The Statfjord oil field, operated by Statoil in the North Sea, is far down its production decline curve. 60% of the STOOIP has been recovered during the past 23 years of production with secondary recovery techniques, leaving complex distributions of by-passed oil reserves, water and gas.
In order to maintain cost-effective production of the remaining reserves, an aggressive drilling and intervention programme is necessary. Future field development might also include a pressure blow down of the reservoir. So far the Statfjord Field has exhibited a fairly mild scale potential. Sulphate scale has been detected in several wells downhole; in the near well bore area, in the perforation tunnels or in the well bore. Well bore accumulation has been observed both as a thin layer along the tubing or as massive build up in the wire-line re-entry guide. Carbonate scale, when present, is mainly observed above the wireline retrievable downhole safety valve (DHSV). Statfjord wells are typically completed with wire-line retrievable downhole safety valves, which are function tested every 6 months. In wells prone to scaling this testing frequency is increased to 3-months.
In well C-36 scale build up had been confirmed in the 5 1/2" re-entry guide during production logging in June 1998. Following deterioration in well performance, a successful scale dissolver and inhibitor squeeze was performed early in 1999. Later in 2001 the DHSV failed, but this was corrected by an acid job. However, the safety valve failed again in June 2002 and proved this time to be immune to acid treatment and the well had to be shut in. The well condition was evaluated by running gauge rings into the well and this showed extensive scale build up over the DHSV. New and old technologies were evaluated for scale removal.
Statfjord has had good experience of scale removal with Coiled Tubing (CT) milling. This approach would probably have been successful in well C-36 but, due to long delivery time and the cost of deferred oil production, a wire-line solution was sought. An operation program was designed for mechanical removal of scale with wire-line (WL) broaches in combination with a stiff wire-line brush. Wire-line tractor (WLT) milling and string shots were included as back up options.
As the operation proceeded it became evident that mechanical broaching and backup WLT-milling were inefficient at removing the hard Ba/SrSO4 scale. However, use of explosives on string-shot runs was successful in removing the scale and getting the well back in production.
In order to evaluate the scale removal process a multi finger caliper (MFC) was run several times during the operation.
The MFC showed a scale layer with variable scale thickness up to a maximum of 1".
A successful scale-removing job was performed and the well came on stream with approximately 700–1000 Sm3/d of oil after the operation. A CT milling operation was avoided.
The Statfjord Field was discovered in 1973, declared commercial in August 1974, and started production in 1979. The field is over 25 km long and averages 4 km in width, and is the largest producing oil field in Europe. Statfjord is located in the Tampen Spur area, in the northern portion of the Viking Graben and straddles the border between the Norwegian and UK sectors. The field is developed by three fully integrated Condeep concrete platforms. All three platforms have tie-ins, as shown in Figure 1. Production is from the Brent, Dunlin and Statfjord reservoirs, with the main reserves in the Brent and Statfjord reservoirs. As of December 2002 the cumulative oil production is 618 million Sm3, giving a current recovery of ca 61% of the STOOIP. The expected recovery factor at abandonment is 65%.