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Underbalanced Drilling Operations 215

To ease this problem, side-jet subs have been used to divert some of
the gas flow in the cased hole section (Figure 10.2). Another disadvantage
of the drill string injection method is that it prevents the use of conven-
tional measurement while drilling (MWD) tools for directional drilling.
This is because the compressibility of the aerated fluid in the drill string
masks the mechanical telemetry signal from the MWD tool. Electric-
magnetic measurement while drilling tools, or EMWD, are required with
the drill string injection method. To solve this problem and to increase
pressure stability, annular injection methods were developed.
The first annulus injection method developed was the parasite string
injection method shown in Figure 10.3. Gas is injected into the cased annu-
lus through a small tubing string attached to the casing string. The gas injec-
tion sub connecting the tubing string to the casing is shown in Figure 10.4.
With this gas injection method, only the cased hole section of the annulus is
aerated. It allows for the use of conventional MWD in directional drilling

Figure 10.2 Drill string gas injection with a side-jet sub.