What are Hole Openers for Drilling?

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

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,

Crew sets up the under reamer to enlarge the hole, (tamu.edu, 2003)

Holes that have previously been drilled can be re-opened using a fixed diameter hole opener. In this case, a “bull nose” will be run beneath it. Otherwise, a fixed diameter hole opener can be used to drill a new hole, usually alongside a drilling assembly. This assembly will usually be made up of a drill bit, a short drill collar, the hole opener, and so on. One potential use of this method drilling is to use a series of hole openers, with each one growing increasingly wider in diameter. This is often used when “spudding” a new well in order to set the conductor, or surface casing. It can also be run on the bottom of a mud motor- one situation necessitating this might be where there is no surface drive available.

 Hole openers may be used for drilling large diameter holes in order to set platform legs, before grouting takes place. They might also be used to wipe out a key seat, or to deliberately drill an oversized hole if it is necessary to add some additional cement behind a subsequent casing string.

Most rigs feature fixed diameter hole openers as standard. However, they are not always used with the same degree of care and precision as other specialized pieces of drilling equipment. Shoulder areas and connections need to be inspected routinely, and the same goes for welds. They are just as important as a remainder for keeping the drill string in good condition.


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Getech Equipments International Private Limited. (2018). Mud Rotary Drilling Accessories – Tricon Rock Roller Bits Manufacturer from Hyderabad. [online] http://www.getechequipments.com/mud-rotary-drilling-accessories.html#tricon-rock-roller-bits. Available at: http://www.getechequipments.com/mud-rotary-drilling-accessories.html [Accessed 15 Apr. 2018].

Slb.com. (2018). Fixed-Diameter Hole Opener | Schlumberger. [online] Available at: https://www.slb.com/services/drilling/tools_services/reamers_stabilizers/fixed_hole_opener.aspx [Accessed 15 Apr. 2018].

Anon, (n.d.). 210_4_66. [online] Available at: http://www-odp.tamu.edu/public/life/210/week4/pages/210_4_66.html[Accessed 15 Apr. 2018].

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