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Gas Compressors 139

• Calculate the flowing pressure at the KOP inside the drill string
using Eq. (6.29) or repeatedly using Eq. (6.26).
• Calculate the flowing pressure in the standpipe using Eq. (6.18).
• Calculate the gas injection pressure using Eq. (6.28).

Illustrative Example 6.4
5
A well is to be cased to 2,057 ft with a 9 / 8 -in, 40-lb/ft (8.835-in ID) casing.
Starting from the kickoff point at 2,084 ft, the hole will be drilled with mud
o
7
using a 7 / 8 -in bit to build the inclination angle at a constant build rate of 5 /
100 ft until the maximum inclination angle of 74.05 degrees is reached at a
depth of 3,538 ft. Then the drilling fluid will be shifted from mud to air to drill
slant to the TD of 8,145 ft, while the inclination angle is maintained at 74.05
degrees. Additional data are given.

1. Assuming negligible pressure loss in the surface injection line, determine
the minimum required gas injection pressure using a kinetic energy index
safety factor of 1.2 for the design gas injection rate.
2. Plot profiles of the pressure, velocity, gas density, and kinetic energy
index along the flow path.
Drill string data
Drill pipe OD: 4.5 in
Drill pipe ID: 3.643 in
Drill collar length: 247 ft
Drill collar OD: 6.25 in
Drill collar ID: 3 in
Material properties
Specific gravity of rock: 2.8 (water = 1)
Specific gravity of gas: 1 (air = 1)
Gas specific heat ratio (k): 1.25 (water = 1)
Specific gravity of misting fluid: 1 (water = 1)
Specific gravity of formation fluid: 1
Pipe roughness: 0.0018 in
Borehole roughness: 0.3 in
Environment
Site elevation (above mean sea level): 2,000 ft
Ambient pressure: 13.665 psia
Ambient temperature: 75ºF
Relative humidity: 0.8 fraction
Geothermal gradient: 0.01 F/ft
(Continued )