Physico-Chemical Interactions Perturbing the Effectiveness of an ATBS-Based Fluid Loss Polymer Used in Oil Well Cementing

Abstract Water-soluble acrylamide tert-butylsulfonic acid (ATBS)-based copolymers are commonly used to provide fluid loss control for oil well cement slurries. In our study, we investigated the behavior of a CaATBS-NNDMA copolymer synthesized by aqueous radical polymerization with respect to its fluid loss performance in presence of sulfate, chloride and Welan gum and at high temperature (80°C). First, we found that CaATBS-co-NNDMA adsorbs in high amounts on both cement and silica flour. Its effectiveness relies exclusively on high adsorption onto these mineral surfaces. FLA adsorption may, however, be perturbed by several physico-chemical effects. Inorganic anions present in high concentrations such as e.g. chloride, as well as organic anionic admixture molecules such as Welan gum, can negatively impact the effectiveness of CaATBS-co-NNDMA. They were found to compete with the polymer for adsorption sites on the surfaces of cement and silica. This way, they reduced the adsorbed amount of the ATBS copolymer. Their impact on the ATBS copolymer generally depends on their anionic charge density, the quality of their anchor group to the cement/silica surface and their concentration. Elevated temperature (80°C) causes a significant increase in the concentration of solved sulfate present in cement pore solution. As a result of the higher ionic strength, the ATBS copolymer changes its solved conformation from stretched to coiled. Coiling of the macromolecule, however, results in lower adsorption. Through this mechanism, higher temperature reduces effectiveness of the FLA.