Troll West, North Sea Northern, Norway

Although Troll is primarily a gas field, it also possesses significant quantities of oil, in thin zones under the gas cap, to the west of the field. West Troll can be divided into two major zones - the Troll West oil province and the Troll West gas province.

Regular oil production from the 22- to 26m-thick reservoirs, in the Troll West oil province, began in late 1995. Now, the extensive use of advanced drilling technology and the experience of production and well management have allowed the development of the Troll West gas province to begin - where zones are only 12 to 14m thick.

TROLL WEST LOCATION

The field is located in blocks 31/2 and 31/5, within production licence 85. It lies approximately 75km from the shore and 32km from the Troll A platform. The water depth ranges from 315-345m.

DISCOVERY

Oil and gas were discovered by well 31/2-1 in 1979. This was followed by the drilling of 20 appraisal wells between 1980-93, all of which encountered quantities of oil and gas. In 1990, the horizontal test well 31/2-T1 was drilled in the oil province and this was followed, in 1991, by the horizontal test well 31/5-T 1 in the gas province.

DEVELOPMENT STRATEGY

The Troll West reservoirs are being drained by two platforms, Troll B and Troll C. These are both semisubmersible production units. Since it came on-stream, Troll B has been used to produce oil from the oil province and the southern part of the gas province. When it came on-stream in late 1999, Troll C started to deplete the Northern part of the gas province.

TROLL B - OIL PROVINCE

Production commenced on Troll B in September 1995, using eight predrilled wells in the oil province. Current production is 40,000-42,500m³/day, from 21 wells, in the four oil province well groups known as D, E, F and G.

TROLL B - GAS PROVINCE

The platform has also brought wells from the south part of the gas province on-stream. Altogether, there are 24 subsea satellite oil producers tied back to the Troll B platform - 12 wells are in the H-cluster and 12 in well group 1.

FUTURE PLANS

The remaining oil reserves of the gas province will be developed by 58 horizontal wells. Plans envisage that 14 of these will be drilled with two branches. These 58 wells will be arranged in ten well groups, each normally containing two templates. Between two and four wells will be drilled from each template. Three of these well groups - J, K and L, which will contain a total of 18 wells - will also be tied back to Troll B and use the spare tie-in capacity on the platform.

TROLL C

The remaining seven well groups will be routed to a new Troll C platform, which is located in the Northern part of the gas province. There will be no drilling facilities on Troll C. In early 1998, a second rig started pre-drilling wells in the gas province. Production from this part of the field commenced in late 1999. The drilling programme is planned for completion by the year 2002, but it may be extended, in order to find out the potential for improved oil recovery.

Troll C has an oil-production capacity of 30,000m³/d, a water production capacity of 40,000m³/day, a liquid production capacity of 60,000m³/d and associated gas production capacity of 9 million m³/d. It has a total displacement of 52,750t and can accommodate 70 people.

TROLL PILOT

Large volumes of water will be produced with the oil from the thinner zones of Troll West. Separating this water from the oil before the wellstream reaches the platform expands the capacity for oil processing. A subsea separator called the Troll pilot was therefore fabricated in collaboration with ABB. This came on-stream in May 2000 and will be used continuously from September/October 2000 onwards.

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