The impact of electrolytes contained in sea water and 20 wt.% NaCl solution on three chemically different fluid loss additives was investigated. As fluid loss additives, a synthetic copolymer of 2-acrylamido-tertiary-butyl sulfonic acid (ATBS) and N, N-dimethyl acrylamide (NNDMA) and two graft copolymers, one based on humic acid-{ATBS-co-NNDMA-co-acrylic acid} and another based on lignite-{ATBS-co-NNDMA} were studied. Rheology and fluid loss of cement slurries prepared from fresh water, synthetic sea water and 20 % NaCl were measured at 27 °C and 150 °C. Significant differences were found between the admixtures with respect to their salt tolerance. Both graft copolymers are particularly tolerant to NaCl, but highly sensitive to sea water. Whereas the ATBS-co-NNDMA polymer performs well in sea water while it fails at higher NaCl contents.

A mechanistic investigation revealed that higher NaCl concentrations decrease adsorption of the ATBS-co-NNDMA copolymer on cement and thus reduce its plugging effect on the pores of the cement filter cake. Whereas, sensitivity of the two graft copolymers towards sea water is caused by the presence of Mg2+. In the highly alkaline pore solution of cement this cation is precipitated as voluminous Mg(OH)2 which can entrap and thus remove a significant amount of the graft copolymer. Such co-precipitation effect does not occur with ATBS-co-NNDMA. The pronounced effect of Mg2+ is extremely surprising, because of its low concentration in sea water (~ 1.3 g/L only).

The study suggests that in order to detect potential incompatibilities of oil well cement additives with salts, their behavior towards individual electrolytes instead of fully formulated salt brines should be tested. Such insight can prevent failures in the field and allows developing novel additives possessing enhanced salt tolerance.

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