Correlation between Hydration Kinetics and Abnormal Thickening of Oilwell Cement under the Effects of Retarders

Summary To investigate the mechanism of abnormal thickening behavior of oilwell cement slurries containing hydroxycarboxylic acid and sugar-based retarders, the effects of citric acid, maltodextrin, and glucose on the early hydration of oilwell cement were studied using isothermal calorimetry (IC) and thickening time (TT) tests at 70°C. Cement slurries with a higher dosage of any of the three retarders typically exhibited abnormal thickening (premature thickening) during the early TT test period, characterized by a “stepwise increase” in consistency. Higher dosage of any of the three retarders can fundamentally alter the typical shape of the cement hydration heat flow curve, resulting in an additional exothermic peak before the main hydration peak. Based on its timing and on previous literature, this peak is tentatively attributed to accelerated hydration of aluminate/ferrite phases. The duration of the initial hydration peak shows excellent agreement with the duration of the stepwise rise in cement slurry consistency, which establishes a clear correlation between macroscopic cement slurry thickening behavior and microscopic cement hydration kinetics. Therefore, the abnormal thickening behavior of the cement slurries is suggested to be associated with early accelerated hydration of aluminate and ferrite phases before the main hydration peak. As a result, a threshold hydration extent of aluminate and ferrite phases may exist beyond which a cement slurry will exhibit abnormal thickening regardless of the retarder’s ability to inhibit the tricalcium silicate (or alite, C3S) main hydration peak.