Preparation, performance and the fluid loss control mechanism of a rigid-flexible coupled ultra-high temperature oil well cement fluid loss additive

To effectively control the fluid loss performance of cement slurry in ultra-deep wells under ultra-high temperature conditions, this study used nano-silica as the raw material, modified its surface with vinyltriethoxysilane (VTES) and then grafted long-chain polymers to prepare an ultra-high temperature oil well cement fluid loss additive (UHTF). The UHTF was characterized by FTIR, 1H-NMR, SEM, EDS, TEM, TG, etc. Its fluid loss control effect on cement slurry at ultra-high temperatures was evaluated using a high-temperature and high-pressure filter press. The mechanism of action was studied through a mathematical description of the filtration process, analysis of the filter cake structure, analysis of the slurry stability, and temperature-variable UV spectrophotometric analysis of the UHTF solution. The results show that the UHTF has a rigid-flexible coupling structure and exhibits excellent ultra-high temperature fluid loss control performance. At 240 °C and a pressure difference of 6.9 MPa, the UHTF can reduce the API fluid loss of cement slurry to less than 20 mL without adversely affecting its thickening performance or compressive strength development. The fluid loss control mechanism of the UHTF lies in its ability to improve the dispersion of solid particles, self-adaptively plug pores in the filter cake, and form a polymer film on the surface of the filter cake to increase its compactness, thereby increasing the resistance to liquid phase seepage. Meanwhile, it can also form a network structure through hydrophobic associations to retain the fluid within the slurry, thereby contributing synergistically to effective fluid loss control.