate of air for this example drilling operation.
Since the figures are developed assuming sedimentary rock, the drilling rate
must be adjusted for drilling metamorphic rock. Equation 5-4 shows that the weight
rate of flow of solid rock cuttings is linear with the specific gravity of the rock being
drilled. The metamorphic rock formations to be drilled have an average specific
gravity of 3.0. The adjusted drilling rate to compensate for the heavier rock type to
be drilled is (see Equation 5-4)

 30. 
K a = 30
 27. 
.
K a = 33 3 ft/hr
Using Figure 5-2, the approximate minimum volumetric flow rate for drilling at
a depth of 1,200 ft for an adjusted drilling rate of 33.3 ft/hr is approximately 378
scfm. This is the minimum flow rate for sea level air (API standard conditions).
But the drilling location is at 6,000 ft above sea level and the day time temperature
is 70˚ F. Thus, the above minimum volumetric flow rate must be adjusted for the
atmospheric conditions that exist at the drilling location (i.e., to obtain the actual
cubic feet per minute, acfm). In Illustrative Example 5.1 the specific weight of air at
3
API standard conditions was found to be 0.0763 lb/ft . Table 4-1 gives an average
atmospheric pressure of 11.769 psia for a surface location of 6,000 ft above sea level
(mid latitudes North America) (also see Appendix D). The actual atmospheric
pressure for the air at the drilling location (that will be utilized by the compressor),
P at, is

.
p at = 11 769 psia
P at = p 144
at