Chemical Changes in the Composition of Oil Well Cement with Core/Shell Nanoparticle Addition under CO₂ Geological Storage Conditions

This work studies the influence of incorporating core/shell nanoparticles of TiO2 covered with SiO2 (TiO2@SiO2 nTS) in oil well cement class G, evaluating chemical changes in the cement composition due to CO2 attack. The benefits of nTS, including its self-dispersion, pozzolanic properties, and filler effects, make this nanoparticle an attractive supplementary cementitious material for oil well applications. The cement slurry mixing followed the American Petroleum Institute procedures, and no dispersive method was applied for nTS. Hardened cement pastes with water/binder ratios of 0.44 and 0.35, the last being with 0.15 wt % of polycarboxylate ether additive (PCE), were cured in an autoclave (60 °C and 40 bar) for 24 h and then submitted to degradation tests in CO2-saturated water at high temperature and high pressure (HTHP) (90 °C and 150 bar) during 7, 14, and 21 days. The characterization of the carbonated pastes was conducted using advanced techniques. X-ray microtomography and scanning electron microscopy showed that adding nTS and PCE significantly reduced the carbonation depth and the diffusion coefficient, with the latter decreasing by nearly an order of magnitude. Thermogravimetric and X-ray diffraction (Rietveld) analyses showed 1.32 and 1.90% higher CH amounts after carbonation in the mixtures with nTS, respectively. Moreover, in nTS’s presence, a lower content of CaCO3 was observed. The CaCO3 and C–S–H were characterized using solid-state nuclear magnetic resonance (13C and 29Si). Results indicated that Aragonite is the main polymorph of CaCO3 formed, while C–S–H exhibited a lower decalcification degree when nTS was added (about 4.10% lower). Compressive strength tests of the mixtures with nTS and PCE were approximately 31.6% higher when compared with REF–PCE (without nTS). Therefore, the combined effect of nTS and PCE showed interesting properties for HTHP applications.