Influence of electrolytes on the performance of a graft copolymer used as fluid loss additive in oil well cement

• Most cement additives are negatively affected by electrolytes.
• A graft copolymer was tested in cementing systems based on fresh water, 20% NaCl and sea water.
• Poor performance of polymer in sea water system noticed.
• Reason is entrapment of polymers in voluminous Mg(OH)2 precipitate.

The influence of electrolytes contained in sea water and 20 wt% NaCl solution on a high temperature fluid loss additive (FLA) for oil well cement was investigated. The FLA was comprised of a humic acid-{sodium 2-acrylamido-2-methylpropane sulfonate-co-N,N-dimethyl acrylamide-co-acrylic acid} graft copolymer which was tested at 27 °C and 150 °C. Its performance was compared with that of an industrial lignite-based copolymer. It was found that seawater, in spite of its relatively low electrolyte content (~3.6 wt%), affects fluid loss performance of the graft copolymer strongly whereas 20% NaCl dissolved in the mixing water exhibit a minor effect only. Mg2+ present in sea water was identified as cause for poor admixture performance. Experiments demonstrate that in the highly alkaline environment of cement, Mg2+ precipitates as voluminous Mg(OH)2 which entraps significant amounts of the graft copolymer. Delayed addition of the graft copolymer to the cement slurry presents a mitigation strategy to overcome the negative effect of Mg2+ present in sea water.

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