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 24 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.

Seawater breakthrough in the Etive formation of Statfjord Well C-28 was first recorded during 1999. Following deterioration in well performance, scale dissolver and inhibitor squeezes were performed in 2000 and 2002. Production from the well was increased after these jobs, but scale build up in the 7″ re-entry guide was confirmed during caliper logging in June 2003. A scale control program was initiated which included traditional scale inhibitor squeezes supported by scale dissolver treatments.

Considerable emphasis was initially placed on identifying environmentally acceptable scale inhibitors that would provide cost effective scale control under Statfjord conditions to meet the business needs for green products. Much less emphasis has been placed on the development of environmentally acceptable barium sulphate scale dissolvers. Traditional sulphate scale dissolver formulations are generally based on DTPA and/or EDTA chemistry which are relatively effective at chelating barium and other divalent cation including strontium and calcium but do not display acceptable environmental profiles. These chemistries suffer with poor biodegradation and are thus placed on the substitution list under HCMS guidelines.

This paper describes the development and laboratory evaluation of chelant chemistries of a scale dissolver with environmentally acceptable properties. It was compared in the field against traditional products when pumped into the same Statfjord well under "similar" conditions.

The paper will highlight the identification of a particular molecular structure that gave superior barium and strontium sulphate scale dissolution rates and capacity in the laboratory as compared to traditional EDTA and DTPA based dissolvers whilst fully meeting the environmental requirements.

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