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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