A Class G oil well cement conforming to API Standard Specifications was reported to give considerable variation, between batches, in the thickening time of a cement slurry formulation designed for well application. A fundamental study was undertaken on cement obtained from 3 separate batches in an attempt to determine the cause of the variability. The cement was characterized in terms of the chemical and physical properties and found to be within API Specifications: thickening times at 55°C, and 5200 psi give consistent thickening times of around 99-107 minutes and was well within the 90 -120 minutes limit. When used in a slurry formulated for a well having a bottom hole circulation temperature of 67°C, the 3 batches of cement give significantly different thickening times ranging from 1090 to 1400 minutes. A baseline was determined on the 3 batches of cements hydrated without additives, and properties of the hydration products determined using SEM, XRD, calorimetry, TGA/DTA and FTIR. The study was then extended to investigate the effect of the cement, first, in combination with the individual additives of the slurry formulation and second, with the complete additive composition. Additives of the slurry formulations included, lignosulfonate based retarder, sulfonated naphthalene formaldehyde condensate dispersant, hydroxyethylcellulose based fluid loss and polyether polyalcohol antifoam agent.
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