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Multilateral completion systems allow the drilling and completion of multiple lateral boreholes within a single mainbore. 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
- Risk analysis
- Well-construction parameters.
Advantages of multilateral completion systems
General benefit provided by multilateral well design is barrel cost reduction. Multilaterals help to both reduce CAPEX and OPEX of the project and increase production.
In situations where well cost is high, such as on deep water, offshore or Arctic projects, multilateral well design will provide significant economic efficiencies. On the other hand, for complex petroleum plays with highly heterogeneous or low-permeable reservoir and in oil rims, improved Productivity Indices will help multilateral wells to produce more oil and, potentially, reach higher recovery factors, than conventional well would do.
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.
Installation of intelligent completion into multilateral well gives ability to monitor and control each leg separately which gives benefits of having several wells at cost of one.
TAML level 1
Basic multilateral junction consists of an openhole main bore with one or 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.
TAML level 2
Level 2 junction consists of cased and cemented main bore with uncased lateral (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.
TAML level 3
The Level 3 junction uses a cased and cemented main bore with an openhole lateral (Fig 3). This system offers mechanical support of the lateral junction, and provides access into both laterals.
TAML level 4
Level 4 junction offers both a cased and a cemented main bore and lateral (Fig 4). This gives the lateral mechanical support, but the cement itself does not offer pressure integrity at the junction. There is a potential for failure if the junction is subjected to a pressure drawdown. Zonal isolation and selectivity is possible by installing packers above and below the junction in the main bore.
TAML level 5
The Level 5 multilateral junction has both cased main bore and lateral, which offers great level of mechanical integrity (Fig 5). Pressure integrity has been achieved by using completion to isolate the junction. This junction type 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.
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. TAML6 junction as of today are not considered to be viable because of significant loss of ID and complexity of installation. Contemporary TAML5 systems offer higher pressure ratings and IDs which almost made TAML 6 junctions expired technology.
- 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.
Noteworthy papers in OnePetro
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.
Ruzhnikov, A., Latypov, A., Dubovik, A., & Zvyagin, V. (2016, October 24). TAML-5 Intelligent ERD Offshore Well: A Case Story of Successful Application in the North Caspian. Society of Petroleum Engineers. doi:10.2118/181927-MS
Eliseev, D., Golenkin, M., Senkov, A., Latypov, A., Bulygin, I., Ruzhnikov, A., … Kashlev, A. (2016, October 24). New Vision: IC TAML5 Wells on Caspian Offshore. Reasons, Implementation and Results. Society of Petroleum Engineers. doi:10.2118/181901-MS