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214 Part III Underbalanced Drilling Systems

10.2.1 Aeration Methods
Several gas injection methods have been used in the industry to solve
different types of problems:
• Drill string injection
• Parasite string injection
• Parasite casing injection
• Through completion injection

The last three methods are annulus injection methods. The drill string gas
injection method is shown in Figure 10.1. Gas and liquid are both injected
into the standpipe, and they mix while flowing down the drill string. The
full annulus is occupied by aerated fluid. The advantages of this method
include its simplicity and low cost. The major disadvantage of this method
is the borehole washout in the openhole section because of the high
velocity of liquid and gas mixture around the drill collar.

Figure 10.1 Drill string gas injection method.