The Troll field is a giant gas field but below the gas cap 4 billion barrels of oil are located in an oil layer varying from 4–27m and covering an area of 700km2. The size and complexity of the field together with simultaneous oil and gas production from different parts of the field make development planning and reservoir management a challenge. The oil strategy has been to start developing the most productive and thickest oil layers first, and having gained experience, to subsequently develop cost-effective technology to exploit the thinner and the less productive sands. Drainage of the oil is currently taking place through 106 wells with 149 horizontal branches with a total length of more than 350km. Today oil layers as thin as 8m are economically exploited. Wells located in low-productive sands will be on stream in 2005. Small differences in reservoir pressure between regions have caused movement of the oil column. The challenges to oil production from fluid contact movements and reduced reservoir pressure will be magnified when large-scale gas production start from the gas cap above the oil rim in Troll West. Technology to counteract the negative production effects and enhance oil production into the 2020's is being developed and introduced in the production system. All efforts have so far resulted in over 1.4 billion barrels of expected oil reserves, an increase from 350 million barrels when the first decision to develop the oil reserves was taken in 1991.
The Troll oil and gas field is located offshore Norway and covers an area of 700 km2. It contains 4-27m oil layer between a large gas cap and an aquifer. The field consists of three easterly tilted fault blocks, Troll West Oil Province, Troll West Gas Province and Troll East, Fig. 1. Troll East is currently the main area for gas production, while Troll West is the oil production area1. Gas production from Troll East started in October 1996 from the Troll A platform. Large-scale gas production from Troll West has not yet been approved. Oil is produced by two platforms, Troll B and Troll C, Fig. 2, which came in production in September 1995 and November 1999, respectively. Troll B has processing capacity of 35 000 Sm3/d of oil, 40 000 Sm3/d water and 8.0 million Sm3 of gas, while Troll C has a processing capacity of 40 000 Sm3/d of oil, 40 000 Sm3/d water and 10.3 million Sm3/d of gas. The two platforms have no drilling facilities. Development of Troll West Oil Province (TWOP) was approved in late 1991 after two long-term well tests had proved the production efficiency of horizontal wells in the Troll environment. The area has an initial oil column of 22-27m with an original-oil-in-place (OOIP) of 155 million Sm3 and is drained to Troll B by 20 satellite wells and one gas injector. The wells are divided into four subsea groups with a flexibility to tie up to 26 wells.