Characterization of a fly ash-based hybrid well cement under different temperature curing conditions for natural gas hydrate drilling

Abstract

This paper presents a study on the temperature effect (3°C, 10°C, and 50°C, in water bath) and activator effect on the characterization of a fly ash-based hybrid well cement, with a large fraction of fly ash (approximately 70 %), which has potential for natural gas hydrate drilling due to its low heat release. The mechanical strength, micro/nanostructure characteristics were investigated through various tests, including x-ray diffraction (XRD), thermogravimetry analysis (TGA-DTG), nuclear magnetic resonance (MAS NMR), and scanning electron microscope (SEM), to reveal the hydration mechanism of the fly ash-based hybrid well cement. The results indicate that the low curing temperatures (3°C, 10°C) inhibited both the hydration of the well cement and the pozzolanic reaction of fly ash in the cement system. However, the incorporation of solid Na2SO4 significantly contributed to the pozzolanic reaction during the hydration process by consuming more calcium hydroxide, promoting more ettringite formation and reducing the apparent activation energy of the blend cement (from 53.06 kJ·mol−1 to 41.23 kJ·mol−1), resulting in better compressive strength. MAS NMR results showed that the addition of solid Na2SO4 facilitated the substitution of Al with Si atoms, leading to the formation of more C-A-S-H and C-(N)-A-S-H gels in the system with a denser microstructure. Overall, the fly ash-based hybrid well cement can be developed as a cement system for natural gas hydrate drilling in the ocean. This cement not only exhibits low heat release but also shows improved strength development, with a 72-hour compressive strength of 8.05 MPa when cured at 10 °C.