Troll Oil Development Concept

ABSTRACT Underneath the large gas accumulation of the Troll field lies a considerable volume of oil. However, its development requires an innovative approach because it is located in thin layers sandwiched between a large gas cap and an active aquifer. During the past 4 years Norsk Hydro has carried out extensive technical studies and an ambitious seismic acquisition and appraisal well programme to identify an economic development concept for the oil resource. This has resulted in the submission of a development plan for an extensive subsea development based on horizontally completed wells which will be manifolded on the seabed and tied back to a concrete semi-submersible production platform. INTRODUCTION The Troll field is located on the Norwegian Continental Shelf in 315-340m of water 100 km NW of Bergen as shown in Figure 1. It is covered by production Iicences PL054 and PL085 which are unitized. The owners of the field and their individual percentages are shown in Table 1. In December 1991 the Troll Licensees submitted a Plan For Development and Operation (PDO) for the oil resources of the Troll Field. The plan marked a successful milestone in the extensive development study and appraisal well activities carried out by the operator, Norsk Hydro since 1988. The proposed development concept consisted of 17 horizontal subsea wells tied back in groups to a concrete floating production platform. A schematic of the PDO concept is shown in Figure 2. The PDO was approved by the Norwegian Parliament in May 1992 and during this year and the first quarter of 1993 the operator built up a multi-discipline project team and completed basic engineering and part of the detailed engineering phase. Contracts were awarded for topside facilities design, the subsea production facilities and the production unit living quarters. Detail design of the concrete substructure is in progress and a drilling rig will be contracted medio 1993 to carry out the 3–4 year development well drilling programme. This paper describes the work done to select a development concept for the North Sea's most ambitious subsea project to date, and the changes that have been made to the original plan as a result of subsequent, more detailed work. TROLL FIELD APPRAISAL HISTORY The Troll Field was discovered in 1979 by exploration well 31/2-1. This well was drilled on the crest of the large gas structure of Troll West. By the end of 1984 a total of 15 wells had been drilled on Troll West confirming it to be a significant discovery. In addition to the large quantities of gas the appraisal programme also confirmed the presence of significant quantities of oil below the gas. However, the oil is contained in thin layers which are distributed over a wide area. Development of the oil layers was considered complex and uncertain commercially, therefore development planning activities concentrated on the large gas resource.