Expanding the Options for Complex Multilateral Completions
Authors: Jabs Matthew 
Published: December 2007

Multilateral (ML) well constructions and completions have come to the forefront of industry interest over the last year. The interest has resulted from the continued drive in the industry to reduce costs and improved efficiency in well construction. Multilateral well constructions offer the ability to improve well cost/bbl efficiency and increase recovery percentage of a hydrocarbon field. Further benefits include slot preservation on offshore platforms and reduced environmental impact as a result of reducing the number of surface wells on land locations.

Multilateral completions options are expanding rapidly thanks to "junction creation" technology. Several different methods of ML junction construction have been developed by the industry. While each method has various advantages and limitations, the final goal is to produce the lateral wellbores with desired flow rates, plus wellbore control and re-entry capabilities equal to conventional monobore wells.

The need to have the same level of workover and control flexibility in lateral wells as conventional monobores has fueled development of new completion equipment. Current ML completion equipment allows varied level of well control and re-entry capabilities depending on the type of junction created.

Types of Junctions

Junction construction and isolation design fall into two basic categories: isolated and non-isolated junctions. Isolated junctions have both mechanical stability and hydraulic integrity while non-isolated junctions have only mechanical stability.

Factors that must be taken into account in determining junction and completion design are:–formation characteristics of lateral production zone,–formation characteristics of the zone in which the casing exit is located,–expected pressure differential at the junction, and–pressure differences between upper lateral zone and lower main bore production zone.

These factors - individually or collectively - along with the overall well objectives and operating parameters, can determine what type of junction is required. Each lateral well junction must be evaluated using these parameters to determine production of the well.

The different ML junction creation methods have various levels of complexity, with the cemented junction providing the most versatile platform for completion designs. The cemented junction is a mechanically stable multilateral junction. Specifically, the RootTM junction can be constructed in both new and re-entry wells and forms the basis for a number of multilateral completions, including hydraulically sealed completions. The cemented RootTM junction is formed by milling a window in the main wellbore casing, drilling the lateral, cementing a liner in the lateral, and washing over the liner overlap to leave full bore access at the junction. (Fig. 1)

The casing exit out of the main wellbore is formed utilizing an ML whipstock assembly anchored in a ML packer with orienting profile. (Fig. 1A-C) After the mainbore exit window is completed, the lateral well is drilled and completion liner placed. The lateral liner is cemented with the top positioned approximately 20ft above the exit window in the mainbore casing. (Fig. 1D-E) The ML whipstock assembly remains in place throughout this operation. The liner top is then pilot milled - or washed over, cut and pulled - to approximately 5 ft above the window. The remaining line lap and ML whipstock are removed by washing over. (Fig. 1F-G) The ML washover shoe engages the a control sub located below the ML whipstock and the entire ML Whipstock assembly is retrieved together with the cut liner stub trapped inside the washover shoe.

The result is a cemented RootTM junction with full lateral liner and mainbore ID. The cement around the junction and liner provides mechanical stability and the ML packer provides the datum point for the installation of multilateral completion systems that allow various completion scenarios and re-entry abilities.

This technique can be used in consecutive applications in the same wellbore if desired.

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