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Gas and Liquid Injection Rates 205

Measured depth: 5,000 ft
Drill pipe OD: 4.5 in
Material properties
Specific gravity of solid: 2.7 (water = 1)
Liquid weight: 8.5 ppg
Specific gravity of formation fluid: 0.8 (water = 1)
Specific gravity of gas: 1 (air = 1)
Pipe roughness: 0.0018 in
Borehole roughness: 0.1 in
Productivity index: 3.333 bbl/d/psi
Environment
Ambient pressure: 14.7 psia
Ambient temperature: 70°F
Geothermal gradient: 0.01 F/foot
Operation conditions
Bit diameter: 7.875 in
Rotary speed: 50 rpm
Penetration rate: 60 ft/hr
Choke pressure: 40 psia
Solution
Circulation-breaking bottomhole pressures at various liquid flow rates and gas
injection rates are calculated using Eq. (9.3) with the friction factor set to zero.
The results are plotted in Figure 9.20. A horizontal line is drawn in the plot
2500
Breaking Bottomhole Pressure (psia) 1500 Rate: Rate: Mud flow rate 100gpm
2000

1000
Mud
Gas
Mud flow rate 200gpm
500
425
200
Mud flow rate 400gpm
600
300
Collapse pressure
400
790
0 340 100 Mud flow rate 300gpm
0 500 1000 1500 2000
Air Injection Rate (scfm)
Figure 9.20 Calculated bottomhole pressure curves for circulation break
conditions with identified flow rate combinations to yield borehole collapse:
7⅞-in bit, 4½-in pipe, 5,000-ft depth, 60-ft/hr ROP, 42-bbl/h oil influx.
(Continued )