Hole Openers vs. Underreamers
Most people use the terms “hole openers” and “underreamers” interchangeably without knowing the difference. While both products are used to enlarge a pre-existing well bore, one is fixed diameter and the other is expandable. The type of application and certain parameters will tell you which tool to use. The biggest factor is what restrictions there are in the well bore.
A hole opener is a fixed diameter tool that is commonly used in drilling larger top hole sections where there are no restrictions. Hole openers are used in all facets of drilling a well for actually enlarging the hole, to act as a stabilizer, or to use for a wiper trip (ensure hole is to gauge). Hole openers usually are run with roller cones but as the hole sizes get smaller, some people prefer to use a bladed hole opener which does not use roller cones.
Most people consider a hole opener “dumb iron” because there are no moving parts, when in fact there is a lot of engineering that goes into them. Although hole openers have not changed in appearance very much since their inception, the roller cone cutters have gone through many changes to increase the drilling life of the tool. Hole Opener Corporation has not only designed and created its own cutters, but its own line of hole opener bodies called the RHO, that are absolutely revolutionary in the industry.
Underreamers, as explained in What is Underreaming?, are hydraulically expandable tools that are used for a completely different reason. Underreamers are a much more complex tool than a hole opener and require careful engineering and planning. They are used to expand the wellbore beyond the pre-existing hole, which may be below a restriction. Most of the time the restriction is casing and good cement jobs are necessary to hold the casing in place. Because a larger bit or hole opener cannot fit through smaller casing, an underreamer is needed to sit just below the casing shoe so that the tool can expand beyond the inside diameter of the casing. Once you underream to the desired depth, the pumps are disengaged and the tool’s hydraulic cutting structure collapses into the body, allowing you to pull back through the casing and out of the hole.