The determination of yield points in cement slurries is important in the overall description of slurry flow properties. Yield points affect both the start-up pressure after a temporary shut-down 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, a number of 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 accurately measure yield points. 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 wind-up 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 a number of advantages in making these measurements.