The whipstock is a steel ramp shaped device which deflects the bit off the cement plug. Two types are popular. One is called a permanent whipstock because it is cemented and left in the hole. The other is called a retrievable because after deflecting the bit off the cement plug it is pulled from the hole. The basic problems which often occur are similar for both types. The first problem is that the whipstock is not set on bottom which allows the bit to drill around the end of the whipstock and back into the old hole. This problem is usually caused by bottom fill which was not removed before setting the whipstock. The whipstock is set on the bottom of the hole with exacting precision measurements of the setting string and drilling string to which the setting string must be matched. After pulling out of the pilot hole, the whipstock may turn which prevents the re-entry into the pilot hole. The rock may be of sufficient strength that the drill bit will cut through the steel of the whipstock rather than the surrounding rock.
Sidetracking from a window within casing with a whipstock may be less expensive and faster in some cases. After the window is cut, a whipstock fastened to a hanger is run into the hole and the hanger is set within the casing below the window. The hanger locks the whipstock in place with the assurance of the casing. A cement plug is not required.
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