Determining the best tools for sidetracking completions
This illustration shows a typical big bore packer, latch and whipstock assembly. [73,236 bytes] Sidetracks followed by lateral wellbore sections have become a common completion option. These activities may be a part of the original drilling and completion plan, or may be a means of extending the life of an existing well or field.
State-of-the-art options for lateral sections
Gary M. Briggs
In order to meet the varying configuration demands and competitive project pricing considerations, several equipment systems have been developed by different companies. Included are enhanced conventional monobore systems, through-tubing systems, and a multi-lateral access system for new wells. This article outlines three types of system capabilities and equipment configurations to help users determine the best tools for a given application.
Enhanced sidetractsTo meet the various completion requirements associated with conventional sidetracks, special big bore whipstock packers may be needed. The use of a retrievable packer, or anchor, further expands the completion options once the lateral, or multiple laterals, are drilled.
These big bore packers offer positive setting performance during the sidetracking operation, and a large seal bore to accommodate various completion tools and servicing equipment once the lateral is finished.
They can also be removed, if necessary, to gain even greater access to the wellbore below the sidetrack point. Removing the packer, however, results in the loss of orientation capability during future re-entry operations.
Once the big bore packer has been set, the setting orientation is determined using conventional, meaurement-while-drilling (MWD), or gyro survey equipment. Determining the packer orientation with MWD equipment run when the packer is set can save additional rig time.
The whipstock is next configured at the surface to insure the desired downhole orientation and related kick-off direction. A variety of whipstock and milling systems are available. The choice depends on the particular downhole conditions. To improve the overall project economics, one-trip and combination systems are often used for the actual milling operation.
Completion optionsThere also are a variety of final completion options. By taking advantage of the packers' large bore, and the positive orientation feature of the packer system, it is often possible to selectively produce from the various lateral sections of the well. Preplanning can also allow the completion to be serviced using coiled tubing for remedial and production option change work.
By adding a dual packer to the main wellbore, and taking advantage of the big bore packer seal bore capabilities, the lateral junction could be configured to meet the pressure containment requirements associated with a Type 5 completion.
Due to the high cost and potential shortage of conventional drilling rigs, sidetrack re-completions using coiled tubing can offer significant cost savings for the operator. This is particularly true in areas where large-diameter production casing is used.
System componentsThe through-tubing window milling system consists of three main components: the retrievable casing anchor, the retrievable latch assembly, and the retrievable whipstock. All of the components are designed to be run through standard ID production tubing, set and/or operated in the well's production casing, and then to be fully retrievable. The retrieval can be either as individual components, or as a complete unit.
The retrievable casing anchor can be run and set at any hole inclination angle using appropriate wireline, conventional, or coiled tubing conveyed setting tools. The anchor is a fully functional anchor system with bi-directional, non-rotational slips, and an orientation slot for precise final whipstock orientation.
The setting slips are free to move independently as part of a dovetail slot and expander system. This allows the anchor to adapt to minor variations in the casing ID and different casing weights. There is also assurance that the anchor orientation and location will be maintained.
Once the anchor has been set, it is held firmly locked in position with an internal ratchet ring system. The ratchet ring system is also shear pinned to allow for releasing and retrieval, once the sidetrack work has been completed.
After setting the anchor, its orientation is determined using a survey overshot with either a wireline, MWD, or gyro survey system. The retrievable latch assembly is then adjusted on the surface and made up to the whipstock to allow for the desired downhole whipstock orientation.
Before running the latch and whipstock assembly, it can also be set to allow for either independent component retrieval, or retrieval with the anchor, latch, and whipstock as a complete assembly. This allows for various completion options.
The retrievable whipstock is designed with a hinge mechanism that angles the top and bottom of the whipstock to a position firmly against opposite sides of the casing ID. The whipstock face is then oriented opposite the intended window. This ensures a continuous depth whipstock face, resulting in more consistent window mill positioning and orientation.
The whipstock is locked in the angled position with a pivoting hinge and ratchet ring assembly to insure a solid face during milling operations. The hinge assembly is activated by tension loading, rather than compression loading.
