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Underbalanced Drilling Operations 227

bottomhole pressure is called the maximum allowable pressure for stable
foam drilling, and it can be solved from Eq. (10.4):

4:07Tð1 − Γ min ÞGLR
P max = (10.7)
Q fx
Γ min 0:09358 + 0:13369
Q l

Assuming T = 520 °Rand Γ min = 0.55, the maximum pressures are
calculated and plotted in Figure 10.15 for different values of Q fx /Q l .
The figure indicates that the maximum pressure is less than 2,000 psia
even with a GLR value of 20 scfm/gpm. This does not mean that
wells cannot be drilled with foams when the bottomhole pressures are
greater than 2,000 psia. In fact, many wells have been drilled with
foams at bottomhole pressures higher than 2,000 psia. The explanation
is that although the foams may not be stable at the bottomhole, hole
cleaning can still be achieved with adequate mixture flow velocities in
the annulus.
With the constraint of GLR, the foam quality at the bottomhole will
decrease to its minimum allowable value of quality Γ min at a critical depth.

2000
Q /Q = 0 bbl/hr per gpm
l
fx
1800 Q /Q = 1 bbl/hr per gpm
Maximum Allowable Pressure (psia) 1400 Q /Q = 4 bbl/hr per gpm
l
fx
1600
Q /Q = 2 bbl/hr per gpm
fx
l
Q /Q = 3 bbl/hr per gpm
fx
l
l
fx
1200
Q /Q = 5 bbl/hr per gpm
l
fx
1000
800
600
400
200
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Injection GLR (scfm/gpm)
Figure 10.15 Effect of reservoir fluid influx on the maximum allowable pressure.