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206 Part III Underbalanced Drilling Systems

Illustrative Example 9.5 (Continued )
at a circulation-breaking bottomhole pressure of 1,500 psia. This horizontal
line intersects the pressure curves at the flowing points:
Gas Injection Liquid Flow
Rate (scfm) Rate (gpm)
340 100
425 200
600 300
790 400
These combinations of flow rates define the maximum allowable air injection
rates at given mud rates so the borehole will not collapse. A curve (to be
plotted later) going through these points defines the right boundary of a
GLRW.
Flowing bottomhole pressures at various liquid flow rates and gas injec-
tion rates are calculated using Eq. (9.3), and the results are plotted in
Figure 9.21. A horizontal line is drawn in the plot at a flowing bottomhole
pressure of 2,000 psia. This horizontal line intersects the pressure curves at
the flowing points:

Gas Injection Liquid Flow
Rate (scfm) Rate (gpm)
90 100
200 200
380 300
720 400

The preceding combinations of flow rates are plotted in Figure 9.22, which
shows that the useable liquid‒gas rate combinations are clamped in a narrow
region.
In order to determine the lower boundary of the region, the cuttings car-
rying capacities of the liquid at flow rates of 100, 200, and 300 gpm are calcu-
lated. The results are as follows:
Liquid Flow Unit Kinetic
3
Rate (gpm) Energy (lbf-ft/ft )
100 0.86
200 3.46
300 7.77
3
The unit kinetic energy of 3 lbf-ft/ft corresponds to a liquid flow rate of
187 gpm. These data points are plotted in Figure 9.23 to show their locations
in the flow rate map.