The field comprises the main Troll Øst and Troll Vest structures in blocks 31/2, 31/3, 31/5 and 31/6.
Containing about 40 per cent of total gas reserves on the Norwegian continental shelf (NCS), it represents the very cornerstone of Norway’s offshore gas production.
Troll is also one of the largest oil fields on the Norwegian continental shelf. In 2002 the oil production was more than 400,000 barrels per day.
Statoil operates the Troll A, B and C platforms and the landfall pipelines, while Gassco is operator for the gas processing plant at Kollsnes on behalf of Gassled. Statoil is technical service provider for Kollsnes operations.
The enormous gas reservoirs lying 1,400 metres below sea level are expected to produce for at least another 70 years.
Norske Shell was chosen as operator when block 31/2 was awarded in April 1979. A large gas find with an underlying oil zone was proven later that year. The block was declared commercial in 1983.
The neighbouring blocks were awarded to Statoil, Norsk Hydro and Saga Petroleum in 1983.
Block 31/2 contains 32 per cent of the Troll field’s reserves, while the remaining 68% lies in the three other blocks.
The licence terms for block 31/2 specified that Statoil could take over as operator for this acreage eight to 10 years after a discovery had been declared commercial.
In 1985, the two licences were unitised so that Troll could be developed as a single field.
Statoil took over as production operator for Troll Gas on 19 June 1996, while Hydro started production from Troll Oil in the fall of 1995.
Troll Gas comprises the Troll A platform, the Kollsnes gas processing plant west of Bergen and the pipelines between the platform and the land plant.
Norske Shell was responsible for the first gas development phase on Troll, which received a green light from the Norwegian parliament (Storting) in December 1986. Four years later, Statoil and Shell agreed to move the processing facilities originally due to be installed on the A platform to a plant on land at Kollsnes west of Bergen. This made it possible to build a simpler offshore gas production installation than originally planned, with smaller staffing.
Troll A is the tallest structure ever moved by humans over the surface of the Earth. Its concrete support section has been built for a producing life of 70 years. The platform is the only one of its kind on the NCS that is powered electrically from land.
Statoil took over as operator for Troll Gas from Norske Shell on 19 June 1996 with gas coming on stream in the first half of that year, marking the completion of the gas development. Contractual gas deliveries began flowing from the field to continental Europe under the Troll gas sales agreements on 1 October 1996. Initial deliveries under these contracts, which began on 1 October 1993, were provided from Statoil’s Sleipner Øst development in the North Sea.
Declining pressure in the Troll reservoir means that more compression is needed to help drive gas production through the pipelines to the processing plant at Kollsnes. Two compressors have accordingly been installed on the A platform, and new technology allows these units to be powered from land. That in turn means zero carbon dioxide and nitrogen oxide emissions from the installation or the processing plant.
Troll B, a floating process and accommodation platform with concrete hull, and Troll C, a floating process and accommodation platform with a steel hull, produce from thin oil-bearing layers in the Troll Vest reservoir.
The thin oil layer is between 22 and 26 meters in the Troll Vest oil province and 11 and 13 meters in the Troll Vest gas province. In order to recover oil from the thin layer, it has been necessary to develop advanced drilling and production technology.
All of the more than 110 production wells to be drilled in Troll Oil are horizontal wells. This entails drilling in two phases. First, down to the reservoir, which lies at 1,600 meters beneath the sea bottom, and then to 3,200 meters in a horizontal direction through the reservoir.
A total of 28 of the wells are called multi-lateral wells, which have two or three horizontal sections that radiate out from a conjunctive point in the reservoir.