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Underbalanced Drilling Operations 217

and better pressure stability control at the bottomhole. However, running in
casing with a parasite tubing string is difficult.
The second annulus injection method is the parasite casing injection
method shown in Figure 10.5. The outer casing can be set and cemented
beyond the vertical section. The inner casing is set but is not cemented.
Gas is injected into the drill string–inner casing annulus through the
inner–outer casing annulus. The inner-casing string is usually retrieved
after drilling. With this gas injection method, only a portion of the cased
hole section of the annulus is aerated. It again allows for the use of con-
ventional MWD and better pressure stability control at the bottomhole.
However, complications can occur due to the nonconstrained, noncentralized
inner casing at the bottom.
The third annulus injection method is the through completion injec-
tion method shown in Figure 10.6. The outer casing can be set and

Figure 10.5 Parasite casing gas injection method.