Multilateral Completion: Coming of Age
Though side-track drilling techniques have been with our industry for almost 60 years1 it is as recently as February 1996, that the elusive dream of being able to reliably re-enter the lateral bores while leaving the upper completion in place has become a reality. That the technology remains in a state of flux is illustrated by the new methods which are being developed with celeritous regularity.
The forum on "Technical Advancement - Multi-Laterals" (TAML) has proposed a well classification matrix that, when finally accepted, disseminated, and implemented will engender better communication between operators and suppliers regarding the multilateral well being planned. Using the TAML matrix, Texas A&M University (TAMU), Department of Petroleum Engineering has made an informal survey2 indicating that though there are hundreds if not thousands of Level 1 and 2 completions there are only a handful of Level 5 completions and only four Level 6 completions all of which were created using preformed downhole or wellhead splitters. TAMU is preparing an industry-wide database of case histories of level 3 to 6 multilaterals.
Most of the dynamic activity currently occurring encompasses the TAML Level 3 and 4 completions. The table below shows the TAML descriptions for levels and junctions. Level 3 and 4 junctions. The remainder of this discussion will concentrate primarily on the exciting Level 3 and 4 junctions.
Table 1: Junction Complexity Ranking (Excerpted from the Multi-Lateral Well Classification Matrix, by Eric Diggins, Shell UK E&P on behalf of the participants of the forum on Technical Advancement - Multi-Laterals (TAML).
| Level | Description | Illustration |
|---|---|---|
| 1 | Open/Unsupported Junction Barefoot mother-bore & lateral or slotted liner hung-off in either bore. |
 |
| 2 | Mother-Bore Cased & Cemented* Lateral Open Lateral either barefoot or with slotted liner hung-off in open hole |
 |
| 3 | Mother-Bore Cased & Cemented * Lateral Cased But Not Cemented Lateral liner 'anchored' to mother-bore with a liner 'hanger' but not cemented. |
 |
| 4 | Mother-Bore & Lateral Cased & Cemented* Both bores cemented at the junction. |
 |
| 5 | Pressure Integrity At the Junction (Cement is NOT acceptable) Achieved with the completion. |
 |
| 6 | Pressure Integrity at the Junction (Cement is NOT acceptable) Achieved with the casing. |
 |
| 6s | Downhole Splitter Large main well bore with 2 (smaller) lateral bores of equal size. |
 |
ABrief History of Multilateral Re-entry (MLR) (back to top)
Drilling multilateral wells was a logical extension of side-tracking techniques drillers had developed to salvage holes that would otherwise be abandoned. The difference, of course, was planning combined production from both bores rather than plugging and abandoning the motherbore because of problems. By February 1996, multilateral drilling was an established practice with over 50 economically successful multilaterals having been drilled world-wide. Then the Nederlandse Aardolie Maatschappij BV well RTD-14 was completed. This well was the first to allow through tubing, selective, lateral re-entry into a cased side-track so that reservoir management utilizing cost effective coiled tubing intervention techniques could be accomplished on both the mother bore and the lateral.3 This well was the necessary revolutionary step that cauterized the industry and opened the floodgates of thought to the evolutionary ideas that are currently flowing at an astonishingly fast pace.
The RTD-14 casing program (Figure 1) included a pre-milled, composite wrapped window with a moveable gate. The gate had a mechanical lock to secure a specially designed lateral liner hanger to the primary casing. A sleeve which locks into the same latch profile as the drilling whipstock provides lateral access as well as upper and lower seal bores and locking profiles This windowed sleeve straddles the casing window and contains an internal orienting nipple The orienting nipple accepts a deflection device so the lateral can be accessed. The windowed access sleeve achieves isolation for low differential pressures by being contained between two sets of seals.
Within months, the preferred MLR techniques dropped the mechanical moveable gate in favor of cementing the lateral liner in place. It was soon discovered that the locator/latch for the drilling whipstock, though quite reliable for its intended purpose, was found lacking when used to orient and anchor the completion. After all, it had already withstood the abuse of supporting the drilling of the side-track. A separate orienting nipple, more geared to the reliable placement and orientation of the MLR completion was designed and incorporated. Shown as part of Figure 2, this nipple is carefully oriented in relation to the drilling whipstock placement when the casing components (including the pre-milled window) are made up on the surface.4
After a few more iterations and hybrids the current technology shown in Figure 3 emerged. The casing equipment when planning a new well for today's state-of-the-art MLR should probably include (from bottom to top) at least the following:
- A lower casing nipple in which the bottom packer will set.
