Page 205 - Air and Gas Drilling Manual

Chapter 5: Shallow Well Drilling Applications 5-47
5.2.3 Prime Mover Fuel Consumption
In this section the fuel consumption of the prime mover for the compressor
system will be discussed. Illustrative examples of the fuel consumption was
discussed in detail in Chapter 4. In this section the illustrative examples will be
completed with the calculation of the approximate fuel needed on the drilling
location for the operation of the compressor system.
Illustrative Examples 5.2c and 5.3c describe the implementation of the basic
planning step No. 10 given in Section 5.1 (planning step No. 11 is discussed in
Chapter 8 and will not be addressed here).
Illustrative Example 5.2c Determine the approximate total volume of diesel
fuel needed at the drilling location to operate the compressor system when using
direct circulation to drill the borehole described in Illustrative Examples 5.2a and
5.2b.
a) Sullair Model 840 Rotary Screw Compressor
This primary rotary screw compressor system is integrated into the design of the
portable Tamrock Driltech Model D25K drilling rig (see Figure 4-19). For this
drilling rig design the prime mover is used to operate both the compressor system
and the hydraulic rotary top drive. The prime mover for this compressor is a
Caterpillar Model 3406, diesel fueled, turbocharged, motor. To estimate total diesel
fuel needed at the drilling location to drill the 1,200 ft deep 4 3/4 inch borehole, it
will be necessary to also estimate the power requirements for the operation of the
hydraulic rotary top drive system. The anticipated drilling rate of penetration is
estimated to be 30 ft/hr. Since the vertical depth to be drilled is 1,200 ft, then the
estimated actual drilling time to reach this depth is approximately 40 hours.
In Illustrative Example 5.2b the derated fixed pressure output from the Sullair
Model 840 rotary screw compressor with a volumetric flow rate of 840 acfm was
found to be 284.1 psia. This will be the pressure output of this rotary compressor
regardless of the drilling depth (and, therefore, regardless of the drilling time). Also
in Illustrative Example 5.2b for the depth of 1,200 ft the injection pressure (into the
top of the inside of the drill string) was found to be 96.2 psia. Using this same
injection calculation procedure, the injection pressures for drilling at depths less than
1,200 ft can be obtained.
Figure 5-10 shows the derated fixed pressure output of the compressor and the
injection pressure to the drill string as a function of drilling time (or drilling depth).
Compressor output pressures that are different from the actual injection pressures is a
unique characteristic of rotary compressor systems. The fixed internal design of the
rotary compressor dictates a fixed pressure output from the compressor regardless of
the back pressure resistance (assuming the back pressure is less than the fixed
pressure output). In this case the compressor pressure output is much greater than
the injection pressure. Therefore, as air exits the compressor it decompresses when it
passes into the surge tank which is usually mounted at the exit of the compressor.
This decompression is due to the fact that the back pressure in the flow line to the
drill string is less than the fixed pressure output of the rotary screw compressor.
This decompression in the surge tank (or surface flow line) must occur in order to
allow the pressure in the compressed air flow that exits the compressor to match the
injection pressure resistance at the drill string.