Importance

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Abstract

A sidetrack mill is provided for drilling through a cased borehole to provide a sidetracked wellbore having a final diameter, the mill comprising: a pilot mill having a tapered cutting surface being, at its largest diameter, between about 50% and about 75% of the final diameter; a second stage cutting surface being, at its smallest diameter about the diameter of the maximum diameter of the pilot mill, and being at its largest diameter, at least five percent greater in diameter than the largest diameter of the pilot mill; and a final stage cutting surface being, at its largest diameter, about the final diameter, and at the smallest cutting surface diameter, being a diameter of at least about 5% smaller than the final diameter. This mill is can rapidly penetrated casings without requiring excessive torque. The milling can be accomplished with a single trip. In another aspect of the present invention, a method is provided to provide a sidetracked wellbore through a cased borehole the sidetracked wellbore having a final diameter, the method comprising: providing a whipstock, the whipstock comprising an elongated wedge shaped body having a wedge surface of a hardness greater than the casing, and less than tungsten carbine; securing the whipstock in the borehole with wellbore cement; and milling a window in the casing by rotating a mill having tungsten carbine cutting elements in the wellbore along the wedge surface of the whipstock. The staged mill described above is preferably utilized in this method.

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First Claim