Page 469 - Air and Gas Drilling Manual

Chapter 9: Aerated Fluid Drilling 9-73
The above determined 322.5 horsepower is the actual shaft power needed by
each of the two compressors to produce the 420 psia pressure output at the surface
location elevation of 4,000 ft above sea level (while drilling at a depth of 7,000 ft).
At this surface location, the input horsepower available from the Caterpillar Model
D398 prime mover is a derated value (derated from the rated 760 horsepower
available at 900 rpm). In order for the compressor units to operate at this 4,000 ft
surface location elevation, the derated input power available must be greater than the
actual shaft power needed. Figure 4-15 shows that for 4,000 ft elevation the input
power of a turbocharged prime mover must be derated by approximately 10 percent.
˙
The derated input horsepower, W , available from the prime mover is
i
W ˙ i 760 (1 . 0 10 )
W ˙ i 684 .0
For this illustrative example, the prime mover of the compressor unit derated
input power is greater than the actual shaft horsepower needed, thus, the selected
compressor unit can be operated at this 4,000 ft surface location elevation (while
drilling at a depth of 10,000 ft).
The volumetric flow rate of gas to create an aerated drilling fluid is very
dependent upon the flow resistance in the circulation system. This resistance is the
sum of the major and minor flow friction losses in the flow system. This friction
resistance creates choking of the system and, therefore, generally increases the average
pressure and in turn increases the average specific weight of the aerated drilling fluid
in the annulus and elsewhere in the circulation system. Therefore, if it is required to
maintain a constant bottomhole pressure limit, more gas volumetric flow rate will be
required to reduce the average pressure and the specific weight in the annulus.
A major contributor to this choking effect is the inside diameter of the surface
return flow line (relative to the annulus inside of diameter). In Illustrative Example
9.3 the surface return flow line inside diameter was 5.625 inches. For this example,
the required maximum volumetric flow rate when drilling at 10,000 ft was
determined to be 1,168 acfm. If the surface return flow line inside diameter is
changed to 4.00 inches the required maximum volumetric flow rate becomes 1,575
acfm. This is a 35 percent increase in gas volumetric flow rate required. Thus, the
design of the surface return flow line and the drill string to reduce choking can
dramatically reduce the gas volumetric flow rate needed to provide the required
aerated drilling fluid.

9.5 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 were
discussed in detail in Chapter 4. In this section the illustrative example in this
chapter will be completed with the calculation of the approximate fuel needed on the
drilling location for the compressor system.
Illustrative Examples 9.3 describe the implementation of the basic planning step
No. 12 given in Section 9.1.