The blades of the underreamer remain inside the tool for accessing through
restrictions. Then they expand by centrifugal and hydraulic force when pumping
begins and the downhole motor begins to turn. For recovery, the blades are
retracted again by stopping pumping.
Common applications for underreaming are cement removal after squeeze
operations and scale removal. When fitted with specially hardened knives, the
underreamer can be used for cutting tubing.
Packers and bridge plugs are used to temporarily or permanently isolate zones
downhole. Typical applications of such zonal isolation are
- zone testing
- zone shutoff or abandonment
- selective stimulation
- leak detection
- cement plugs
- velocity strings
- recompletions and workovers
- well control.