The whipstock is an old directional drilling tool that is used today primarily to perform well sidetracking out of the casing. Whipstocks and jetting techniques of various types were the principal deﬂection tools for many years before turbine-type and positive-displacement mud motors were fully developed and made economical for drilling applications. A whipstock is a wedge-shaped steel casting with a tapered concave groove down one side to guide the bit into the wall of the hole to start a deﬂection.
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Drilling Whipstock Types & Mechanisms
For directional drilling purposes, there were two types of whipstocks used in the old days:
- the retrievable whipstock and
- the permanent whipstock.
The ﬁxed whipstock stays in the hole after the desired deﬂection (change in hole-inclination and azimuth angles) has been accomplished, while the removable one is pulled out of the hole with the drill string. A whipstock can be set in an open and cased hole. Fig. 1 shows a removable whipstock in an open-hole operation.
The whipstock edge angle is selected according to the desired deﬂection. A bit of diameter small enough to ﬁt into the hole with the whipstock is then chosen. Initially, the whipstock is ﬁxed to the drill string above the bit. When the whipstock is positioned at the KOP depth, the centerline of the toe is oriented in the desired direction.
Fixed whipstocks typically are used to sidetrack an existing cased hole, as shown schematically in Fig. 2. Typically, the operation is accomplished in three stages. First, a window is cut with the milling tool (check also: Milling In Drilling Operations). Then the starting mill is replaced by a sidetracking mill, which makes a window approximately 8 to 12 ft long. Then the sidetracking mill is pulled out and replaced by a taper mill and a BHA (bottom hole assembly types) with watermelon mills (Check Weatherford Milling Tools) to enlarge the casing window to accommodate a conventional BHA. In some applications, a number of trips and many rotating hours are required to make the desired sidetrack.
Drilling With Retrievable Whipstock
- The retrievable whipstock is run in the hole nearly to bottom.
- The orientation of the whipstock is checked with a single-shot, and the face of the whipstock is oriented in the direction desired.
- The tool is then set on bottom. Depending upon the type of retrievable whipstock, it may be necessary to drop a ball down the drill string in order to divert flow from the bottom of the whipstock to the drilling bit.
- Weight is applied to the assembly, and the pin that holds the whipstock in place is sheared allowing the bit to travel down the face of the whipstock. (See Figure 3)
- The bit is rotated between 40 and 60 rpm’s with low bit weights (Drill String Weight Calculations) while drilling off the whipstock.
- Too much weight can cause the bit to bind and the whipstock to rotate.
- Heavy vibration of the kelly will indicate excessive weight on bit.
- When the bit is one foot below the toe of the whipstock, weight can be gradually added.
- Drill approximately 15 to 20 feet of rathole before retrieving the whipstock.
- The collar at the top of the whipstock has a smaller inside diameter than the bit.
- When the bit is pulled up to the collar at the top of the whipstock, the bit pulls the whipstock from the hole.
- The rathole is then opened to full gauge using a hole opener with a pilot bit or bull nose.
- The whipstock causes a significant change in hole direction and angle in a very short interval; therefore, a very limber assembly should be run following the hole opener.
- It is often necessary to work this assembly around the deflection.
- Care should be taken to minimize rotation especially if the whipstock has been set on top of a cement plug.
- If not careful, the assembly will redrill the old hole.
- After drilling a few feet with the limber assembly and reaming the sidetrack, a standard directional drilling assembly is run (RSS assembly – Mud Motor with bent sub) and drilling resumes.
The whipstock is most applicable for sidetracking in very hard and very high-temperature formations where other deflection methods are not feasible.
Disadvantages Of Retrievable Type
- One disadvantage of the retrievable whipstock is that an extra trip is required to open the rathole to full gauge.
- Also, only 15 to 20 feet of hole can be drilled when the whipstock is in the hole.
Because of this, the whipstock is the least desirable method of changing the hole course.
Permanent Drilling Whipestock
- The permanent whipstock is run in the hole on the bottom of the drill string being attached with a shear pin. Usually, about 30 feet of tail pipe is run below the whipstock to act as an anchor.
- The assembly is run in the hole and landed near bottom.
- The whipstock is then oriented using conventional methods.
- The assembly is landed on bottom and cemented in place.
- Weight is applied shearing the pin that holds the whipstock to the drill string.
- The drill string is pulled from the hole, and the cement is allowed to set.
