Performance of oil well cements before and after being exposed to supercritical CO2: Evaluation of combined chemical additives

Abstract

In this study the individual effects of water/binder (0.44, 0.35, 0.30, and 0.25), polycarboxylate superplasticizer- PCE (0.07 wt%, 0.14 wt%, and 0.21 wt%), defoamer-DF (0.5 % and 1 %), and silica fume-SF (10 wt%, 15 wt%, and 20 wt%) admixtures in oil well cement Class G properties were investigated before and after exposure to CO₂ under high pressure and high temperature (HPHT) of 150 bar and 70°C. Spread diameter, compressive strength, Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) were used for characterization. Moreover, the microstructure of the samples was observed with scanning electron microscopy (SEM). The results reveal that the w/b ratio is a crucial factor for improving cement paste properties, with lower ratios resulting in lower porosity (higher carbonation resistance) and higher compressive strength. PCE enhances dispersion and cohesion, with optimal results achieved at a low w/b ratio (0.25). Although high dosages of PCE increased carbonation depth, they significantly improved the workability. DF reduces air incorporation, increasing cement matrix density, compressive strength, and resistance against CO₂ attack. SF significantly improves mechanical properties and microstructure, especially at higher addition levels. However, as SF is added, the higher the carbonation depth. Additionally, the synergistic behavior of additives was evaluated after determining the optimum value for each one. The optimal mixture with the lowest water/binder ratio and highest PCE, DF and SF amounts achieved improved properties in the hardened state with good slurry workability. It also provided the lowest carbonation depth under geological carbon storage conditions.