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Multilateral wells are new evolution of horizontal wells in which several wellbore branches radiate from the main borehole.
Multilateral well categories
In 1997, Technology Advancement for Multi-Laterals (TAML), an industry consortium of operators and service companies, was formed to categorize multilateral wells by their complexity and functionality. It is important to understand, that TAML Codes refer to junction, but not to whole well design. Junction designated categories (levels) are as follows.
- Level 1— Open / Unsupported Junction
- Level 2— Main Bore Cased & cemented, Lateral Open
- Level 3— Main Bore Cased & Cemented, Lateral Cased, but not Cemented
- Level 4— Main Bore & Lateral Cased & Cemented
- Level 5— Pressure Integrity at the Junction; Achieved by Completion Equipment
- Level 6— Pressure Integrity at the Junction; Achieved by Casing
Advanced multilateral wells application
There are projects in the world where geological complexity demands high end technical solution to be applied to maximize recovery and project value. Classic case for TAML 5 multilateral application is gas fields with thin oil rim. In such a setting gas breakthrough happens sooner or later and operator's aim is to postpone gas production as long as reasonably possible. This means that drawdown should be minimized to reduce gas coning effect. To enable reasonable production levels one must place multiple wellbores to maximize heterogeneous reservoir coverage. Each wellbore must be controlled independently with zonal inflow control valves and isolation packers. Since separation of production flow is a requirement, multilateral junction also has to provide hydraulic integrity (TAML 5).
Such setting is very common on Norwegian Continental Shelf , where hundreds of 2- and 3- lateral wells were drilled over the last two decades. There are projects in Russia with same challenges and solutions.
Technology is matured over last two decades and now more and more operators are willing to utilize multilateral wells.
Drilling multilaterals underbalanced
The setting of the production packer with a mechanical plug allows the lower leg in a multilateral well to be isolated and remain underbalanced while the second leg is drilled. After running the liner in the second leg, the completion can be run, and a second packer can be installed and stabbed into the lower packer. If leg isolation is required, a flow sleeve can be installed at the junction to allow selected stimulation or production as required. Re-entry into both legs is possible by use of a selective system. However, more detail as to the exact requirements from a multilateral system must be reviewed.
Drawbacks of UBD multilateral wells
Drilling a multilateral well underbalanced with the main bore producing can be done, but the drawdown on the reservoir is small. A further setback is that the cleaning up of the lateral is difficult if the main bore is a good producer. Getting sufficient flow through the lateral to lift fluids can be a challenge.
Best practice would be to temporarily isolate mainbore to avoid above mentioned risks.
- Smith, R.C., Hayes, L.A., and Wilkin, J.F. 1994. The Lateral Tie-Back System: The Ability to Drill and Case Multiple Laterals. Presented at the SPE/IADC Drilling Conference, Dallas, Texas, 15–18 February. SPE-27436-MS. http://dx.doi.org/10.2118/27436-MS.
- Bosworth, S. et. al. 1998. Key Issues in Multilateral Technology. Oil Field Review 10 (4): 14-28.
- Pasicznyk, A. 2001. Evolution Toward Simpler, Less Risky Multilateral Wells. Presented at the SPE/IADC Drilling Conference, Amsterdam, Netherlands, 27 February-1 March. SPE-67825-MS. http://dx.doi.org/10.2118/67825-MS.
Noteworthy papers in OnePetro
Golenkin, M. Y., Latypov, A. S., Shestov, S. A., Bulygin, I. A., & Khakmedov, A. M. (2017, November 1). First Intelligent Multilateral TAML5 Wells on Filanovskogo Field. Society of Petroleum Engineers. doi:10.2118/189006-MS