Swab and Surge

Bottom hole pressure effect caused while tripping

Swab and Surge pressures on a wellbore are caused by the movement of pipe in and out of the hole. Swabbing occurs when pipe is pulled out of the hole while surging occurs whilst running into the hole.

The swabbing occurs when the drillpipe is pulled from the well, forcing mud to flow down the annulus to fill the void left by the pipe, if the pipe is moved slowly the fluid can fill this without issue, however if it is moved to quickly it can result in a temporary loss of hydrostatic pressure, surging works the opposite way and can fracture or induce losses into the formation.

The drillstring and BHA effectively acts like a syringe due to its configuration where the bit is equal to the open hole size, although unlike a syringe the bit and stabilisers do have pathways to allow flow. However, these pathways are limited and due to the viscosity of the mud, there is a restriction to the flow of liquid around these items which causes swabing and surging.

Balled up components in the BHA and Bit greatly affect swab and surge as they restrict the ability for the fluid to move through the tight restrictions. The best way to mitigate against swabbing when pulling out of hole is to pump out of hole, this replaces the fluid from below the bit through the drillstring.