- The proprietary orientation nipple from the manufacturer of the MLR sleeve. This nipple is aligned to the proprietary whipstock latch when the casing string is made up. These two proprietary items are usually supplied by different companies (drilling and completion) and early communication between the two is essential.
- The window joint with a pre-milled window. The lateral will be drilled through this window.
- An upper casing nipple in which the top packer will set.
- A casing lock nipple.
The multilateral re-entry sleeve which is the completion that is run into the above casing equipment should include (from bottom to top) at least the components highlighted in Figure 4:
- A lower landing nipple to which any tailpipe or bottom hole assembly will be attached. This nipple often contains a retrievable plug when it is run so the packers can be set.
- The bottom packer. This retrievable packer has seals but no slips. It is spaced-out to land and pack-off into the lower casing nipple. Since it is not sealing in raw casing the construction is more economical, and more specialized than a conventional packer.
- The orientation locator. This critical proprietary item finds the orientation nipple and aligns the MLR access joint (window) into the correct rotational direction.
- A landing nipple used to isolate the lower packer or the window joint for testing.
- An orientation device that will accept a coil tubing run diverter (mini whipstock) so that reliable lateral access can be achieved.
- The access joint. This is the window that lines up with the window in the casing.
- A landing nipple used to isolate the window joint for testing.
- The top packer. This retrievable packer has seals but no slips. It is spaced-out to land and pack-off into the upper casing nipple. Since it is not sealing in raw casing the construction is more economical, and more specialized than a conventional packer.
- A casing lock. This lock sets into the casing nipple after the access joint is aligned and before the packers are set.
New MLR Horizons (back to top)
As it should be with any newly emerging technology, MLR is driven by economics and restrained by imagination. Today's environment includes increasing crude prices, shortages of drilling rigs and existing platform slots, and the renewed emphasis on reservoir management techniques to increase the productivity of the field and the well during its entire economic life. These facets, and others, come together to expand the operator's interest in the solutions afforded by MLR. It seems that suppliers and designers are getting asked to break new ground every week.
Figure 5 shows a well with a downhole injection pump suspended from a permanent coiled tubing string. The pump injects fluid from the lower aquifer to two zones via lateral branches. Either lateral branch can be accessed for maintenance by removing the ESP and running a diverter into the orientation nipple associated with that branch. The diverter is run on coiled tubing.
With the arrangement shown in Figure 6, flow control or logging tools can be run into the insert nipple using the diverter which lands in the MLR sleeve. When the MLR sleeve is removed during a workover, full diameter access to the laterals is possible by running the original drilling whipstock and retrieving the casing lock from the full bore nipple.
In some planned completions, the drilling rig will drill the main bore and the lateral and then move off location. Coil tubing can then be used to set the fabricated diverter shown in Figure 7. This diverter gives full access to the lateral if there is any cleanup or completion work required. The diverter is then retrieved and the MLR sleeve is run on coiled tubing. The sleeve can be retrieved and the
diverter re-run on coiled tubing if intervention becomes necessary later in the life of the well. The full access diverter (from Figure 7) run on coiled tubing has opened some interesting possibilities for coiled tubing drilling within the lateral. Later drilling can either extend the lateral or side-track from it as shown in Figure 8. Figure 9 shows an ambitious project currently being planned that allows intervention access to any number of laterals without pulling the electric submersible pump.
The Future of MLR Technology
The potential of multilateral technology is undisputed.5 As always, when the drilling specialists and the completion specialists approach any area where they must interface, there is a difference in perception of the key issues. The industry, as a whole, recognizes that an economically viable and reliable multi-lateral exit system will evolve to become standard practice if some issues are resolved. The disagreement revolves around the definition of those issues.
The Junction
Some believe that the great barrier to multilateral success is sealing the window junction. Designers labor to find a reliable mechanical seal whilst chemists seek new sealants.6 TAML seems to reinforce the view that better junction integrity is necessary by assigning its highest complexity rating to wells with "Pressure Integrity at the Junction (Cement is not applicable)." Others point out that the majority of multilaterals exit into the same reservoir and pressure differential at the junction is nil. Their highest priority is assigned to increased production, fit-for-purpose junction integrity, and the ability to manage lateral branches for the life-of-the-well.7
The Casing Window
There are numerous schools of thought in this area. Mill through standard casing referenced from an inexpensive casing nipple, many casing nipples can be incorporated into the casing design so that the operator need not choose where to side track until he is ready to do so. The MLR sleeve references off the same casing nipple.
