Patent ID: 6880631

Claim:
A whipstock casing milling system comprising: a whipstock having a whipface, the whipface comprising a first ramp surface and a second ramp surface or parallel surface meeting the first ramp surface at a juncture, the first ramp surface being relatively steep compared to the second ramp surface, said surfaces being ramped or parallel relative to the longitudinal axis of the whipstock such that the first ramp surface is disposed at a greater angle to said longitudinal axis than the second ramp surface; a window mill secured to the whipstock adjacent the first ramp surface and operable in use to form an opening in a wellbore casing in which the whipstock casing milling system is located, milling blades of the window mill being directly engaged with and deflected by the first ramp surface laterally into the casing as the window mill is rotated and forced along the first ramp surface toward the second ramp or parallel surface; and a protrusion provided on the whipface, the protrusion forming an extension of the first ramp surface of the whipface and being directly engaged by said milling blades as said milling blades travel along said first ramp surface onto said extension so as to reduce damage to the first ramp surface at the juncture of the first ramp surface and the second ramp or parallel surface during use of the system, the first ramp surface and the extension together forming a mill deflecting slope; wherein the length of the extension being such that, in use, the surface area of the milling blades directly engaging said first ramp surface section of said mill deflecting slope reduces as a consequence of the mill moving along and projecting beyond said first ramps surface section of said mill deflecting slope, an increase in stress in said first ramp surface section of said mill deflecting slope due to said reduction in surface area being maintainable below a level which would cause damage to said first ramp surface section of said mill deflecting slope until the mill has moved beyond the relatively steep first ramp surface and is no longer in engagement therewith.