An underreamer is a device used to enlarge the

borehole A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petro ...

below an existing casing or restriction, during a

well drilling Well drilling is the process of drilling a hole in the ground for the extraction of a natural resource such as ground water, brine, natural gas, or petroleum, for the injection of a fluid from surface to a subsurface reservoir or for subsurfac ...

operation. It can be positioned either above the

drill bit Drill bits are cutting tools used in a drill to remove material to create holes, almost always of circular cross-section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. In order ...

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

("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 correctly, either due to restrictions (swelling shales, tortuosity) or to be able to run a larger casing size. It is commonly used for

offshore drilling Offshore drilling is a mechanical process where a wellbore is drilled below the seabed. It is typically carried out in order to explore for and subsequently extract petroleum that lies in rock formations beneath the seabed. Most commonly, the te ...

operations, exploration wells,

extended reach drilling Extended Reach Drilling (ERD) is directional drilling of very long horizontal wells. The aims of ERD are: a) to reach a larger area from one surface drilling location, and b) to keep a well in a reservoir for a longer distance in order to maximize ...

or to increase the size of the

production liner Production may refer to: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in the outline of industrial organization, the act of making products (goods and services) * Production as a stati ...

. 3 primary designs have been developed over the years. 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. 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.

External links

Drillstar Z-Reamer technical sheet

Schlumberger Rhino Reamer XS technical sheet

Near-bit reaming-while-drilling animation with compact underreamer
Drilling technology

See also

External links