Working mechanism of nano-SiO2 sol to alleviate the strength decline of oil well cement under high temperature

Strength retrogression of cement stone is one of the major challenges in oil well cementing under high temperatures. Nano-SiO2 exhibits the potential to mitigate this problem, but there is lack of reports about the role of nano-SiO2 playing within cement stone under high temperatures. Therefore, in this study, we experimentally determined the effect of nano-SiO2 sol on the compressive strength of Class-G oil well cement stone cured under high temperature (150 °C/35 MPa). Then we provided the explanations for the cement strength variation by analyzing the mineral compositions and microstructures of cement. The experimental results are summarized as follows: (1) The cement stone with 2% nano-SiO2 sol does not perform a strength reduction after cured for 5 days, and its XRD patterns indicate that low-dose nano-SiO2 sol will yield little effect on the cement hydration products; (2) While with a high dose, nano-SiO2 can react with calcium hydroxide, resulting in microstructure change of cement and the generation of a novel honeycomb calcium silicate hydrate gel (Ca/Si ratio is about 1.5); (3) Nano-SiO2 particles make the microstructure of cement stone more compact through impaction and filling into the pore among cement hydration products, which helps to prevent cement stone from strength retrogression under high temperatures. Generally, this study lays a theoretical foundation for the nano-SiO2 used as cement additive and sheds light on the design of high-temperature cement slurry system. Keywords: Nano-SiO2 sol, High temperature, Class-G oil well cement, Cement stone, Compressive strength, Strength retrogression, Hydration product, Microstructure