New cement slurry formulations containing colloidal silica have been used in North Sea primary cementing operations to help control annular gas migration, enhance slurry "turbulent flow" characteristics without solids settling, and aid cement bonding to formation and casing. Colloidal silica slurries have been successfully placed as lightweight lead cements in intermediate casing operations, and have effectively sealed annuli in gas storage wells and in horizontal completions.
Silica cementing additives have been used often for gas migration control, slurry stability, and high-temperature applications in the North Sea. However, because of its extensive specific surface area (500ml/g), colloidal silica adds additional benefits. Colloidal silica is more efficiently handled in offshore operations because less bulk material is required. The reduced volumes also reduce overall slurry cost. Because colloidal silica particles are so fine, there is virtually no settling of the base material in containment offshore.
A discussion of colloidal silica slurry properties is contained in this paper. Performance properties are presented in relation to each of its various well applications. A case history of a horizontal well is also given that describes the practical use of colloidal silica cement.
In 1988, Grinrod et al. introduced the use of microsilica in cement formulations to help prevent annular gas migration. The successful application of this technology was also confirmed by Coker et al. in North Sea shallow gas control. Current use of microsilica in cementing operations not only provides anti-gas flow properties, but also enhances slurry rheologies, slurry stability, compressive strength development, and fluid loss control. The drawbacks to the use of microsilica in some operations have been its cost and also its bulk handling in offshore operations, namely high storage volumes and the potential for solids settling in the microsilica suspension.
To reduce costs and make offshore logistics more efficient, a new cement slurry design using colloidal silica has been developed. Colloidal silica is a pure form of amorphous silica containing only SiO and traces of NaOH.