Through-tubing whipstock to exit two casing strings

A specific need was identified on the Montrose platform in Amoco`s North Sea drilling operations for milling through 7-in. and 9-5/8-in. casing at 60{degree} inclination using coiled tubing. All equipment would be required to pass through 3.75-in. restrictions in the 4{1/2}-in. tubing completion string. This operation would be a first for the industry through these two casing strings using through-tubing whipstock technology. A variable-angle test fixture was designed and constructed at Amoco`s Drilling Technology Test Facility at Catoosa, Oklahoma. Available through-tubing whipstock systems from three service companies were evaluated, including: (1) TIW/The Red Baron--whipstock OD (3.67 in.), mill OD (3.75 in.), system designation (T); (2) Weatherford Enterra, Inc.--(3.75 in.), (3.75 in.), (W); and Baker Oil Tools--(3.625 in.), (3.70 in.), (B). Results are discussed.