The whipstock was the first widely used deflection tool for changing the wellbore trajectory, and is seldom used in open-hole deflections today. A whipstock is selected according to the wedge needed to effect the desired deflection. A bit that is small enough to fit in the hole with the whipstock is then chosen; at the start of the running mode, the bit is locked to the top of the whipstock. When the whipstock is positioned at the kickoff depth, whether it is the total depth of the wellbore or the top of a cement plug, it is carefully lowered to bottom, and the centerline of the toe is oriented in the desired direction by a conventional nonmagnetic collar with a mule-shoe sub and by a single-shot survey. With the whipstock assembly oriented, enough weight is applied to the toe of the wedge so that it will not move when rotation begins.
Additional weight is applied to shear the pin that holds the drill collars to the wedge; then rotation can begin. Forcing the bit to cut sideways as well as forward, the wedge deflects the bit in an arc set by the curvature of the whipstock. When the bit reaches the end of the wedge, it ordinarily continues in the arc set by the wedge. Drilling continues until the top of the whipstock assembly reaches the stop.
The whipstock is then retrieved and the hole is opened with a pilot bit and a hole-opener. The wellbore is enlarged to the original hole size, and the assembly is pulled again. A full gauge directional BHA is then run and standard drilling is resumed.
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