Abstract: An apparatus for finding a pre-existing downhole window includes a body having a deflector ramp and a cavity, and an extendable dog disposed in the cavity, the dog mechanically moveable between a retracted position and an extended position into the pre-existing window. In some embodiments, the apparatus includes an axially moveable sleeve disposed adjacent the extendable dog. A downhole window finding apparatus includes a deflector including a window finding assembly, and a running tool removably coupled to the deflector, wherein the window finding assembly includes an extendable member and an axially moveable sleeve retaining the extendable member. In certain embodiments, a fluid flow path extends through the running tool and the deflector to the window finding assembly. In some embodiments, a shear bolt removably couples the running tool to the deflector and includes a fluid passage fluidicly coupling a flow line in the running tool to a flow line in the deflector.

Abstract: An underreamer opens a borehole below a restriction that is larger than the restriction itself. The underreamer includes cutters which engage the formation by traversing outward and upward. The force pushing the cutters to the extended position is supplied by a piston built into each cutter support. Pressure acting on these pistons comes from the pressure differential between the annulus and drill string during circulation of drilling fluid. The cutters are supported on both sides of the cutting structure by arms that are supported by and in sliding contact with the body. A spring opposes the upward and outward motion of the cutter mechanism and returns the cutters to their collapsed position in the absence of differential pressure. The body of the tool incorporates by-pass ports for transmitting fluid through the tool to the remainder of the bottom hole assembly.