Definition
Underreamer — An underreamer is a downhole tool with expandable cutter arms that enlarge the borehole below a restriction such as existing casing, opening the hole to a diameter larger than the tool could pass through in its collapsed state.
Also known as: underreamer, under-reamer, hole opener, expandable reamer
What an underreamer does
An underreamer enlarges a wellbore beneath a smaller-diameter section. Its cutter arms or blocks stay retracted so the tool can run through the existing casing or restriction, then extend hydraulically once below it to ream the hole to a larger gauge. This lets operators drill a pilot hole with the bit and simultaneously, or subsequently, open it to the size needed for casing and cementing.
Where it is used
Underreaming is common where a larger annular clearance is required for good cement, for casing-while- drilling programs, and in directional and deepwater wells that need enlarged hole for liner running. A related fixed hole opener enlarges from the top down where no restriction exists. Reliable arm actuation and full-gauge cutter dressing are critical so the enlarged hole stays to size.
Why it matters for rentals
Underreamers are interval-specific, mechanically complex, and consumable in their cutting structure, which makes them well suited to rental.
Explore our downhole tools for rental underreamers, or contact us to match cutter type and enlarged diameter to your casing program.