Swabbing
If a drill string, casing, or logging tool is pulled out of the hole too fast, the string creates a piston effect pulling some mud out of the hole with it. This tends to reduce hydrostatic pressure in the well. The fluid in the annulus travels downwards to fill the void. This downward movement of fluid creates friction with the wellbore wall that acts upwards. The resultant pressure reduction created by this situation is called 'Swab Pressure'. If the swabbing reduces the pressure too much and the net hydrostatic pressure falls below the formation pressure, it may invite a kick (wellbore influx) into the wellbore. This may require carrying out well control procedures to secure the well.
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e-Learning resources on Surge & Swab Pressures while drilling