Troll, the largest gas field on the Norwegian shelf, is divided into two main structures; Troll Øst and Troll Vest. A thin oil zone runs through both of these structures. The oil zone in Troll Øst was too thin for it to be technically feasible to produce the oil before production of gas commenced in 1996.
In Troll Vest, which consists of a gas province and a pure oil province, the zone is thicker. In the gas province the oil zone varies from 11-14 meters. In the part of Troll Vest which mainly contains oil, the zone varies from 22-27 meters.
The oil in the oil province is produced on the Troll B facility. The oil in the gas province is produced on the Troll B and Troll C facilities.
So far, production of the gas in Troll Vest has not yet started (with the exception of the gas that automatically follows the oil). The timing for dedicated gas production from Troll Vest will be evaluated by the Troll licensees this autumn.
- "Today Troll is the largest gas field in Western Europe, measured in remaining recoverable reserves. But the field also contains significant amounts of oil" says NPD director Tormod Slåtsveen.
With the current approved plans, Troll's total recoverable oil reserves amount to 230 million Sm3. Of this total, 137 million Sm3 were produced as of January 2004. The remaining approved reserved are equivalent in size to a field such as Norne.
In relation to the total oil resources in place in Troll Vest, the production of 230 million Sm3 oil is equivalent to a recovery rate of 35.7 per cent. In other words, the potential for enhancing oil recovery is very considerable.
The authorities have a goal of achieving an average recovery rate of 50 per cent for all oil found on the Norwegian shelf. The Norwegian Petroleum Directorate has great expectations that Troll Vest will contribute its share towards reaching that goal. If so, the recoverable reserves will be increased to about 300 million Sm3 oil.
As the NPD-director responsible for development and resource management in the Troll area, Slåtsveen is thus eager to see what decisions the Troll field licensees will make as regards gas development of Troll Vest.
- "Extraction of gas reduces the possibility of going back later on to produce oil. If we are to produce as much oil as possible, we therefore have to wait before commencing significant gas production. As soon as we start to produce gas, reservoir pressure will fall and the balance between the gas cap, the oil zone and the water which is at the bottom will be disturbed."
A new project team in the Norwegian Petroleum Directorate has the task of defining and making recommendations on the challenges associated with gas production from Troll Vest. The list of challenges already contains a number of items beyond the direct consequence that increased gas production will have on future oil production, according to principal engineer Svein Olaf Nesse:
- A decision to phase in gas from Troll Vest is not just a question of higher revenues for the licensees, it also has socio-economic importance, and an impact on large parts of Norwegian gas export.
- Troll currently has great flexibility as regards delivery level, and the field plays an important role as a swing producer in the European gas market. For example, the gas lifted in December of last year was three times greater than in June, and while the gas lift is lower than the installed capacity, market fluctuations can be accommodated. How will that be affected by increased gas production from Troll Vest?
- What impact will a potential decision to develop more gas from Troll have on the activity level on the rest of the Norwegian shelf?
- Troll has guaranteed for development of smaller and more uncertain gas deposits. How will Troll's role in supporting other fields be affected over the shorter and longer terms?
- What is the correct gas lift level for Troll from the viewpoint of the society at large? High plateau production with shorter duration, or lower plateau production with longer duration? In addition to the consideration for the best possible production of the oil resources, this will in part depend on what the market is willing to pay for flexible and secure delivery of gas in the years to come.
- Do the interests of the licensees and the Norwegian society coincide?
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