A new multilateral system has enabled Norsk Hydro to increase production from a deepwater field.

Norsk Hydro has successfully used multilateral wells in the Troll Olje field (the oil rim of the giant Troll gas field) to produce more oil using the existing subsea template structure. An innovative multilateral system was developed to provide mechanical and hydraulic isolation at the junction as well as re-entry capabilities into the main and lateral wellbores. This system also is mechanically simple to run, requiring a minimal number of trips to establish the junction and achieve mechanical and hydraulic isolation. The implementation of multilateral technology has decreased drawdown pressures and increased sweep efficiencies within the reservoir, resulting in a dramatic increase in production.
Troll was the first deepwater development to use multilateral technology successfully, even though these installations were operationally complex. Through the application of lessons learned and subsequent improvements to the system, installation times were improved significantly. However, it was felt that more improvements were possible, especially for expensive, hostile operating areas. In addition, there were several system capability issues to resolve. Norsk Hydro and Sperry-Sun decided to jointly develop a new system that would incorporate the features and benefits necessary to develop the Troll field properly. The new system incorporates many of the design features from existing, proven systems as well as lessons learned during previous operations. The system was developed to:
• reduce installation time in comparison to the early Troll Olje installations;
• hydraulically isolate the window junction;
• avoid the use of resin treatments as in previous systems;
• mechanically tie back the lateral liner to the main casing string;
• optimize the flow area of each producing string; and
• minimize the number of steps and components to complete the junction.
The Isolated TieBack System (ITBS) was designed to use a standard retrievable multilateral system premilled window and drilling whipstock to facilitate casing exit. The permanently installed deflector is retained in place by the standard nipple profile, the Sperry-Sun latch system used in many multilateral systems. The deflector contains an internal sealing element to accept the polished stinger of the flexible hanger. The lateral leg of the flexible hanger is designed to bend over the junction, allowing the main bore stinger to enter the deflector and seal off, creating a mechanical and hydraulic seal between the lateral and main bore.
A vector block assembly above the flexible hanger ensures re-entry tools travel through the main bore leg of the flexible hanger. Lateral re-entry is accomplished by setting a tubing exit whipstock in the internal landing profile. This tool is designed to guide the bull nose of the service tool into the lateral. Re-entry tools can be run on either coiled tubing or wireline.
The initial system specifications were:
• 95/8-in., 47-lb/ft casing;
• 8 1/2-in. lateral wellbore drillout;
• 7-in. lateral liner;
• 1,000-psi pressure rating (burst/collapse);
• 4 3/4-in. bore equivalent flow area through each leg; and
• a maximum tool size for through-tubing intervention of 33/8-in.
As of October 2001, seven wells had been completed with this system at Troll. Junction construction times had been reduced from 22 to 3.6 days. In addition, these systems incorporated several benefits over previous systems, including enhanced junction stability and flow isolation at the junction. With the success of these installations, several more wells are planned for the Troll Olje area this year.
But the true impact of these wells has been more than just operational simplicity and success. These wells will contribute incremental production in the amount of 75 million bbl of oil. These are reserves that likely would have gone unproduced due to the fact that drilling these targets with conventional wells instead of multilateral wells would have been cost-prohibitive. At US $20/bbl, the additional recovery is worth $1.5 billion, a substantial contribution to the development's overall economic worth. Hydro is planning to continue the development of the oil rim in the Troll Olje field with 20 additional ITBS installations. This program demonstrates the large financial impact on this and other deepwater developments.
Many other places around the world have potential use for the ITBS system and other emerging multilateral technologies. Frontier areas such as other North Sea fields, West Africa, Brazil, the Caspian Sea, the east coast of Canada and the Gulf of Mexico can benefit from the application of this system. In fact, any area that has high rig rates, facilities costs or other capital costs should examine the impact the application of multilateral technology might have on their field development plans.
The multilateral service industry is advancing rapidly in terms of systems capabilities and operational efficiencies. These advanced systems have dramatically increased the capability to deliver a viable, commercial multilateral product that can have a large impact on field and reservoir development strategies. These increases are especially pertinent in the deepwater arena with its high finding costs and development capital requirements.

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