Swabbing Effect | Well Control | IWCF Level 4 | BHP | Drilling | Tripping

Swabbing Effect | Well Control | IWCF Level 4 | BHP | Drilling | Tripping | IWCF

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Swabbing
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Some of the drilling fluid moves up with the pipe and up the walls of the well as the drill string is lifted off bottom. This swabbing effect, much like a piston moving within a cylinder, occurs when the pipe is moved upward faster than the mud can fall below it. The result is a reduction of bottom hole pressure.
The degree to which swabbing occurs depends on,
1. The flow properties of the fluid (density and viscosity),
2. The clearance between the moving pipe and the walls of the hole or casing, and
3. The speed, or rate at which the string is pulled.
It can be seen that only one of these three causes can be controlled by the driller. The driller alone runs the rig and is therefore responsible for the speed at which the pipe is pulled; however the likelihood of swabbing is also significantly increased when pulling the pipe in thick, heavy mud, or when pulling the bottom hole assembly (BHA) off bottom, especially if the bit and BHA are balled up with sticky clay.
When the drill string is in a well and full of mud it is displacing some liquid, just like a solid rod inserted into a glass of water. As the rod is removed the level of water in the glass falls. The same thing happens when the drill string is pulled out of a well on a trip. The column of mud in the well falls as the pipe is lifted out of the well. Since bottom hole hydrostatic pressure is a function of the vertical height of the fluid as well as the density of the fluid, bottom hole pressure is reduced as the vertical height of the column is reduced. The only way to maintain hydrostatic pressure within a safe range on a trip out is by replacing a volume of mud into the well equal to the closed-in displacement of the pipe that is removed. (See Figure 5) This explanation describes a “wet” trip, since the inside of the pipe is filled with mud. No crew likes to make a wet trip. Even though a “mud bucket” may be used to save as much mud as possible, when a pipe connection is broken mud splashes everywhere, the floor gets slick, creating a slipping hazard, crew morale sags, and it becomes much more difficult to accurately measure the mud pumped back into the well to replace the pipe that has been removed.