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218 Part III Underbalanced Drilling Systems

Figure 10.6 Through completion gas injection method.

cemented at any depth. The inner casing is set with a packer inside the
outer casing. Gas is injected into the drill string–inner casing annulus
through the inner-outer casing annulus and a port at the lower section of
the inner-casing string. The inner-casing string can be retrieved when
necessary. With this gas injection method, again only a portion of the
cased hole section of the annulus is aerated. It allows for the use of con-
ventional MWD, better pressure stability control at the bottomhole, and
prevention of complications at the bottom of the inner-casing string.

10.2.2 Pressure Stability Control

As shown in Figure 10.7, making a pipe connection or changing a choke
setting can cause severe fluctuation in the bottomhole pressure. Figure
10.8 illustrates the effect of the liquid injection rate on the pressure