What occurs when there is a momentary decrease in bottom hole pressure during drilling?

When there is a momentary decrease in bottom hole pressure during drilling, the phenomenon that typically occurs is referred to as swabbing. This situation arises when the pressure in the wellbore drops below the formation pressure due to various operational activities, such as rapidly pulling the drill string or other interruptions in circulation.

Swabbing can lead to a temporary influx of formation fluids into the wellbore, as the fluid from the formation may flow into the void created by the reduction in pressure. Effectively, this means that swabbing is associated with the potential for drawing hydrocarbons or other formation fluids into the wellbore, which can pose risks if not adequately controlled.

Understanding swabbing is essential for maintaining well control, as it highlights the importance of balancing the pressure system during drilling activities to prevent navigating conditions that could lead to further complications, such as kicks or blowouts. Proper practices and monitoring are necessary to mitigate risks associated with pressure fluctuations in the well.