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Underbalanced Drilling Operations 223

3
100 gpm & 500 scfm
2.5 200 gpm & 1000 scfm
Instability Factor (ΔP BH /ΔP S ) 1.5 2

0.5 1

0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Depth (ft)
Figure 10.12 Effect of combination of liquid and gas flow rates on the instability
factor.

Figure 10.12 shows the calculated pressure instability factor profiles for
two different liquid‒gas rate combinations while the injection gas‒liquid ratio
is kept the same. It indicates that it is the combination of the liquid and gas
injection rates, not the injection GLR, that determines the pressure instability
factor. Figures 10.10 through 10.12 show that the magnitude of the pressure
instability factor increases with depth.

10.3 FOAM DRILLING

Foam has much better pressure stability. The major issues in foam
drilling operations are foam stability in the borehole and foam handling
at the surface. Foam stability is controlled by foam formulation, the
gas‒liquid ratio (GLR), and backpressure. Foam handling at the surface
can be improved using closed-loop foam regeneration systems.

10.3.1 Foam Formulations
The best performance formulae of foams depend on the foaming agents
(surfactants) used. Pilot tests are always required when a new foaming