Borehole enlargement is a drilling process whereby a wellbore is enlarged by a tool. This tool is called under-reamer and it has expandable cutter arms, activated and deactivated by a steel ball, RFID or mud telemetry and hydraulically actuated. Arms could be locked closed or locked open, depending on the requirements.
Swelling formations and significant inclination of the drilling angle require underreaming of the borehole, in order to reduce the risk of a pipe being stuck. In addition, it is used during gravel pack completions in high sand-content reservoirs, as well as top-hole section enlargements, when surface restrictions exist to drill a larger well.
Two major types of under reamers are:
- Roller-cone for various formations
- Drag type, mostly for soft to medium applications
Underreaming while drilling (or back-reaming) proved to be a very effective way of enlarging a wellbore, in particular in highly swellable formations and directionally steered wells, sidetracking and multilateral drilling. Not only it saves rig time by fewer trips to under-ream a well, but significantly reduces the risk of lost equipment down-hole in highly swellable formations.
Enlarging the borehole in the reservoir section for gas wells is another area where underreamer technology was successful.
The technology is still evolving and went through several iterations over the last two decades. Because the expandable cutter arms are used on demand, having confirmed data on activation or deactivation has been a challenge for many years. There are several solutions available today that overcome this problem.
The technology itself is very cost effective and contributes to less than 1% of a well cost.