Surge and swab pressures have been for a while an issue in the drilling of oil wells. The drill string causes a flow for displaced fluid and a pressure change in the borehole when running in or pulling out of the hole. When the string moves upward creates a swab pressure and it creates surge pressures when moves downward. They will reduce the mud hydrostatic pressure below the formation pressure and induce a kick if swab pressures are high enough. The formation can be fractured by high surge pressures and lead to circulation loss. It is always important to remove the drill string during the drilling operations especially to change the bit.