The determination of yield points in cement slurries is important in the complete description of slurry flow properties. Yield points affect both the start-up pressure after a temporary shutdown and void filling properties of cements. Values for the yield point are normally obtained by extrapolation of rotational or tube flow rheometer data. This technique is subject to both experimental and analytical errors.

Recently, several papers have described a more direct method of measuring yield points in slurries. A rotational viscometer equipped with a vane rather than a bob can be used to measure yield points accurately. The yield surface with the vane test fixture is in the fluid rather than at a fluid-solid interface as it is with a solid bob or a tube flow apparatus. Experiments with a vane fixture using a shear rate controlled rheometer can be subject to errors caused by windup of the torque spring. In this study a controlled stress rheometer was used to circumvent these problems.

Yield points for various oilfield cement slurries are presented. Data reflect the differences in additives, aging time and water content. The vane technique can be used to produce accurate and reproducible measurements of yield points in cements. Using a controlled stress instrument has several advantages in making these measurements.

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