Installing a new Level 5 multilateral junction at the Troll Olje field took only two trips after drilling the lateral.

is business development manager for Sperry-Sun Multilateral Systems in Houston, Texas.
Since their debut in the early 1990s, more than 500 multilateral installations classified as Technology Advancement for MultiLaterals (TAML). Level 3 and higher have been deemed operationally and economically successful. The earliest systems were perceived as too complex, time-consuming, expensive and risky. However, in the past 2 years, remarkable developments have occurred in Level 5 and Level 6 multilateral technologies.
The Isolated Tie-Back System (ITBS) developed by Sperry-Sun is a TAML Level 5 assembly that provides mechanical and hydraulic isolation at the junction without cement, which makes it especially suited to solve sand control problems at the junction. This system also allows re-entry into the main and lateral wellbores. Fast and easy to run, the ITBS requires only two trips after drilling the lateral to establish the junction.
A first for Troll
The first field installation for this system was at the Troll Olje field, about 62 miles (100 km) northwest of Bergen in the Norwegian North Sea. Norsk Hydro is developing two areas of a thin oil rim in waters 1,034 ft to 1,116 ft (315 m to 340 m) deep: the Troll Olje Oil Province, in a 72- to 85-ft (22- to 26-m) oil zone; and the Troll Olje Gas Province, in a 43-ft (13-m) oil zone. The goal is to produce oil from the thin oil rim before the gas reservoir blows down, then the gas above the rim can be produced. The combined development is expected to recover 1.33 billion bbl of oil.
At the Troll Olje field, 69 wells have been drilled, completed and tied back to one of the two floating production platforms, Troll B and Troll C. Four multilateral well completions have been installed at Troll Olje to increase the total drainage area from the existing subsea template structures. Another 20 multilateral wells are planned to increase oil reserves by 75 million bbl. Tapping these oil reserves using conventional wells would have required another six wellhead templates, which would have been prohibitively expensive.
Concept and design
Incorporating design features from proven systems and lessons learned from previous Troll operations, the ITBS was developed to:
• reduce installation time;
• provide hydraulic isolation of the window junction;
• avoid resin treatment;
• tie back the lateral liner to the main casing string;
• optimize the flow area of each producing string; and
• minimize the number of steps to complete the junction.
The ITBS incorporates a flexible hanger with two semicircular sections to maximize the cross-sectional flow area and eliminate sand production at the junction. The system features Sperry-Sun's premilled Retrievable MultiLateral System (RMLS) window joint with the standard orienting latch coupling, as well as a deflector with an internal sealing component.
The flexible hanger's lateral leg is designed to bend over the junction, allowing the main bore stinger to enter the deflector and pack off, creating a mechanical and hydraulic seal between the lateral and the main bore. A vector block assembly above the flexible hanger allows for nonrig intervention with coiled tubing or wireline.
The ITBS is designed for use in new subsea and platform wells and offers the primary benefit of not requiring cementing operations to achieve hydraulic integrity at the junction.
After the 103/4-in. by 95/8-in. liner was installed and cemented in place at Troll Olje's Y-22 well using the semisubmersible West Vanguard, the main bore was drilled into the reservoir and completed with prepacked screens. The multilateral installation process involved six steps:
• Drill the lateral. Install a drilling whipstock with a pinned mill and mill out the premilled window to initiate the lateral wellbore. Drill the lateral and remove the drilling whipstock in preparation for running the lateral liner and completion.
• Install the deflector. The system deflector assembly then is run into the lower latch assembly to automatically orient the deflector toward the lateral window. The external packer element is energized to seal the main bore from the rest of the well.
• Run the lateral screens. The lateral screen liner and system hanger assemblies are run into the hole. A bull nose below the screen deflects off the deflector assembly and into the lateral.
• Orient the liner running tool. The alignment key sub above the flexible hanger engages the key slot in the self-alignment crossover and orients the main bore leg to the deflector.
• Set the hanger assembly. The liner running tool strokes through the orienting latch assembly as the hanger sets into the deflector assembly and engages the deflector's internal sealing element. Then the liner hanger packer is set.
• Remove the liner running tool. The lateral junction is completed.
Note that there were only two trips into the hole once the lateral was drilled: once to set the deflector assembly and once to run that lateral screen and flexible hanger assembly.
Operational efficiencies
As of late March, two wells at Troll Olje had been completed with the ITBS. Junction construction times have been reduced by 50% to 75% compared to Halliburton's previous Level 4 multilateral system installations, and the new system has exhibited enhanced stability and flow isolation at the junction.
"Norsk Hydro considers the ITBS installation at 31/2-Y-22 to be a major step forward in our use of multilateral technology on Troll Olje - both with respect to the time savings and the hydraulic isolated junction," said Tore Grønås, Norsk Hydro project leader. Hydro is planning to continue the development of the oil rim in the Troll field with 10 installations from three rigs this year.
Acknowledgments
The author wishes to thank members of the Norsk-Hydo/Sperry-Sun multilateral team for their contributions, not only to this article, but also for their efforts in promoting the application of this rapidly emerging technology.