Underreamer with sequentially expandable cutter blades

Abstract

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 piston moves between a first position substantially closing the upper passageway and a second position opening in upper passageway responsive to fluid force acting thereon. The lower piston moves between a first position substantially closing the lower passageway and a second position opening the passageway responsive to the upper passageway being opened by the upper piston whereby the lower cutter blades are extended only after the upper cutter blades have been extended and, after both the upper and lower cutter blades have been extended, a continuous fluid flow path is established from the upper end of the tool to a drill bit at the lower end of the tool.