Multilateral completions

Multilateral completion systems allow the drilling and completion of multiple wells within a single wellbore. In addition to the main wellbore, there are one or more lateral wells extending from the main wellbore. This allows for alternative well-construction strategies for vertical, inclined, horizontal, and extended-reach wells. Multilaterals can be constructed in both new and existing oil and gas wells. A typical installation includes two laterals; the number of laterals would be determined by:

• The number of targets
• Depths/pressures
• Risk analysis
• Well-construction parameters.

Advantages of multilateral completion systems

Multilateral systems combine the advantages of horizontal-drilling techniques with the ability to achieve multiple target zones. The advantages of horizontal drilling include:

• Higher production indices
• The possibility of draining relatively thin formation layers
• Decreased water and gas coning
• Increased exposure to natural fracture systems in the formation
• Better sweep efficiencies.

Depending on the type of multilateral design used, the target zones can be isolated and produced independently—or produced simultaneously, if commingled production is allowed or if a parallel string completion is used.

Multilateral systems categories

The various degrees of multilateral systems have been categorized by the Technology Advancement of MultiLaterals (TAML), a group of operators and suppliers with experience in developing multilateral technology. The TAML system for multilateral-well classification is based on the amount and type (or absence) of support provided at the lateral junction. There are six industry levels defined by TAML. This categorization system makes it easier for operators to recognize and compare the functionality and risk-to-reward evaluations of one multilateral completion design to another. As the TAML level increases, so does the complexity and cost of the system.[1]

TAML level 1

The most fundamental multilateral system consists of an openhole main bore with multiple drainage legs (or laterals) exiting from it (Fig 1). The junction in this design is left with no mechanical support or hydraulic isolation. The integrity of the junction is dependent on natural borehole stability, but it is possible to land a slotted liner in the lateral or the main bore to help keep the hole open during production. The production from a Level 1 system must be commingled, and zonal isolation or selective control of production is not possible. Re-entry into either the main bore or the lateral may be difficult or impossible should well intervention be required in the future.

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  Fig. 1—TAML Level 1: openhole main bore and lateral; unsupported slotted liner or screen may be set in lateral or main bore.

TAML level 2

This system is similar to Level 1, with the exception that the laterals are drilled off of a cased and cemented main bore (Fig 2). The cased main bore minimizes the chances of borehole collapse and provides a means of hydraulic isolation between zones. As with Level 1, there is no actual mechanical support of the lateral junction, but it is possible to run a slotted liner into the lateral to maintain borehole stability.

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  Fig. 2—TAML Level 2: cased and cemented main bore with an openhole lateral; an unsupported slotted liner or screen may be installed in the lateral.

TAML level 3

The Level 3 system also uses a cased and cemented main bore with an openhole lateral (Fig 3). However, in this design, a slotted liner or screen is set in the lateral and anchored back into the main bore. This system offers mechanical support of the lateral junction, but the advantage of hydraulic isolation is lost, and the zones must be commingled to be produced. The production from the zone below the junction must flow through the whipstock assembly and past the slotted liner to reach the main bore. This system provides easy access into the lateral for coiled-tubing assemblies, but re-entry into the main bore below the junction is not possible.

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  Fig. 3—TAML Level 3: cased and cemented main bore with an openhole lateral; slotted liner or screen is set in the lateral and anchored in the main bore, offering mechanical support of the junction.

TAML level 4

This system offers both a cased and a cemented main bore and lateral (Fig 4). This gives the lateral excellent mechanical support, but the cement itself does not offer pressure integrity at the junction. While the cement does protect the junction from sand infiltration and potential collapse, it is not capable of withstanding more than a few hundred psi of differential. There is a potential for failure if the junction is subjected to a pressure drawdown, as might be experienced in an electrical submersible pump (ESP) application. Zonal isolation and selectivity is possible by installing packers above and below the junction in the main bore. Systems are available that also offer coiled-tubing intervention, both into the lateral and into the main bore below the junction.

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  Fig. 4—TAML Level 4: cased and cemented main bore and lateral; cemented lateral provides mechanical support, but the cement does not provide pressure integrity for the junction.

TAML level 5

The Level 5 multilateral is similar in construction to the Level 4 in that it has both a cased and a cemented main bore and lateral, which offers the same level of mechanical integrity (Fig 5). The difference is that pressure integrity has now been achieved by using tubing strings and packers to isolate the junction. Single-string packers are placed in both the main bore and lateral below the junction and connected by tubing strings to a dual-string isolation packer located above the junction in the main bore. This system offers full access to both the main bore and the lateral. The zones can be produced independent of one another, or the completion can be designed to allow them to be commingled.

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  Fig. 5—TAML Level 5: cased and cemented main bore and lateral; pressure integrity across the junction is achieved through use of tubing and isolation packers.

TAML level 6

In the Level 6 multilateral system, both mechanical and pressure integrity are achieved by using the casing to seal the junction (Fig 6). Cementing the junction, as was done in the Level 4 system, is not acceptable. The Level 6 system uses a premanufactured junction. In one type of system, the junction is reformed downhole. In another, two separate wells are drilled out of a single main bore, and the premanufactured junction is assembled downhole.

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  Fig. 6—TAML Level 6: mechanical and pressure integrity at the junction is achieved by using casing to seal the junction.

References

1. Jump up↑ Hogg, C. 1997. Comparison of Multilateral Completion Scenarios and Their Application. Presented at the Offshore Europe, Aberdeen, United Kingdom, 9-12 September. SPE-38493-MS. http://dx.doi.org/10.2118/38493-MS.

External links

Technology Advancement of MultiLaterals (TAML)

General references

Allen, T. and Roberts, A.P. 1993. Production Operations, fourth edition, I and II.

Factors and Conditions Which Cause Seal Assemblies Used in Downhole Enviornments to Get Stuck. Baker Oil Tools—Engineering Tech Data Paper No. CS007.

Patton, L.D. and Abbott, W.A. 1985. Well Completions and Workovers: The Systems Approach, second edition, 57–67. Dallas: Energy Publications.