- A full gauge bit and drilling stabilizer are run to drill off the whipstock.
- The method for drilling off the permanent whipstock is the same as for the retrievable whipstock.
Advantages Of Permanent Type
The advantage of the permanent whipstock is a full gauge hole that can be drilled off with the whipstock.
Disadvantages Of Permanent Type
- A disadvantage is the pin holding the whipstock to the drill string may shear prematurely after hitting a bridge or tight spot in the hole. A costly fishing job will result.
- Also, the whipstock may turn or fall over in the hole at some time after drilling off the whipstock. Any hole below the whipstock will be lost.
The permanent whipstock should be run only if no other method is available.
Whipstock Sidetracking Drilling From a Casing
The majority of whipstocks used today are used to sidetrack out of the casing. The whipstock is hard enough to allow milling a window in the casing rather than drilling out the whipstock. There is a myriad of whipstocks available to the industry. Figure 4 is an illustration of a common whipstock. Below are the steps for drilling with whipstock to sidetrack from the casing:
- In this case, a cast iron bridge plug (CIBP) was set with a wireline five feet above a casing collar to avoid milling the casing collar while drilling off the whipstock.
- A mule shoe sub is placed above the whipstock, and the assembly is run as shown to approximately 15 to 20 feet (4.5 to 6 m) above the CIBP.
- The face of the whipstock is oriented in the desired direction with a gyroscopic survey tool.
- If the inclination is above 5 o, it can be oriented with an MWD.
- Lower the drill string until the whipstock tags bottom (do not set slips).
- Check the orientation of the whipstock again. If the orientation is OK, set the slips on the whipstock by applying weight.
- After setting the slips, set down enough weight to shear the shear bolt.
- Pick up and rotate the pipe to ensure that the whipstock is set and the shear bolt has sheared.
- Once the whipstock is set, it is time to drill off the whipstock. The starter mill is used to start cutting the window. The starter mill only drills a short distance (approximately 2 feet or 0.6 m) as illustrated in Figure 5.
- The starter mill is then pulled from the hole. A window mill along with a watermelon mill is run to actually cut the window in the casing as illustrated in Figure 6. Sometimes the BHA may also contain a string mill.
- Once the well has been sidetracked and rathole drilled, the string is reamed through the window several times to clean it up.
- When finished, it is a good idea to run the BHA through the window a few times without rotation to make sure that it is not going to hang up.
This is just one example of a permanent whipstock with a bottom trip. There are also whipstocks that can be set on a packer and where all the milling is accomplished in a single trip. The single trip whipstocks are more expensive but the savings in rig time may offset the additional cost of the single trip whipstock ( check also oil well drilling well cost per foot). There are also many whipstocks that are retrievable for drilling multilaterals wells. Running procedures vary with every whipstock and the manufacturer of the whipstock should be consulted.
Weatherford Whipstock Specifications
|Whipstock OD (in./mm)||Part number||Concave Face Angle (degree)||Concave Pin Connection (in.)||Setting|
|Setting Tool Box Connection (in.)||Stationary Pad Height in / mm||Assembly Stack Height (in./mm)||Running OD Nominal|
|Running OD Maximum|
|Open Hole ID|
|64309-001||3.00°||3-1/2 IF||64889-001||3-1/2 IF||1.059|
|64309-002||3.00°||3-1/2 IF||64889-001||3-1/2 IF||0.997|
|64065-001||3.00°||3-1/2 IF||64889-001||3-1/2 IF||1.136|
|64065-002||3.00°||3-1/2 IF||64889-001||3-1/2 IF||1.282|
|63949-001||3.00°||4-1/2 IF||64889-001||3-1/2 IF||1.285|
|63949-002||3.00°||4-1/2 IF||64889-001||3-1/2 IF||1.137|
|64557-002||3.00°||4-1/2 XH||64559-001||4-1/2 IF||1.059|
|64557-004||3.00°||4-1/2 XH||64559-001||4-1/2 IF||1.206|
|64638-001||3.18°||4-1/2 XH||64559-002||4-1/2 IF||1.132|
|64635-001||3.58°||4-1/2 XH||64559-002||4-1/2 IF||1.138|
|64611-001||3.88°||4-1/2 XH||64559-003||4-1/2 IF||1.202|
|65029||3.88°||6-5/8 Reg||65026||4-1/2 IF||1.800|
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