Underreamer

An underreamer is a device used to enlarge the borehole below an existing casing or restriction, during a well drilling operation.

It can be positioned either above the drill bit or above a pilot assembly run inside the existing borehole.

This device is typically composed of a body connected to the drillstring, and of mobile parts ("arms", "blocks" or "blades", typically 3 of them) retracted in the body during the descent and extended downhole for the reaming operation.

Underreaming is primarily used to allow a wider clearance for running and cementing the casing correctly, either due to restrictions (swelling shales, tortuosity) or to be able to run a larger casing size.

In chronological order : - "Roller-cone" type underreamers, based on roller cone cutters technology, were often plagued by low reliability and limited bottom hole time - "hinge-and-pin" PDC - "block-type" PDC, commonly used today, suitable for reaming-while-drilling operations.