Cement Slurry Flow Properties and Mud Displacement
Washes, Preflushes, and Spacers
Intermediate fluids are sometimes pumped as a buffer ahead of the cement to prevent it from contacting and mixing with the mud in the annulus, which can cause inefficient displacement. Drilling mud contamination of cement can also reduce its compressive strength. These intermediate fluids can be:
- Chemical washes or preflushes that contain no solids and have a density and viscosity comparable to that of water (such as a 5% hexametaphosphate solution to help disintegrate the mud cake).
- Spacer fluids that contain solids and are mixed at various densities and viscosities. One variation on a preflush is a cement scavenger slurry – a dilute mix of cement and water pumped ahead of the cement slurry in turbulent flow to help remove gelled mud and filter cake.
Common washes contain surfactants and dispersants, often similar to those used in cement slurries.
Spacers typically contain viscosifiers like polyacrylamides, guar gum, cellulose derivatives, or biopolymers. The density and rheological properties of the spacer should lie between those of the drilling mud and the slurry. The volume of spacer pumped should be enough to extend across at least 500 feet of annular distance. Any expected reduction in hydrostatic head due to a spacer being pumped must be carefully considered. If the overbalance is calculated to be less than the minimum required overbalance, at any stage of the cement job, a weighted preflush should be used instead of a water spacer.
Just as the mixing of mud and slurry in the casing is prevented by the use of wiper plugs, similar plugs can be used to prevent the mixing of the spacer with both the mud (ahead) and the slurry (behind) in the casing. Surface flowlines should also be flushed thoroughly to ensure that any cross-contamination of mud, spacers, and cement slurry is minimized.
