Page 180 - Air and Gas Drilling Manual

5-22 Air and Gas Drilling Manual
nominal, EU, NC 31, Grade E drill pipe (see Table B-4). The anticipated drilling
rate is assumed to be 90 ft/hr and the maximum drilling depth of the well is 2,400
ft. The formations to be drilled are a sequence of limestone rock. The drilling
location (where the drill rig will sit on the surface) is at 4,000 ft above sea level and
the day time air temperature during operations is assumed to be approximately 40˚F.
This is a typical mining or geotechnical borehole geometry.
Figure 5-5 gives the minimum volumetric flow rates of air for a 6 1/4 inch
openhole borehole with a 2 7/8 inch drill pipe. The flow rate is determined for the
maximum depth of the well which is 2,400 ft. 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 5-5 can be used to determine the approximate
minimum volumetric flow rate of air for this example drilling operation.
Using Figure 5-5, the approximate minimum volumetric flow rate for drilling at
a depth of 2,400 ft and a drilling rate of 90 ft/hr is approximately 835 scfm. This is
the minimum flow rate for sea level (API standard conditions) air. But the drilling
location is at 4,000 ft above sea level and the day time temperature is 40˚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 API standard
3
conditions was found to be 0.0763 lb/ft . Table 4-1 gives an average atmospheric
pressure of 12.685 psia for a surface location of 4,000 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 used by the compressor), P at, is
p at = 12 685 psia
.
P at = p 144
at
2
,
P at = 1 827 lb/ft abs
The actual atmospheric temperature of the air at the drilling location, T at (that will be
used by the compressor), is

t = 40˚F
at
.
T at = t at + 459 67
o
.
T at = 499 67 R
Thus, P g and T g become

P = P at = 1 827 lb/ft abs
2
,
g