To enlarge the size of the borehole of the well by the use of an under-reamer (q.v.). A tool with expanding arms or lugs that, when lowered into the hole, can be released at any depth to ream the hole with steel or insert cutters.Often the borehole, which has penetrated the producing formation, is under-reamed to enlarge the exposed area of the hole and increase the flow of oil into the hole. A belling tool, a type of under-reamer, is sometimes used to excavate a bell-shaped hole just below the production string of tubing or casing for the same purpose, to enlarge or increase the face area of the hole.
