Key requirements

• Cement slurry density
• Fluid loss control (only required for zonal isolation or occurrence of annular/pipe restrictions)
• Compressive strength (Release casing weight, 500 psi; dress off or drill out cement top, 500 psi; perforation shots, > 2000 psi; side-track cement plugs, > 5000 psi. NOTE: Keep in mind the compressive strength in lab report is mostly for pure cement slurry under API schedule in a standard reference of 24 hrs, however actual downhole compressive strength would be likely lower due to contamination lead/tail – mud/spacer and/or heating rate slower than API schedule.
• Loss circulation.

Secondary requirements

• Fluid loss control additive increases cement slurry rheology select a concentration in the lower range, as per the technical document of the chosen fluid loss control additive (see temperature limit). Not applicable for Kick of cement plugs, go to next.
• Select dispersant, keep in mind that unnecessary dispersant concentrations promote free water and sedimentation. 
• Select a compatible retarder, concentration as per the technical document of the retarder and/or laboratory database for the expected thickening time. Keep in mind both the fluid loss control additive and the dispersant would tend to increase thickening time, and the retarder might have an added dispersing effect. Recommendation to start with a concentration 25 – 30% lower than the recommendation in technical documents or database. Having a shorter that required thickening time in the first iteration would allow moving quickly to the next and have an idea of the slurry response – compare your result with the database and select next concentration accordingly.
• Select the appropriate antifoam agent.
• If there is a need for an antisettling agent, the slurry design is faulty. Consider the following.
  • The cement slurry is unstable (as determined in rheology, free water and/or sedimentation test).
  • Lower the dispersant concentration, increase the retarder, keep the fluid loss control additive. See the effect of rheology and free water and opt to increase fluid loss if necessary.
  • Other options: increasing slurry density a fraction; increasing concentration of Ceramic or glass (gas-filled) microspheres (low density slurries), decreasing concentration of weighting agent (or changing weighting agent), changing from silica sand to silica flour (or combine both, in some cases the presence of silica flour may call for extra dispersant – surface mixing – in this case, the recommendation would be to use silica sand, to lower the amount of dispersant).

Tertiary requirements

• These are the result of chemicals incompatibility, poor quality control of chemicals/materials and/or low-quality cement (contamination with construction cement) and/or water.
• Most common effects are slurry gelation (detected at mixing, during rheology or thickening time) and delay/poor compressive strength development.
• Main actions in most cases include:
  • Changing chemicals, batch number or supplier,
  • Use different source of water (only available for small to medium slurry volume),
  • Small amounts of sodium silicate (would damage other properties),
  • Changing cement is not usually an option, it might be required to completely redesign the cement slurry,
  • If Latex is in the slurry, remember the presence of latex does not ensure gas migration control; all slurries would need to be validated with static gel strength development and CHA testing equipment (preferably). Check the quality of the latex and the need of latex stabilizer. NOTE: Doesn’t matter what you have in the slurry to control gas or how expensive it is, if not properly placed in the annulus, it will do nothing to prevent gas migration – Cement Coverage + gas migration capabilities is the key.