Effect of Size-Controlled Nanofluid on Mechanical Properties, Microstructure, and Rheological Behavior of Cement Slurry for Oil Well Cementing.

The optimal design of cement slurry by balancing various cement additives and cement is critical for effective oil well cementation job. However, given adverse circumstances of application, existing additives may not be sufficient to perform suitably in challenging conditions, leading to premature cement hydration, formation of microcracks, and gas channeling pathways. Thus, this study explores the use of a single-step silica nanofluid (NP size: 5-10, 90-100, and 250-300 nm and concentration: 1, 3, and 5 wt %) as an additive and explores its effect on thickening time, fluid loss, and rheological behavior of class G cement slurry at high-pressure and high-temperature (HPHT) conditions (135 °C and 3625 psi). The improvement in thickening time, fluid loss, and rheology of conventional slurry was greater for low NP size than the nanofluid of high NP size: the nanofluid size, e.g., 5-10 nm, and concentration (1 wt %) were found to accelerate the thickening time by 30-40% while reducing fluid loss from 38 mL (no silica, slurry CS) to 30 mL (with silica, slurry C1). The rheological behavior was studied via shear (viscosity) and dynamic (elastic moduli, ') modes to evaluate the viscosity, hysteresis, and elastic response of slurry with and without nanofluid. The inclusion of nanofluid slightly reduced the slurry viscosity; however, all slurries exhibited shear thinning with superior fitting with the power law model. As compared to slurry CS, hysteresis of slurry C1 was least dependent on shear deformation, and thus, it showed that it almost matched viscosity profiles during loading and unloading cycles. The addition of silica was found to maintain the original properties of cement slurry, establishing that cement had not agglomerated, and no sedimentation was observed even at shear rates of 1000 s. The results of this study greatly promote the use of silica nanofluid as an important additive in class G cement for cementation operations, which is unlikely with a two-step nanofluid where nanoparticles are expensive, and upon mixing, they tend to agglomerate and make large size clusters.