Whipstocks

The whipstock was the main deflection tool from 1930-1950. A standard whipstock is seldom used nowadays, but it has not disappeared completely. Whipstocks are used in coiled tubing drilling for re-entry work. There are 3 types of whipstock used in conventional directional drilling:

Standard removable Whipstock: 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.
WhipstocksCirculating Whipstock: The “Circulating Whipstock” is run, set and drilled like the standard whipstock. However, in this case the drilling mud initially flows through a passage to the bottom of the whipstock which permits more efficient cleaning of the bottom of the hole and ensures a clean seat for the tool. It is most efficient for washing out bottom hole fills.

Permanent Casing Whipstock: The “Permanent Casing Whipstock” is designed to remain permanently in the well. It is used where a “window” is to be cut in casing for a sidetrack.A special stinger at the base of the whipstock slips into the packer assembly, and a stainless steel key within the packer locks the whipstock’s anchor-seal and prohibits any circular movement during drilling. The normal procedure is to orientate the system and then set the packer. After this, the starting mill is pinned to the whipstock and the whole assembly run slowly in hole and seated in the packer. Although the packer has already been orientated, it is good practice to orientate the whipstock in the same manner as the packer. This will ensure that a faster “latch up” will take place without endangering the shear pin. After the whipstock has been “seated” in the packer, the pin is sheared and circulation and rotation started. The starting mill is used to make an initial cut through the casing and mill approximately 2 feet of the window. The lug that held the starting mill to the whipstock must also be milled off. This assembly is tripped out and the mill changed. A tungsten carbide or diamond speed mill is used to cut the rest of the window. Once the window has been cut, approximately 5 feet of formation is cut before pulling out of hole. Next, a taper mill is run with a watermelon mill immediately above it. This is done to “clean” the top and the bottom of the window. Finally, another trip is made to change over to the drilling assembly which is used to drill ahead. The advantage of using this system, instead of the normal method of milling a section and sidetracking, is that the operation usually takes less time. The main disadvantage is that it gives a sharp dogleg, and as such the casing whipstock assembly is not recommended if there is a considerable distance to drill below the sidetrack. This is because problems can occur when trying to pull stabilizers, etc back into the casing through the window. On the other hand, if there is only a short distance to be drilled below the sidetrack point, then the casing whipstock is well worth considering. In recent years, improvements in the design of the system have eliminated the need for so many trips in and out of the hole.

Video: Whipstock 

Sidetracking Whipstocks: Sidetrack services for your wellbore requirements. For exiting cased or open holes, Schlumberger sidetracking and milling services deliver faster, higher-quality windows and sidetracks to reduce cost and risk. By matching specific technologies to your exact wellbore requirements, our sidetrack services can meet your well departure objectives.

Casing Exits: Achieve a high-quality window for exiting standard steel casing.

Openhole Sidetracking: Sidetrack efficiently in open hole, with maximum probability of success on the first attempt and isolation of lower zones if required.

Thru-Tubing Sidetracking: Exit standard steel casing below the production tubing.

Section Milling: Single-trip milling to ensure rock-to-rock isolation.

Multilateral Completion Systems: Additional laterals for cost-effective increase in reservoir exposure.

Whipstock Types

Whipback: The Whipback tool is a retrievable or non-retrievable, mechanically-set non-hinged whipstock for use in cased holes. It can be oriented and is ideally suited for drilling multiple laterals from a single hole. The most unique feature of the Whipback tool is that in a deviated hole it can be set traditionally to mill up and to the right or it can be set on the low side so that it will mill downward. The top of the tool is concave and locks against the casing bore regardless of orientation. The Whipback tool has been designed with a bypass area allowing sidetrack, while producing. It has circulation slots around and ports through the whipstock for more by-pass area.

Packwhip Tool: The Packwhip for cased holes is available either as a retrievable or as a non-retrievable, non-hinged type. Both are designed to be set into a permanent packer. If orientation is desired, the Packwhip can use a whipstock packer and stinger with an orienting system. This type of whipstock is easily oriented and ideal for running in deep holes.

Bottom Trip: The Bottom Trip for cased holes is a mechanically set, hinged, non- retrievable whipstock that, when set, is highly resistant to rotation. It is specially designed to be set on any plug, cement retainer, liner hanger, junk, or on top of a fish. It is run the same way as the Whipback but is not retrievable.

Cement Type: The Cement Type is usually set in an open hole. It is the simplest type designed to accommodate setting and cementing operations. It has a standard thread connection on the bottom to allow the addition of a tail pipe. Once the Cement type is set in the open hole, a drill bit can be used to continue down the concave face of the whipstock.

Stiffback Whipstocks: Based on extensive research and field experience, key company has engineered the premier solution in whipstock technology. Key’s “Stiffback” whipstocks are made of treated 4140 material, which offers greater protection from wear and makes exiting the pipe easy. Threads cut on the concave make the tool easy to retrieve and a bi-directional slip anchors its positioning. The whipstock can be set at any azimuth to cut your window in any direction in your wellbore.

Whipstock Mills: Key’s whipstock mills are dressed with tungsten carbide for fast cutting of all steel grades of casing. The highly engineered mills create fine particles of cuttings that can be quickly and easily circulated and flowed to the surface. This results in more efficient cleaning of the hole and faster penetration.

Note: The removable whipstock is a reliable deflection tool that is usually used when jetting or when downhole motors cannot be used. It is a cylindrical steel casting, five to thirteen feet long, with a ring at the top, a concave inclined groove formed on one side and a chisel point bottom. The ring at the top is smaller than the bit and provides the means of transporting the whipstock in and out of the hole. The chisel bottom holds the whipstock stationary when it is set, and the indented groove, (usually 3 degrees) guides the bit to a new course.

