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In the context of drilling, ballooning is the phenomenon in which fluids are lost to the rock during over-pressured operations, such as found in increased pressures from equivalent circulating density operations, and then flow back when pressure is reduced. This may be confused with a kick.
In the context of pipes, balooning is an increase in pipe O.D. as internal pressure is applied (shortens pipe) or a decrease in diameter (Reverse Ballooning: lengthens pipe) as external pressure is applied.