Page 470 - Air and Gas Drilling Manual

9-74 Air and Gas Drilling Manual
Illustrative Example 9.4 In Illustrative Example 9.3 the semi-trailer mounted
unit with the Dresser Clark CFB-4, four-stage, reciprocating piston compressor
system was selected for the drilling operation. In Illustrative Examples 9.3 the
compressor unit was utilized to drill the interval from 7,000 ft to 10,000 ft. The
compressor unit has a volumetric flow rate of 1,200 acfm. The compressed air
flowing to the drilling operation varies over the drilled interval from 120 acfm to
1,168 acfm. But the compressor must output 1,200 acfm. The volumetric flow rate
to the borehole must be varied and the excess is vented to the atmosphere.
Therefore, to estimate the total diesel fuel needed by the compressor unit it is
necessary to estimate the fuel consumption of the compressor units’ Caterpillar
Model D398, diesel fueled, turbocharged, prime mover. The anticipated drilling rate
of penetration is estimated to be 60 ft/hr. The vertical interval section to be drilled
is from a depth of 7,000 ft to a depth of 10,000 ft, or a 3,000 ft length of borehole.
Therefore, the estimated actual drilling time to drill this interval is approximately 50
hours.
The reciprocating piston compressor is not a fixed pressure ratio machine like
the rotary compressor. As long as there is sufficient power available from the prime
mover the reciprocating piston compressor will match the back pressure resistance.
Thus, Figure 9-6 shows only one curve indicating the injection pressure is the same
as the pressure output of the compressor unit.
In Illustrative Example 9.3 it was found that the actual shaft horsepower
required by the reciprocating piston compressor unit to compress air to 420 psia was
approximately 322.1 (at 7,000 ft of depth). Also in Illustrative Example 9.3 the
derated horsepower available from the Caterpillar Model D398 prime mover at the
surface elevation of 4,000 ft was found to be 684. Drilling at a depth of 7,000 ft,
the prime mover power ratio is

322 1 .
PR ( 100) 47 1 .
684 0 .
Entering the abscissa of Figure 4-17 with the power ration percent, the approximate
fuel consumption rate can be read on the ordinate using the diesel fuel curve. The
approximate fuel consumption rate at this power level is 0.650 lb/hp-hr. The total
weight of diesel fuel consumption per hour is
˙ w . 0 650 (322 1 209 .4 lb/hr
. )
f
The diesel fuel consumption rate (in United States gallons) for the drilling depth of
7,000 ft is

.
209 4
.
q 30 8 gal/hr
f
(. ) ( .
0 8156 8 33)
Using the data in Figure 9-6 and similar calculations as those given above, the
diesel fuel consumption rate as a function of drilling time (or drilling depth) can be
obtained. Figure 9-8 shows the diesel fuel consumption rate as a function of drilling