Cementation of casing string depends on composition and properties of cement slurry. The properties of Portland cements must often be modified to meet the demands of a particular well application. These modifications are accomplished by the admixing of additives that effectively alter the hydration chemistry. Silica (SiO2) is used most frequently for the prevention of strength retrogression. It can have a different particle size (“silica sand”, with an average particle size of about 100 μm; “silica flour”, with an average particle size of about 15 μm; and “silica fume”, with mean particle size between 0,1 μm and 0,2 μm). Commercially available additive “Microblock” was used in lab tests. It is a liquid cement additive made from a finely divided, high surface-area silica (D50: cca 0.15 μm; D90: cca 0.75 μm). “Microblock” can help prevent high-temperature strength retrogression, control lost circulation as well as gas migration and can provide a degree of fluid-loss control. The Portland cement slurries with 10%, 20%, 30% and 40% of “Microblock” have been tested. Results of laboratory tests have shown that silica fume (also known as microsilica) affects the slurry properties such as thickening time, rheology, fluid loss, free water, slurry stability, and set cement compressive strength. The development of high early compressive strength is important to ensure structural support to casing and hydraulic/mechanical isolation of downhole intervals. The development of compressive strength of Portland cement slurries with and without “Microblock” at different curing temperature (90 °C, 120 °C and 150 °C) has been determined by Ultrasonic cement analyzer. Results have shown that “Microblock” affects the properties of cement slurry and set cement. The compressive strength has been higher with the addition of “Microblock” than compressive strength of neat PC slurry, but negative effect has been exhibited on slurry rheology and early strength development at elevated temperatures.

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