US Patent 7231979 Short radius whipstock system

A short radius exit from a window milled in casing is possible using a whipstock with a sloping surface in excess of 3.5° and a window mill whose diameter is reduced to a percentage generally below about 95% of the casing inside diameter in a mono-bore or non-through tubing application. The system provides a greater flexibility in choosing the window location and eliminates having to penetrate adjacent formations as compared to previous techniques using a longer exit radius. The decrease in mill diameter, as compared to previous techniques, limits stresses on the milling equipment to minimize equipment failures during window milling and subsequent drilling of the lateral.