The circulating whipstock differs from the standard whipstock, in that there is a control sub immediately above the bit that prevents the flow of circulating fluid through the bit and deflects it through a hollow shear pin used to pin the whipstock to the drill pipe assembly. The flow of fluid passes through the shear pin and down to the bottom of the whipstock via a bored circulating channel in the back of the whipstock. The fluid then flows out the bottom of the whipstock. This permits the bridges in the hole to be washed through, and fill up on the bottom of the hole to be circulated clean. Prior to setting the whipstock, a ball is dropped through the drill pipe and seated in the control sub. The drilling fluid is then diverted from the bottom of the whipstock and established through the bit. The whipstock can then be set and drilling commenced. A 15′ – 20′ rat hole is usually drilled below the whipstock setting point. The diameter of this hole is smaller than the open hole, and must be reamed out to the full hole size by means of a hole opener. Therefore, setting a whipstock is slower than other deflection tool methods because more trip time is needed to set and ream the rat hole out to full gauge. A combination universal knuckle joint and short drill collar may be run in conjunction with the whipstock when extreme angles are required to sidetrack an enlarged hole.

Directional Drilling: In an operation designed to sidetrack the hole around a non-retrievable fish, such as a broken drill string which has become stuck in the hole, it is necessary to place a cement plug at a specific depth to help support the whipstock for directing the bit into the desired area. Another example of a controlled change in the drilling direction to help reach a specific target area is in shoreline drilling operations for offshore production.

Conventional Re-entry: Deepening and sidetracking wells cover the bulk of the conventional re-entry market, and CTD is suited to a portion of these operations. For sidetracks, a whipstock is set at the kick-off depth, and a window is milled in the casing. In such types of wells CTD has limitations of maximum hole size (6″) and build up rates (>30 degree/100 ft). For horizontal sidetracks the drain hole length is limited by the required WOB. Here too CTD units have to compete with depreciated drilling rigs with low day rate. Here too CTD has the same environmental advantages as for new wells. CTD is also useful for offshore where UBD is required due to depleted reservoir pressures.

Whipstock in Cement (WIC): This technique is slightly more complicated than the CS technique. The casing is filled with cement (preferably to the tailpipe) and a through-tubing directional assembly is used to drill a hole to the inside of the casing at the proper tool face angle. Enough straight hole is drilled adjacent to the casing to make a straight rathole for the whipstock. The procedure described above can be used if the well is theoretically deviated more than 3 degrees at the point where the whipstock is to be set. From a practical stand point, a hole deviation of more than 10 degreesmay be required for stable tool face readings while drilling (this applies to the CS technique also). A bottom trip whipstock is run in with a bottom hole assembly (BHA) of; (a) whipstock, (b) starter mill, (c) steering tool, (d) CT

Through-Tubing whipstock (TTW): Several versions of through-tubing whipstock exist. All consist of an anchor that reacts torsional and axial loads and are designed to allow the small through-tubing diameter to span from the high side to the low side of the much larger casing inside diameter. All TTW are normally used for a near high side exit. This is to allow gravity to force the upper whipstock taper to lay against the low side of the casing.

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1 Comment

  1. Definition of Directional Drilling: Directional drilling can generally be defined as the science of directing a wellbore along a predetermined trajectory to intersect a designated subsurface target.

    Applications

    Multiple wells from offshore structures: The most common application of directional drilling techniques is in offshore drilling. Many oil and gas deposits are situated well beyond the reach of land based rigs. Drilling a large number of vertical wells from individual platforms is both impractical and uneconomical. The obvious approach for a large oilfield is to install a fixed platform on the seabed, from which many directional boreholes can be drilled. The bottomhole locations of these wells are carefully spaced for optimum recovery.

    In conventional development, wells cannot be drilled until the platform has been constructed and installed. This can mean a delay of several years before production begins. Such delay scan be considerably reduced by predrilling some of the wells through a subsea template while the platform is being constructed. These wells are directionally drilled from a semisubmersible rig and tied back to the platform once it has been installed.

    Relief Wells: Directional techniques are used to drill relief wells in order to “kill” blowouts. Relief wells are deviated to pass as close as possible to the uncontrolled well. Heavy mud is pumped into the reservoir to overcome the pressure and bring the wild well under control.

    Controlling Vertical Wells: Directional techniques are used to “straighten crooked holes”. When deviation occurs in a well which is supposed to be vertical, various techniques can be used to bring the well back to vertical. This was one of the earliest applications of directional drilling.

    Sidetracking: Sidetracking out of an existing wellbore is another application of directional drilling. This is done to bypass an obstruction (“fish”) in the original wellbore, to explore the extent of a producing zone in a certain sector of a field, or to sidetrack a dry hole to a more promising target.
    Wells are also sidetracked to access more reservoir by drilling a horizontal hole section from the existing well bore.

    Inaccessible locations: Directional wells are often drilled because the surface location directly above the reservoir is inaccessible, either because of natural or man-made obstacles.

    Fault Drilling: Directional wells are also drilled to avoid drilling a vertical well through a steeply inclined fault plane which could slip and shear the casing.

    Salt Dome Drilling: Directional drilling programs are sometimes used to overcome the problems of drilling near salt domes. Instead of drilling through the salt, the well is drilled at one side of the dome and is then deviated around and underneath the overhanging cap.

    Shoreline Drilling:
    In the case where a reservoir lies offshore but quite close to land, the most economical way to exploit the reservoir may be to drill directional wells from a land rig on the coast.
    These are only some of the many applications of directional drilling. Although it is not a new concept, one type of directional drilling, horizontal drilling, is the fastest growing branch of drilling, with major advances occurring in tools and techniques. As with directional drilling, there are numerous specific applications for horizontal drilling.

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