Abstract

Norsk Hydro's Troll Olje field will require about 25 multilateral wells in order to drain the field using the existing sub-sea template structure.

Prior to the development of the Isolated Tie-Back System (ITBS™) described in this paper, three multilateral wells had been installed on the Troll Olje field using System 4503. On these TAML Level 4 wells, junction isolation was achieved with special cement and a resin squeeze. Junction related production problems were experienced on two of the wells with lumps of resin flowing back and plugging surface equipment. One well cleaned up rapidly, whereas the other is currently shut down. The MLT operations on these wells had been completed in 22 days, 17.9 days and 14.6 days, respectively. Although all wells were technically successful and the trend was good there would be a lower limit around 12 days due to all the milling operations required establishing the initial exit and the re-entry to the main bore. In order to further improve the reliability of the junction and the installation time, a new system would be required.

After an evaluation period, the operator (Norsk Hydro) accepted a proposal from Sperry-Sun/ Halliburton to further develop their existing prototype Isolated Tie-Back System for use on Troll Olje. ITBS™ will provide both hydraulic and mechanical isolation, a combined flow area that is larger than that of the tubing above, and an acceptable solution for access to main bore and lateral. The long term objective was to install this TAML Level 5 system in less than six days.

ITBS™ is a multilateral system based on a pre-milled window and on a latch coupling placed in the 9-5/8-in. liner section. After installation of screens in the main bore reservoir section, a whipstock is installed in the latch coupling and the window opened up to drill the lateral. To complete the well, the whipstock is retrieved and replaced by a deflector; and a multilateral junction system is then installed as an integral part of the lateral bore completion. The main component is a flexible hanger with D-shaped legs. This component is attached to the lateral liner on one leg and has a stinger on the other leg. The stinger is oriented and lands in a seal stack in the main bore deflector. On Troll Olje, screens are installed in both the main and lateral bore and the lateral liner is not cemented across the junction. An early prototype of the system was developed in 1998/99. The system was then further developed during 2000 and the first installation conducted for Norsk Hydro in the Troll Olje field in Norway. This paper will discuss the development of the ITBS™ system and also the first installation from a floating rig in the Troll Olje field.

Introduction

The Troll Olje Field is located approximately 100 km north west of Bergen, in the Norwegian Sector of the North Sea. The water depth is 315 – 340m. Troll Olje is part of the Troll gas field in which Norsk Hydro is responsible for development of two areas of a thin oil rim: the Troll Olje Oil Province (22 - 26m oil zone) and Troll Olje Gas Province (around 13m oil zone). The combined development is estimated to recover a total 1.33 billion barrels of oil.

By March 2001, a total of 70 wells had been drilled and completed on the Troll Olje Field, including five multilateral wells. The wells are tied-in to one of the two floating production platforms - Troll B and Troll C.

The multilateral well concept has been introduced on Troll Olje primarily to increase the total drainage area from the existing sub-sea template structures. Further development of Troll Olje includes another 20 multilateral wells. These multilateral wells contribute additional reserves in the region of 75 million barrels of oil. Drilling all these targets with conventional horizontal wells would not be practical. An alternative solution with four wellhead templates (16 slots) would contribute about 25 million barrels of oil less than the multilateral solution and also add greatly to the development costs.

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