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142 Part II Gas Drilling Systems

Illustrative Example 6.4 (Continued )

Kinetic Energy Index
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
0
Annular profile
1000 In-pipe profile
2000
3000
Depth (ft) 4000

5000
6000
7000

8000
9000
Figure 6.4 Calculated kinetic energy index profile.

6.5 HORSEPOWER REQUIREMENTS

Most compressors used in gas drilling operations are centrifugal com-
pressors. These compressors are often specified by manufacturers in horse-
power. Selected compressors should have adequate horsepower to meet
operation needs. Assuming that the gas obeys the ideal gas law and the com-
pression follows a polytropic process, the compressor power is expressed as
2 3
k−1
n s k
n s kp i Q gi
HP c = 4 p o − 1 5 (6.50)
229:17ðk − 1ÞE p p i
where
HP c = compressor power, hp or W
n s = number of compression stages
k= specific heat ratio of gas
2
p i = gas pressure at inlet condition, psia or N.m
3
3
Q gi = gas flow rate at inlet condition, ft /min or m /min