Hole openers are used to increase size of well bore and there are two broad categories of hole openers: fixed diameter hole openers, and under-reamers.
Fixed diameter hole openers are usually made up of three “cutters” arranged around a mandrel, and mounted on “saddles” by strong retaining pins. Cutters may be milled tooth, PDC, or TC inserts, which will vary depending on the formation to be cut.
Fixed Diameter Hole Opener (Getech Equipments International, 2018)
Under-reamers, on the other hand, are hydraulically actuated hole openers that possess two or three arms. They are primarily used when a hole needs to be opened to a diameter larger than the casing which has already been set. Both of these forms typically feature a series of fluid passages, or “jets”, which are arranged to keep the cutters lubricated and help with the removal of cuttings. These need to be set up properly before use, to ensure a balanced mud flow both through and out of the hole opener.
Crew sets up the under reamer to enlarge the hole, (tamu.edu, 2003)
Not only are reamers important for directional drilling, but they can also be useful in straight hole applications. Reaming assemblies can straighten out and smooth over crooked holes, restore undergauge holes to gauge, and get rid of any irregularities or keyseats. They also help to prevent excessive hole curvature in short intervals, which may be experienced when entering and exiting a section of hole which forms a sharp curve. Finally, reamers can reduce the rotational torque in a wellbore, and may therefore be used as a substitute for a conventional string or near-bit stabilizer.
Reamers are made by almost all major downhole tool manufacturers, and have the same core features: sealed or open (mud lubricated) bearings, cutter types – either “nobbly” or “smooth”, and either one (so called “3-point”) or two (“6-point”) sets of cutters in a tool.
Reamer (Courtesy of NOV, 2017)