Page 228 - Air and Gas Drilling Manual

5-70 Air and Gas Drilling Manual
drilled is a competent unfractured basalt (igneous rock). The drilling location (where
the drill rig will sit on the surface) is at 6,000 ft above sea level (in the mid latitudes
of North America) and the day time air temperature is approximately 40˚F. This is a
typical environmental monitoring or geotechnical borehole geometry.
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.
The borehole diameter, D h, is
.
d = 5 875 inches
h
d
D = h
h
12
D = 0 490. ft
h
Since Figure 5-16 is independent of rock type, then Equation 5-4 can be used to
determine the solids weight rate of flow for igneous rock formation drilling. This is
π
   40 
2
.) (. ) 
˙ w = (. 0 490 ) (62 4 2 8 
s     (60 ) (60 )  
4
˙ w = . 0 366 lb/sec
s
In this example the solids weight rate of flow calculated above is less than 1.0
lb/sec, therefore, Figure 5-16 can be used to determine the approximate minimum
volumetric flow rate of air. The innertube of the dual wall pipe has an inside
diameter of 2.375 inches (see Table 3-6). Entering Figure 5-16 at a diameter of
2.375 inches and moving vertically to the depth of approximately 1,200 ft, the
approximate minimum volumetric flow rate is found to be 155 scfm.
3
The specific weight of air at API standard conditions is 0.0763 lb/ft . To obtain
the adjusted minimum volumetric flow rate for this example, the specific weight of
the air at the actual atmospheric conditions at the drilling location must be obtained.
The actual atmospheric pressure for the air at the drilling location (that will be
utilized by the compressor), P at, is (see Table 4-1 or Appendix D)
.
p at = 11 769 psia
P at = p 144
at
2
,
P at = 1 695 lb/ft abs
The actual atmospheric temperature of the air at the drilling location, T at (that will be
used by the compressor), is