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Underbalanced Drilling Operations 221

where all the parameters are defined by Eqs. (9.4) through (9.10) in
Chapter 9. Although the gas–liquid rate windows (GLRWs) can be
prepared in the design stage, it is a good practice to optimize the combi-
nations of gas and liquid flow rates in field operations. Such an optimiza-
tioncanbeeasilyperformed usingsimplecomputerprogramssuchas
Pressure Instability.xls in this book.

Illustrative Example 10.1
For the following given conditions, investigate the range of the pressure
instability factor.
Total depth: 10,000 ft
Depth of the surface choke: 0 ft
Annulus OD: 6.28 in
Drill String OD: 3.5 in
Inclination angle: 0 deg
Surface temperature: 520 R
Rock specific gravity: 2.65 (water = 1)
Liquid weight: 8.4 ppg
Gas specific gravity: 0.97 (air = 1)
Formation fluid specific gravity: 0.8 (water = 1)
o
Geothermal gradient: 0.01 F/ft
Hole roughness: 0.0018 in
Formation fluid influx rate: 0 bbl/hr
Bit size: 6.125 in
Rate of penetration: 30 ft/hr
Liquid injection rate: 200 gpm
Gas injection rate: 500 scfm
Backpressure at choke: 50 psia
Solution
This problem can be solved with computer program Pressure Instability.xls.
Figure 10.10 presents the calculated pressure instability factor along depth for
three different liquid pumping rates while other parameters are fixed. It
shows that the instability factor ranges from 1 to 2.5, and the higher the
liquid flow rate, the lower the pressure instability factor. In order to achieve
instability factor values below 1.5, the liquid injection rate should be above
200 gpm.
The effect of the gas injection rate on the instability factor is presented in
Figure 10.11. It indicates that the higher the gas injection rate, the lower the
pressure instability factor. However, for gas injection rates higher than
500 scfm, the instability factors at all depths are below 1.5.
(Continued )