One trip milling system and method

A method of sidetracking including the steps of:

• lowering a pair of serially connected mills, releasably connected to a whipstock into a borehole casing said mills each having a plurality of circumferentially disposed radially extending blades each having a tapered portion and said whipstock having at least three axially spaced ramps provided thereon, each said ramp being interspaced by a substantially axially extending portion, each ramp having substantially the same angle of inclination to the longitudinal axis and the taper on said tapered portion of said blades having a similar angle of inclination, the distance between the ramps being substantially the same as the distance between the tapered portion of blades,
• orienting the whipstock so that the ramps are angled toward a desired orientation for cutting a window in the casing and cutting through said formation to a desired new location, releasing the connection between the mills and the whipstock,
• rotating the mills and moving said mills downwardly so that the tapered blades of each respective mill abrade a respective ramp, downward movement of said mills against said ramps causing an upstream one of the mills to first cut the casing and continued downward movement causing the downstream mill to cut the casing, continued downward movement causing a window to be cut into the casing and sidetracking operations to be performed through formation.