This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 95721, "Subsea Smart Multilateral Wells Increase Reserves at Gullfaks South Statfjord," by V. Haugen, SPE, A.-K. Fagerbakke, SPE, B. Samsonsen, and P.K. Krogh, SPE, Statoil ASA, prepared for the 2006 SPE/DOE Symposium on Improved Oil Recovery, Tulsa, 22-26 April.
Production from the Statfjord formation in Gullfaks South (GS) field was limited by low connectivity, reservoir heterogeneity, and gas breakthrough. Surface-operated downhole inflow control and multilateral wells were identified as solutions to these limitations. Implementing a downhole-instrumentation and -control system (DIACS) and multilateral technology (MLT) in three horizontal subsea wells led to a two-fold increase in the reserves estimate to 5.4×106 m3 oil in the reservoir management plan for 2005.
The GS field, operated by Statoil ASA, is in the Norwegian sector of the North Sea, approximately 150 km northwest of Bergen. The field is a subsea satellite tied back to the Gullfaks A and C plat-forms and consists of two independently developed reservoir units, the Brent and Statfjord formations. Production from the Statfjord formation, which began in 1999, is routed to Gullfaks A.
Fig. 1 shows the structurally complex Statfjord formation with many fault segments and indications of locally varying fluid contacts. The formation is approximately 300 m thick. Production experience, however, has shown less connectivity than expected. The initial reservoir pressure at 3300 m true vertical depth was 476 bar, and the reservoir temperature was 128°C.
In 1995, oil recovery from the GS Statfjord field was estimated to be approximately 40%. In 2001, after drilling the first three wells, further development of the field was postponed because of poor production performance, and the expected recovery factor was reduced considerably. In 2003, a project was established to improve the reservoir knowledge, identify cost-effective well solutions, and implement available technology suitable for further development of the reservoir. The project, with considerable support from the field partners, identified the need for more drainage points and robust solutions, particularly the use of MLT to account for geological complexity and reservoir uncertainty.
Increasing exposure to the reservoir by increasing the number of drainage points is essential to maximize oil production and enable access to reserves that otherwise might be left behind. This solution minimizes the localized drawdown to help maintain an even pressure depletion in the reservoir.