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Abstract Three oil producing fields in the Danish sector of the North Sea, Gorm, Dan and Halfdan, have been assessed for downhole scale control. All three fields have water injection. To date downhole scaling has not been perceived as a significant production problem in these fields. This paper presents the results of a review of the well service history for each well in the three fields which identified when and where scales had been encountered and estimated the impact of scale on well intervention operations. A thermodynamic equilibrium model for mineral solubilities was used to assess the scaling tendencies of sulphate and carbonate scales from bottom hole to wellhead. The observational data for downhole scale has been compared with the scaling predictions and a risk assessment for carbonate and sulphate scales developed for each well. The implications of this study on industry trends are discussed.
Introduction Downhole scaling of mineral deposits has been a feature of oil production since the beginning of the modern industry. Self-scaling of produced waters, primarily calcite has been a persistent problem, however, with the advent of large seawater injection projects in the North Sea and elsewhere the potential for downhole scale was greatly increased. For over 20 years in the North Sea the management of downhole scale has been a significant challenge. Whilst the cost of scale has been high, both in terms of deferred production and remedial treatment, downhole scale management has generally been successful, even in the most severe scaling environments. Table 1 shows the revenue from worldwide sales of scale control chemicals by region and application in 2002, the North Sea is clearly seen to be a major user of this technology, particularly for downhole scale control.
The initial challenge of downhole scale prevention in the North Sea saw major advances in the modelling of mineral solubilities with several software packages being available. This modelling capability, together with improved scale inhibitor chemistries and understanding of the ‘Squeeze’ (injection of scale inhibitor chemical into producing wells) process itself was sufficient in most cases for effective management of downhole scale. The development of fields with very high barium concentrations (>500 mg/l) such as Brae, Miller, T-Block, Njord and South Arne forced downhole scale management into a new direction; the removal of sulphate ions from the injected seawater by reverse osmosis (RO). Whilst RO units can be difficult to operate they have been effective in maintaining scale control in the most challenging of environments. The Miller field chose not to use RO and, by a combination of water management and squeeze inhibition, maintained production well beyond initial design; however it is doubtful if this approach would be taken in the future. RO technology is now being exported outside of the North Sea region; in 2002 there were 11 installed units worldwide, this number is expected to grow to 40 or 50 by 2010.
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