Summary
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 the tested microsilica 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 stability and morphology is also
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 mechanical properties. Only moderately compacted microsilica
with a bulk density of approximately 300 kg/m3 have been found to
offer the adequate compromise between proper handling characteristics and good
slurry and set cement performance.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
10 December 2007
- Meeting paper published:
4 March 2008
- Revised manuscript received:
19 March 2009
- Manuscript approved:
15 April 2009
- Published online:
10 September 2009
- Version of record:
23 December 2009
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