Method and apparatus for accurate milling of windows in well casings

Abstract

A method and apparatus for predictable downhole milling of a casing window having predetermined location, orientation, dimension and contour geometry. An elongate substantially rigid milling shaft has at least one casing window milling element in fixed relation therewith and has a pilot mill in articulated and rotary driven connection with the milling shaft. The milling shaft is in articulated and rotary driven connection with a rotary drive mechanism. The articulated connection of the pilot mill and milling shaft may incorporate an articulation control system to permit the pilot mill to be maintained substantially coaxial with the milling shaft so that its trajectory at a predetermined stage of window milling can be controlled by the milling shaft when positive guiding by a deflecting tool can no longer be ensured. The deflecting tool is adapted to be set within the well casing and defines an inclined pilot mill guide surface for guiding the pilot mill along a predetermined inclined trajectory for milling into the well casing. The deflecting tool incorporates a generally cylindrical bearing for guiding and providing rotational stabilization to the pilot mill during initial window milling to ensure the accuracy of the pilot bore being milled through the well casing and into the surrounding formation. During window milling the pilot mill guides the milling shaft so that the string mills of the milling shaft remove a portion of the pilot mill guide bearing and form a guide face of predetermined contour on the deflecting tool for guiding other tools through the casing window and into the lateral bore. The deflecting tool may be of tubular geometry so as to guide not only the pilot mill but also the string mill and may also receive the rotary drive motor for guiding and stabilizing thereof.