The effect of different additives on the permeability of cement slurries to formation gas has been studied with the aid of a gas flow apparatus. The performance of two commercial additives (polymer latex and silica fume) has been compared to that of a novel additive (carbon black) that has been developed in our laboratories with the aim of simplifying the cement slurry composition and reducing field operational costs. Data on the thickening time, fluid loss, rheology and compressive strength are also presented to provide a clear picture of the potential of carbon black as a substitute for silica fume and polymer latex in some field applications. Finally, the paper describes the results of a field application using carbon black as a gas-block additive in the cement slurry.

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