In Central Graben, North Sea, there is generally no free water production, most of the produced water is condensed water; however wells have been treated against scale build up at perforation level by performing acid washes using mainly acetic acid. Wells reacted differently after acid treatment: Some wells showed a significant productivity improvement, others showed good results but limited in time, some wells presented no gain after acid treatment and some other wells are scaling on a more aggressive basis after acidification. This paper shows that these behaviours are linked with the lithology of the different reservoirs. A better understanding on the scale formation and mechanism is essential in order to optimize the well intervention planning and timing to treat the wells. This study showed that calcium carbonate and sodium chloride are the only scale deposits susceptible to precipitate at the bottomholes of wells. The precipitation of sodium chloride has been induced by the high salinity of water associated with a high temperature at bottomholes as well as an inversion of the Joule- Thomson coefficient. Calculations showed that evaporation is slightly higher for Elgin reservoirs comparing to Franklin reservoirs, this is due to Joule-Thomson effect slightly stronger and a higher temperature reservoir for Elgin reservoirs. These high temperatures at the bottomholes induced the formation of calcium carbonate precipitation, this conducted to the loss of productivity. From the subsurface safety valve (SCSSV) to the surface facilities, the reduction of temperature caused a reduction of solubility of zinc sulphide and lead sulphide. In terms of chronology, halite scale occurred first, followed by carbonate scale and finally formation of sulphide scale at lower temperature.

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