Resolving the Adsorbed-Dissolved Phase Distribution of Fluid Loss Additives in Cement Slurry via Centrifugation-Based Pore Solution Isolation

Summary

Fluid loss additives (FLAs) are crucial for controlling the filtration properties of oilwell cement slurries, but the true mechanism linking their adsorption capacity to performance remains contentious due to limitations in conventional measurement methods. With this study, we introduce a novel, rigorous methodology to accurately determine the adsorbed-dissolved phase distribution of FLAs by combining centrifugation-based pore solution isolation and experimental correction for effective free water volume (Vf) depletion. Comparative analysis showed that the traditional filtration method severely underestimates the equilibrium concentration (Ce) by 61–83%, leading to artificially inflated adsorption capacities (Qe). Two-factor analysis of variance (ANOVA) confirmed that centrifugation speed and time did not significantly affect the measured Qe (P-values > 0.05).

Using this validated method, two FLAs (FLA1 and FLA2) were investigated. FLA1 exhibited a saturated adsorption capacity (Qe,max=11.392 mg/g) significantly higher than FLA2 (5.608 mg/g). However, contrary to the adsorption-dominant mechanism, FLA2 demonstrated superior fluid loss (FL) control performance. Further analysis of the phase distribution revealed that the dissolved (unadsorbed) FLA fraction becomes the dominant component at medium to high additive concentrations (≥50% at initial concentrations of 17.05 mg/mL and above). Critically, when plotting FL against the dissolved concentration (Ce), the curves for both FLAs converged into a single, overlapping curve (R2=0.970), whereas FL vs. Qe curves were distinctly separated. This trend supports a dissolved-concentration-guided interpretation for the tested polymers: Their FL control efficacy appears to correlate more closely with the equilibrium concentration (Ce) of the dissolved fraction in the pore solution than with the apparent adsorption capacity. The dissolved fraction is consistent with, and may contribute to, pore-throat sealing by polymer-rich microaggregates within the filter cake. This work establishes a reliable method for phase distribution measurement and provides a new perspective for understanding, screening, and optimizing 2-acrylamido-2-methylpropane sulfonic acid (AMPS)-based FLAs in cement slurries.