In applications with a large tubing/casing diameter differential, reactive torque may cause the motor assembly to fall off the whipstock. For these situations, the whipstock can be constructed to include a double hinge and parallel starter ramp section.
This allows the downhole motor to be started up away from the casing exit point, and insures that the mill and motor stay trapped on the whipstock face during the start of the milling operations. Either special starter milling systems, or one-trip milling systems, can be employed for the window milling operation.
Several key points regarding the system operation should be considered. These include:
- Option of deploying and using coiled tubing through standard 3 1/2-in. or 4 1/2-in. production tubing.
- Setting and sidetracking is possible in a variety of production casing sizes.
- Full retrievability of the various system components either as individual components or as a complete unit, subject to the completion needs for the well, and with proper advanced planning.
- Positive bi-directional and non-rotational anchor.
- Ability to set with all conventional running and setting systems.
Multi-lateral accessIn many new well and recompletion applications, operators know in advance that sidetracks will be required. These may be part of the initial, original completion, or may be planned for some time in the future when the initial well production has declined. With this level of advanced planning, a multi-lateral access system offers potential cost saving for the operator.
The multi-lateral access system (MLAS) includes two main parts. The first half of the system combines the orientation profile sub, and a composite joint, if applicable. The second half of the system includes the locator sub, orientation sub and whipstock assembly.
The basic objectives of the system are to provide rotational and azimuth control during a sidetrack, and, at the same time, offer a normally unobstructed casing ID. This unobstructed ID can be re-entered, precisely regaining the previous orientation.
The use of a composite casing joint further enhances the system benefits by reducing milling loads when exiting the casing. This allows for the use of conventional downhole motors and PDC bits to make the sidetrack.
Drilling of the lateral can then continue without pulling out of the hole once the casing has been exited. Since the system creates minimal loading during setting and drilling operations, coiled tubing can also be used in many cases to further reduce the overall job cost.
Orientation subThe orientation sub is run as a part of the well casing string, and has an ID equivalent to that of standard casing. On the lower end of the sub ID an upward facing biased edge is used to force the locator sub to line up with the locator slot. This slot provides the necessary specific depth location and azimuth orientation for the initial sidetrack, and any subsequent reentry.
The composite casing joint is a hybrid carbon/fiberglass filament wound joint, with wound-in metal end connections and permeation barriers on the ID and OD of the tube body. This configuration provides strength properties compatible with most well operations.
It is, however, possible to run the profile sub with a steel exit joint, and accomplish the sidetrack with conventional milling tools. The choice to use the multiple optional lateral entry (MOLE) system is dictated by cost and well condition constraints, which determine the casing material to be used.
Locator sub/whipstockThe locator sub, orientation sub, wedge and whipstock make up the Retrievable Tool Assembly (RTA). This complete assembly can be run on conventional drill pipe or coiled tubing. Prior to running the RTA, the orientation sub locator slot azimuth is determined with conventional survey tools or a gyro system.
When the RTA is run in the hole, the locator dog is in the closed position. Once bottom, or other appropriate restriction, has been reached, the dog is activated with a bottom trip system to the extended position. The RTA is then pulled back up into the orientation locator slot.
Proper orientation is assured by the upward facing biased edge on the profile sub. At this point, the RTA can only be moved back downhole if it is first pulled up out of the profile sub, and rotated out of the slot alignment position.
Once the RTA has been positioned in the locator slot, overcoming the wedge shear pins activates the wedge below the whipstock. This forces the whipstock to the side of the casing opposite the whip face. The casing exit can then be made with the appropriate mills, or PDC bit if composite casing has been run.
When the initial sidetrack has been completed, the RTA can be retrieved with upward tension, leaving the wellbore again unrestricted for other completion activity.
After all the desired sidetracks have been completed, the well can be re-entered with deflector tools to facilitate completion of the lateral.
Operational benefitsThe main operational benefits are as follows:
- Main wellbore is unobstructed when sidetrack activity not underway.
- Sidetrack tools are retrievable and reusable.
- Depth location and azimuth orientation can be reestablished.
- System can be run and operated with conventional drill pipe or coiled tubing.
- Multiple orientation exit subs can be run in a well.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.