The Standard Removable Whipstock is mainly used to kick off wells, but can also be used for sidetracking. It consists of a long inverted steel wedge which is concave on one side to hold and guide the drilling assembly. It is also provided with a chisel point at the bottom to prevent the tool from turning, and a heavy collar at the top to withdraw the tool from the hole. It will usually be used with a drilling assembly consisting of a bit, a spiral stabilizer, and an orientation sub, rigidly attached to the whipstock by means of a shear pin.
The whipstock assembly is lowered to the bottom of the hole and orientated. Weight is applied to set the whipstock and shear the pin. The bit is then drilled down and forced to deflect to one side. A 12 to 16 foot "rat hole" is drilled below the toe of the whipstock and the assembly is then pulled out of hole, taking the whipstock with it. A hole opener is run to open the rat hole out to full gauge. The hole opener assembly is then tripped out and a rapid angle build assembly run in hole to "follow up"the initial deflection. This whole procedure may have to be repeated several times in the kick-off.
It is obvious that the major disadvantage of the standard whipstock is the number of "trips" involved. The other important disadvantage is that the whipstock produced a sudden, sharp deflection - in other words, a severe dogleg - which may give rise to subsequent problems with the hole. The advantages are that it is a fairly simple piece of equipment which requires relatively little maintenance and has no temperature limitations.
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