Preparation of cellulose nanofibrils and their effects on the rheological properties and compressive strength of oil-well cement paste

In this study, an efficient hybrid preparation method was developed for isolating cellulose nanofibrils (CNFs) from cellulose fibres using ball milling and tetramethylpiperidine-1-oxyl (TEMPO) oxidation. The chemical structure and microstructure of the obtained CNFs were characterised using Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Additionally, the effects of CNFs on the rheological properties, thickening time, and compressive strength of oil-well cement paste were investigated. Ball milling for 30 min at a revolution speed of 500 r/min changed the microstructure of the cellulose fibres to a 'fibrous-sheet structure', reduced their diameters to several micrometres, and increased their specific surface area. Furthermore, this change in the cellulose fibre microstructure reduced the TEMPO oxidation reaction time, thereby improving the preparation efficiency of the CNFs. The TEM and XPS results indicate that CNFs can be obtained via TEMPO oxidation for 2 h. The surface of the CNFs contained up to 5.08% C = O groups. Additionally, compared to the pure oil-well cement sample, the yield stress of the oil-well cement paste containing 0.2% CNFs increased from 2.31 Pa to 9.84 Pa, and its consistency increased from 10.6 Bc to 28.0 Bc, which confirmed that the CNFs formed a 'fabric structure' to increase the structural force of the oil-well cement. Moreover, the compressive strength of the oil-well cement paste containing 0.2% CNFs cured for 2 d at 60 °C increased by 50.4%, while that of the paste cured for 14 d increased by 53.0% compared to that of the pure oil-well cement paste. These achievements can provide some sight for designing the high-performance oil-well cement paste.