Cement Slurry Performance and Set Cement Properties vs. Microsilica Densification

Abstract

Abstract Microsilica is the extremely fine noncrystalline silica obtained in electric furnaces as a byproduct in the manufacture of elemental silicon and ferrosilicon alloys. This waste material, also improperly called silica fume, is available in different grades of various handling characteristics. It has been used for years as cement admixture both in the oil field and in construction and civil engineering industries. Because of its extremely fine nature and high reactivity as a pozzolanic material, microsilica has been used to improve slurry stability and the mechanical properties of the hardened material. This paper reviews typical uses of microsilica in oilfield cements. Particular attention is given to the influence of material grade, in terms of degree of densification, on slurry behavior and set cement properties. The physical properties of microsilica tested include specific gravity, loose and compacted bulk densities, and particle size distribution before and after being submitted to an ultrasonic treatment. Slurry behavior is evaluated in terms of rheology and stability. The observation of set microsilica cement microstructure with a scanning electron microscope coupled with an energy dispersive spectrometer clarifies the question concerning the dispersability of densified microsilica in cement slurry. An unambiguous correlation between physical properties of microsilica, slurry performance, and set cement morphology is then established, leading the way to correctly specifying the quality of microsilica to be used in oilfield applications. This work demonstrates that, contrary to common belief, not all grades of microsilica are equal, and that densified microsilica does not redisperse into small particles when mixed in a cement slurry. Therefore, densified grades, which are popular for their ease of handling, do not provide the performance required for adequate zonal isolation as is normally expected from microsilica slurry. In particular, densified material improves neither the slurry stability nor the set cement permeability and mechanical properties.