Page 230 - Air and Gas Drilling Manual

5-72 Air and Gas Drilling Manual
equipment often dictates the drilling rig selection. This will be demonstrated in the
examples that follow.
Illustrative Examples 5.5b, and 5.6b describe the implementation of the basic
planning steps Nos. 7 through 9 given in Section 5.1.
Illustrative Example 5.5b Select the appropriate compressor system for the
reverse circulation drilling operation data given in Illustrative Example 5.5a. 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 5.5a it was
found that the drilling location adjusted minimum volumetric flow rate for the 12
1/4 inch diameter, 2,400 ft deep borehole was 614 acfm (for the surface drilling
location of 2,000 ft above sea level). Also determine the injection pressure, the
input power required by the compressor, and the derated output power available from
the primer mover while drilling at 2,400 ft.
The borehole depth and the weight of the drill string in this example requires
that a double drilling rig like that shown in Figure 1-4 be used to drill the well.
Since this drill rig does not have an on-board compressor, a stand alone compressor
unit must be used to provide the compressed air. The semi-trailer mounted Dresser
Clark Model CFB-4, four-stage, reciprocating piston primary compressor is selected.
This compressor unit is powered by a Caterpillar Model D398, diesel fueled,
turbocharged, prime mover capable of a 760 peak horsepower at the operating speed
of 900 rpm (see Figure 4-24). This compressor is capable of producing a volumetric
flow rate of 1,200 scfm and a maximum pressure of 1,000 psig at API standard
conditions. When this compressor unit is placed at the 2,000 ft surface elevation
drilling location, it will still produce the same volumetric flow rate as at sea level
conditions. Thus, the actual volumetric flow rate of this compressor at the 2,000 ft
surface elevation is 1,200 acfm (see Chapter 4). This volumetric flow rate is greater
than the required adjusted minimum volumetric flow rate of 614 acfm, thus, the
injection flow rate into the annulus of this well will be 1,200 acfm (factor of safety
of 1.95).
The reason the small single Tamrock Driltech Model D25K drilling rig with the
on-board Sullair Model 840 compressor (rated at 840 scfm) cannot be used in this
operation is because this drilling rig has marginal hoisting capability to lift the drill
string in this example.
A comparison must also be made between the injection pressure required to drill
this borehole and the pressure capability of the selected compressor system. The
injection pressure is obtained by a sequence of calculation steps that determine the
bottomhole pressure inside the drill string above the drill bit, the bottomhole
pressure at the bottom of the annulus, and then the injection pressure (upstream
through the circulation system). In these calculations, the actual volumetric flow
rate from the compressor is used, the 1,200 acfm. Also, in these calculations, the
actual atmospheric conditions at the drilling location will be used (i.e., 13.662 psia
and 80˚F) (see Table 4.2 or Appendix D). The atmospheric pressure of the air, P at,
entering the compressor is

.
p at = 13 662 psia