Page 226 - Air and Gas Drilling Manual

5-68 Air and Gas Drilling Manual
In remote areas (particularly in the arid regions), water for conventional mud
drilling operations is often difficult to find and transport to the drilling location.
Air drilling operations utilize atmospheric air. Therefore, the volume of compressor
fuel used in drilling operations becomes an important logistical issue.
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 = 12 25 inches
.
h
d
D = h
h
12
D = 1 021. ft
h
Using Equation 5-4 the approximate solids weight rate of flow for this drilling
operation can be obtained. This is
π
   30 
. ) ( . ) 
˙ w = . (1 021 ) 2 (62 4 2 7 
s     (60 ) (60 )  
4
˙ w = . 1 15 lb/sec
s
The above value is greater than the 1.0 lb/sec limit of Figure 5-16, but is
between the 1.0 lb/sec to 2.0 lb/sec limit of Figure 5-17. Therefore, using Figure
5-17 the approximate minimum volumetric flow rate can be obtained. The
approximate minimum volumetric flow rate is estimated at the maximum depth of
the borehole. Entering Figure 5-17 with the drill pipe inside diameter of 4.670
inches, and moving vertically to the approximate depth of 2,400 ft, the approximate
minimum volumetric flow rate is found to be 550 scfm.
3
The specific weight of air at API standard conditions is 0.0763 lb/ft (see
Chapter 4 or Appendix D). 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 of 2,000 ft above sea level 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 = 13 662 psia
P = p 144
at at
2
,
P at = 1 967 lb/ft abs