Chemical resistance and mechanical properties of nanosilica addition in oil well cement

Abstract

The aim of this study was to evaluate the chemical resistance and mechanical properties of cement class G with n-SiO2 addition after being exposed to CO2-saturated water at HPHT, simulating geological carbon storage condition. Four different amounts of n-SiO2 (0.5, 1, 1.5 and 3 wt%) and a standard cement (STD Cement) were tested with CO2-saturated water at 150 bar and 90 °C for 7 and 56 days. The workability of the slurries was evaluated by mini slump test and helium gas pycnometry was used to measure the specific density of unreacted hardened cement systems. Zones affected by CO2 reactions (bicarbonated, carbonated and portlandite depleted zones) and unreacted core were analyzed using optical and scanning electron microscopes, energy dispersive spectroscopy by line scan, X-ray microtomography and atomic force microscopy. Vickers microhardness and uniaxial compressive strength were used to obtain information about alteration in mechanical properties. The results showed that the addition of n-SiO2 reduced the workability of the slurries and had insignificant influence on specific density of the hardened cement. After 7 days of exposure to CO2 medium, the 1.5% n-SiO2 was the most effective cement system to reduce CO2 degradation, decreasing the chemical altered thickness to 2.63 mm when compared to STD Cement (3.06 mm). Results from 56 days of exposure to CO2 show that only 0.5% n-SiO2 cement system is similar in terms of carbonation to STD Cement. For other n-SiO2 amounts (1%, 1.5% and 3%) the thicknesses of chemically altered layer are bigger than STD Cement. However, changes in chemical composition, microstructure and density from periphery to the core of the cement system were less accentuated in the cement systems with n-SiO2 addition after 56 days of cement systems exposure to CO2. Furthermore, the n-SiO2 cement systems presented a lower loss in compressive strength values when compared to STD Cement after reaction with CO2.