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140 Part II Gas Drilling Systems

Illustrative Example 6.4 (Continued )
Operating conditions
Surface choke/flow line pressure: 15 psia
Rate of penetration: 120 ft/hour
Rotary speed: 50 rpm
Misting rate: 10 bbl/hour
Formation fluid influx rate: 10 bbl/hour
Dewatering efficiency: 0.95 fraction 1/32nd
Bit orifices: 20 in
20-1/32nd in
20-1/32nd in
Solution
These problems can be solved with computer program GasDrill-09.xls.
1. A gas injection rate of 1,430 scf/min gives a kinetic energy index of 1.0 at
the shoulder of the drill collar. A gas injection rate of 1,615 scf/min gives
a kinetic energy index of 1.2. The minimum required injection pressure is
calculated to be 136 psia.
2. Using the design gas injection rate of 1,615 scf/min, the profiles of
the pressure, velocity, gas density, and kinetic energy index along the
flow path are calculated and plotted in Figures 6.1, 6.2, 6.3, and 6.4,
respectively.

Pressure (psia)
0 20 40 60 80 100 120 140 160
0
Annular profile
1000 In-pipe profile
2000

3000
Depth (ft) 4000

5000

6000
7000
8000
9000
Figure 6.1 Calculated pressure profile.