Barium sulphate scale formation can occur readily in the wellbores of the Forties field when produced water containing a high barium ion concentration mixes with injection sea water of a high sulphate ion concentration. Typically scale formation begins with the onset of sea water breakthrough into a wellbore and can lead to very rapid production declines.

To date, seven coiled tubing scale millouts have been performed on producing wells in the Forties field. These operations have resulted in a combined 39,000 bbl/d (6201 m3/d) increase in production.

This paper describes the mechanism of barium sulphate scale formation and the mechanical removal of the scale using coiled tubing with downhole motors, bits and underreamers. A scale millout case history is presented and the successful prevention of scale formation via scale inhibitor treatments is discussed.

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