Troll West Oil Field - A Giant Gas Field Becomes the Largest Oil Field on the Norwegian Continental Shelf

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 112616, "Troll West Oilfield Development-How a Giant Gas Field Became the Largest Oil Field in the NCS Through Innovative Field and Technology Development," by Richard Dyve Jones, StatoilHydro, and Erland Saeverhagen, SPE, Arve K. Thorsen, SPE, and Sveinung Gard, SPE, Inteq, prepared for the 2008 IADC/SPE Drilling Conference, Orlando, Florida, 4-6 March. The paper has not been peer reviewed. The Troll West oil field has been developed with more than 110 horizontal subsea wells including 53 multilateral (ML) wells. The thin oil rim was developed through extensive use of multilateral drilling and wells containing up to 7 horizontal branches. The process of drilling the ML wells and the benefit/risk evaluation for the ML process are discussed. Introduction The Troll field is offshore Norway on the Norwegian Continental Shelf (NCS) in 300-m water depth, as shown in Fig. 1. The field was discovered in 1979 and covers an area of 750 km2. Drilling of the production wells began in 1994, and production began September 1995. Oil production to September 2007 was 188.3×106 m3 of oil, all from horizontal wells. The 250 horizontal sections have drilled 900 000 m, including 560 000 m of reservoir section. Field Description and Challenges Gas and oil are found mainly in the Sognefjord formation and in the underlying Fensfjord formation. The oil in the Troll West is from a 22- to 26-m thick oil column under a small gas cap. In the Troll West gas province, there is a 12- to 14-m-thick oil column and a gas column up to 200 m thick. The Troll reservoir sands are of two distinct types, with large grading between. The C sand is a clean coarse sandstone, with permeability ranging from 1 to more than 10 darcies. The M sand is micaceous, with a finer grain size than the C sand, and, in general, has permeabilities in the range of 1 to 100 md. The sorting in this sand is poorer than in the C sand; therefore, the permeability is less. Recovery Strategy. Oil production is through long horizontal wells drilled just above the oil/water contact (OWC) in the thin oil zone. The main recovery strategy is pressure depletion, but with simultaneous expansion of the gas cap above and the water zone below the oil. In the Troll West oil province, some of the gas produced has been injected back into the reservoir to optimize the oil production. One important aspect is to recover the oil quickly, because less oil can be extracted when the pressure declines in Troll East. Therefore, limits were placed on gas extraction from Troll East.