View source for Underreamer

[[File:Z-Reamer.png|thumb|Compact block-type PDC underreamer (Drillstar Z-Reamer)]]
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.
Numerous designs exist, in sizes varying from a couple of inches to above 40". It is typically used in hydrocarbon drilling operations, but also geothermal or water wells drilling.
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. Reaming itself can be conducted either in an existing [[borehole]] ("pilot reaming") or during the drilling operation ("reaming while drilling").
Underreaming is primarily used to allow a wider clearance for running and cementing the [[Casing (borehole)|casing]] correctly, either due to restrictions (swelling shales, tortuosity) or to be able to run a larger [[Casing (borehole)|casing]] size. It is commonly used for [[offshore drilling]] operations, [[Hydrocarbon exploration|exploration wells]], [[extended reach drilling]] or to increase the size of the [[Casing (borehole)#Intervals|production liner]].
3 primary designs have been developed over the years. In chronological order :<br />
- "Roller-cone" type underreamers, based on roller cone cutters technology, were often plagued by low reliability and limited bottom hole time<br />
- "hinge-and-pin" PDC <br />
- "block-type" PDC, commonly used today, suitable for reaming-while-drilling operations.
Underreamers are also required for directional casing-while-drilling (or "drilling with casing") services, to allow the directional BHA to be retrieved through the casing, either at the end of drilling, or when a change of equipment is needed.
==See also ==
*[[Drilling rig]]
*[[Driller (oil)]]
*[[Hole opener]]
*[[Drill bit]]
*[[Drilling stabilizer]]
== External links ==
* [http://drillstar-industries.com/wp-content/uploads/2017/11/drilling_z_reamer_web.pdf Drillstar Z-Reamer technical sheet]
* [http://www.slb.com/services/drilling/drilling_services_systems/rhino_system/rhino_xs.aspx Schlumberger Rhino Reamer XS technical sheet]
* [https://www.youtube.com/watch?v=QtCbOabh-BU Near-bit reaming-while-drilling animation with compact underreamer]
[[Category:Drilling technology]]