Run a composite casing window. The drilling whipstock and the MLR sleeve are spaced from a nipple or nipples below the window. Steel is not milled but the casing string strength may be compromised.
Pre-milled windows in casing or coupling stock advocated by many provide tensile strength without necessitating the downhole milling of casing. The window may be run with a removable sleeve or may be wrapped with a readily drillable material. Again, whipstock and MLR sleeve deployment is to carefully placed casing nipples.
Though most are allowing the lateral stub to encroach into the mother bore, cementing the lateral in place and then washing over the stub to restore the full diameter to the mother bore there are still some companies exploring ways to tie the lateral liner back to the mother bore. In this area there are two goals being pursued, mechanical tieback and/or pressure integrity.
High Cost of Multilateral wells
Multilateral wells are commonly thought to be too expensive because of their high perceived cost. When numerous multilateral wells are drilled in a field, records indicate that, though the first multilateral well is relatively expensive, the cost per well quickly drops to approach the cost of a single horizontal well.8 High cost of multilateral wells is a myth.
Multi-purpose Multilateral (back to top)
Some companies are doing ambitious forward planning and slotting today's production multilaterals as tomorrow's injectors. One of the key lessons is that multilateral wells should never be planned in a vacuum. It is always in the best interest of the operator to plan the entire reservoir as a unit so that multilateral technology can be exploited most efficiently.
Working Together
Reservoir planning must be involved at an earlier stage along with drilling, production, and completion planning so that each group's capabilities are known to the others. It has been my observation that the reservoir group is often brought into the planning for multilateral wells too late in the process. By the time they understand the capabilities of the drilling team and the completion team opportunities have already passed. The supplier of the junction, the supplier of the completion, the directional driller, and all involved parties must work together early in the planning stages. It might be wise for operators to rethink their current competitive bid process. If they developed a committed relationship with suppliers on a field-by-field basis all could benefit from the comfort and efficiencies of working as a team.
Thinking Multilaterally
If it is not planned it won't happen. If it is not planned properly major benefits and efficiencies will be lost. When planning your production for the life of the field, it is no longer sufficient to be a lateral thinker. One must think multilaterally throughout the process. This allows you to examine the applicability of multi-lateral completions at a time when the greatest benefits can be achieved. Don't fall into the trap of wanting to drill a multilateral well just because everyone else is doing one. Each well must be economically justified, not as a stand-alone project, but, rather, as part of the whole reservoir exploitation plan.
Footnotes
1. US Patent 2105722, Barrett, Well Boring Apparatus.
2. Status Survey for Multilateral Wells, 1997 Forum on Multilateral Completions Technology, 15 July 1997.
3. Antczak, E., Smith, D., Roberts, D., Lowson, B., Norris, R.: "Implementation of an Advanced Multi-lateral System with Coiled Tubing Accessibility," paper SPE 37673 presented at the 1997 SPE/IADC Drilling Conference.
4. Brooks, R., Stratton, J.: "Development and Application of a Through Tubing Multi-Lateral Re-entry System," paper OTC 8538 presented at the Offshore Technology Conference, 5-8 May 1997.
5. Salas, J., Clifford, P., Jenkins, D.: "Multilateral Well Performance Prediction," article in Journal of Petroleum Technology, September 1996, pp. 838-839 (SPE 37374).
6. Xenakis, H., Vijn, P., Covington, R.: "A New Sealant for Multilateral Junction Geometries," paper SPE38495 presented at Offshore European Conference, 9-12 September 1997.
7. Hills, C.,: "Future Considerations for Multilateral Completions," article in Hart's Petroleum Engineer International, August 1997, pp57-60.
8. Brooks, R., Stratton, J.: "Development and Application of a Through Tubing Multi-Lateral Re-entry System," paper OTC 8538 presented at the Offshore Technology Conference, 5-8 May 1997.
About the Author
Thomas A. Turcich, is world-renowned as a design, development, and product engineer. During the past 25 years, he worked for Baker-Hughes, Camco, Weatherford International, Bowen Tools, and Pressure Control Engineering (Tuboscope. He died unexpectedly March 25, 1999. (back to top)
The basis of this paper was presented at the Fourth European Coil Tubing Roundtable held in Aberdeen, Scotland, November 19-20, 1997.