Page 181 - Air and Gas Drilling Manual

o
T =
T
.
g
at
Using Equation 4-11, the specific weight of the gas entering the compressor is
)
(1 827, = 499 67 R Chapter 5: Shallow Well Drilling Applications 5-23
) (1 0.
γ =
g )
(53 36. ) (499 67.
γ g = 0 0685 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 illustrative 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 = 835
 0 0685. 
q = 930 acfm
g
The above volumetric flow rate is the adjusted minimum value for the atmospheric
conditions at this drilling location (4,000 ft above sea level).
5.2.2 Injection Pressure and the Selection of Compressor Equipment
The planning process given in Section 5.1 requires that the borehole
requirements be compared to the capabilities of the compressor units available.
Specifically, these comparisons are made between; a) the required borehole
volumetric flow rate and the compressor volumetric flow rate capability, b) the
borehole injection pressure (using the compressor volumetric flow rate) and the
pressure capability of the compressor and, c) the horsepower required by the
compressor and the input capability of the prime mover. For shallow drilling
operations the selection of the appropriate compressor equipment often dictates the
drilling rig selection. This will be demonstrated in the examples that follow.
Illustrative Examples 5.2b, and 5.3b describe the implementation of the basic
planning steps Nos. 7 through 9 given in Section 5.1.
Illustrative Example 5.2b Select the appropriate primary compressor system
for the direct circulation drilling operation data given in Illustrative Example 5.2a.
This selection will give the actual volumetric flow rate of air to be delivered to the
drilling operation. The selection of the appropriate compressor will be made from
the example compressor systems listed in Section 4.7 of Chapter 4. In Illustrative
Example 5.2a it was found that the drilling location adjusted minimum volumetric
flow rate of air for the 4 3/4 inch diameter, 1,200 ft deep borehole was
approximately 481 acfm (for a surface drilling location of 6,000 ft above sea level).