WAG Injection at the Statfjord Field, A Success Story

Abstract

Abstract Water-alternating-gas (WAG) injection technology is extensively used to improve oil recovery by increasing the macroscopic and microscopic sweep efficiency. WAG injection was implemented as a supplement to water injection at the Statoil operated Statfjord Field in February 1997. The field was then well into its production decline with a recovery factor of 54% and 70% water cut. This paper outlines details of the project from its conception with a pilot area, to implementation and field performance after 5 years of WAG injection. The WAG injection project in the Brent reservoirs includes 15 injection wells, and the WAG potential accounts for a large portion of the remaining oil reserves. Introduction The Statfjord Field was discovered in the North Sea in 1973, and it is the largest oil discovery to date in Europe. The original oil in place (STOOIP) is estimated to approximately 1 billion Sm3 and the expected recovery factor is 65%. The field is developed by three fully integrated Condeep concrete platforms. Figure 1 shows all three platforms and their tie-ins. Statfjord A came on stream in 1979, Statfjord B in 1982, and Statfjord C in 1985. Gas sales to the United Kingdom and to the European Continent commenced in October 1985(1,2). At plateau, production of 110.000 Sm3/day of oil was a normal daily rate. Today oil production is on a rapid decline and the current oil rate is approximately 29.000 Sm3/day.The challenge now is to identify and develop the remaining oil. In an attempt to chase these volumes, WAG injection started in the Brent reservoirs with the B-05 well pilot in February 1997, and following successful results was expanded to cover the entire field area. Large oil volumes were identified specially at the roof and attic areas. This paper describes a successful story of WAG injection, focussing mainly on well and field behaviour, the WAG Surveillance program, and working methods. Reservoir Description The Statfjord field is 25 km long and 4 km in width. The Field is located in the Tampen Spur area, in the northern part of the Viking Graben on the border between the Norwegian and the UK sectors. Most of the original reserves are found in a half-graben fault block structure (also termed Main Field), which is tilted at about 7° to the west. Due to instability across the graben-bounding fault, a multiple slide system developed in the eastern part of the field. This slump area is called the East Flank(1,2). The East Flank is a very complex, highly faulted area. In addition to challenging structural conditions, the sediments have been reworked through time due to repeated sliding/slumping. Typically Brent members can therefore often not be recognized in the East Flank. However, the pressure communication is generally good, with pressure measurements indicating good communication between the East Flank and the Main Field. Figure 2shows a cross section of the Statfjord Field.