Underreamer with simultaneously expandable cutter blades and method

摘要

An underreamer tool has a body with upper and lower portions having an upper cutter blade slot extending transversely through the upper portion and a lower cutter blade slot extending transversely through the lower portion at a right angle thereto. The upper and lower portions have central bores separated by a wall and connected by upper and lower fluid flow passageways bypassing the cutter blade slots. An upper pair of cutter blades mounted in the upper cutter blade slot and a lower pair of cutter blades mounted in the lower cutter blade slot move between a retracted position within the slots and an extended position outward therefrom. An upper piston in the central bore above the upper cutter blade slot has its bottom end engaged on the upper cutter blades and a lower piston in the central bore above the lower cutter blade slot has its bottom end engaged on the lower cutter blades. The upper and lower pistons move simultaneously between a first position where the cutter blades are retracted to a second position where the cutter blades are extended, the upper and lower passageways being unobstructed through the entire range of movement. A continuous fluid flow path is maintained from the upper end of the tool to a drill bit at the lower end of the tool at all times.