Swab Pressure

The temporary reduction in the bottomhole pressure that results from the upward movement of pipe in the hole. The movement of the drillstring or casing through the wellbore is similar to the movement of a loosely fit piston through a vertical cylinder. A pressure reduction or suction pressure occurs below as the piston or the pipe is moved upward in the cylinder or wellbore, and a pressure increase occurs below as they move downward.