Scale Type II (Sulphate Scale) Why: Sulphate scale will form where formation water, rich in barium, strontium
and calcium ions, mix with injected seawater, rich in sulphate ions. The
problem becomes worse at lower temperatures for barium sulphate and is less
severe for strontium sulphate.
The largest mass of barium sulphate scale usually forms within 0-10% SW
breakthrough. However, the highest
supersaturation tends to occur closer to
the 50:50 FW/SW mixture. Although less scale forms at the maximum
supersaturation, it is harder to prevent due to the larger supersaturation, i.e.
the larger thermodynamic driving force which is precipitating the scale.
Where: Sulphate scale usually forms a coating on production tubing, safety
valves and in the topside facilities – or, indeed, at any location after FW/SW
mixing occurs.
Sulphate scale formation within the formation is less significant in terms of
mass than in the production tubing but its impact on production rates can be
more severe. Sulphate scale has been recorded in perforation tunnels, in
formations where vertical flow is common and in reservoirs where crossflow
between layers at different pressure occurs.
Sulphate scale formation within gravel packed wells is due to the flow
characteristics of both fluids within the gravel pack and can be very severe and
may result in significant production impairment.
Sulphate scale within topside facilities will occur on the edges of metal
surfaces valves and gauges and in areas where different production streams
mix prior to discharge.
