Rheological evaluation of compatibility in oil well cementing

In primary cementing of an oil well, the oil-based drilling mud (lubricant) is displaced by sequential pumping of an aqueous surfactant 'spacer' fluid, and then the aqueous cement slurry. The cement sets to seal the annular space between the geological formation and the steel wellbore casing. In the displacement process, there will be some intermixing of the fluids. Compatibility between the drilling mud, the spacer, and the cement slurry is necessary to achieve successful zonal isolation. In this study, steady shear and dynamic oscillatory shear were used to investigate the changes in rheology that occur as a result of this intermixing. For the steady shear measurements the Herschel-Bulkley model shows good agreement with measured stress-strain data, accurately capturing the yield stress and the plastic viscosity over the range of shear rates from 0.75 to 520 s-1. The viscoelastic properties, which are related to the microstructure of the slurry were examined by using dynamic oscillatory shear and it was demonstrated that this measurement could be utilized to evaluate the compatibility. Moreover, a close relationship between yield stress and storage modulus was observed, which enabled a correlation relating the steady shear and the dynamic oscillatory results.