Cement Slurry design Basics

First, here is a handy table to simplify the process of cement slurry design:

Additive Type	Purpose	Considerations
Extenders I Water, Bentonite (clays), pozzolans, amorphous silica, Sodium Silicate, etc.	Reduction of hydrostatic or greater economy. Lower slurry density from less than 15.8 down to 11.5 ppg, typically. Higher slurry yield (more slurry volume per sack).	Change slurry rheology (viscosity). Increase porosity and consequently the thickening time. Compressive strength: delay the 50-psi value, slow development and reduce the final or max. value at any test time. NOTE: Pozzolans and amorphous silica are reactive and have a positive contribution to compressive strength and permeability.
Extenders II Ceramic or glass (gas-filled) microspheres and foamed cement.	Reduction of hydrostatic while maintaining relatively good, set cement, mechanical properties and durability. Very low specific gravity component.	High cost for the final customer, but great value if properly conceived. Increased logistics. Higher job complexity (quality assurance and safety during job preparation and execution). Additional laboratory work. Cons (microspheres): Gravity segregation, in blend or downhole. Slurry density tied to blend design. Cement Placement (mud displacement/rheology) known to have been an issue Surface mixing for lower densities. Cons (foamed cement): More surface equipment and coordination required Gas is compressible (final density linked to depth and foam quality).
Weight material Less water (max. 17.5 – 18 ppg) Ilmenite – 4.45, Hematite – 4.95, Barite – 4.33, Manganese Tetroxide – 4.84 (sg).	Higher slurry density Depending on the weighting agent, up to 22 ppg.	Increases slurry viscosity. Sedimentation (too viscous at surface > over-dispersed at bottom, duality).
Accelerator CaCl2, NaCl <10 % BWOW, Sodium Silicates.	Shorter thickening time. Higher early compressive strength.	Modifies slurry viscosity Interferes with fluid-loss additives and dispersants. Issues with surface mixing (exothermic reaction) leading to gelation and early setting. Reduced sulphate resistance (CaCl2).
Retarder Lignosulfonates Hydroxycarboxylic acids Organic Salts Inorganic compounds Cellulose-based Combination.	Increases thickening time sensitivity to temperature and/or concentration, secondary effects depending on the mechanism of action: Adsorption, Precipitation, Nucleation, Complexation.	Gelation, Issues with gelling itself or consequent mitigations might lead to delayed compressive strength development. Dispersion effect or chance to synergize with dispersants leading to over-dispersion. Increased fluid loss incompatibility Slow the development of compressive strength.
Fluid-loss additive	Prevents slurry filtration. Dynamically: leading to density increase, impaired rheology and ultimately friction pressure increase, and premature setting. Static: Prevent excessive growth of filter cake. Overall, prevent formation invasion and increase slurry stability.	Plastic viscosity and yield stress increase. Some fluid-loss control additives have a retarding effect. Lowers early and final compressive strength.
Dispersant	Reduces viscosity and yield point, Reduce friction pressures, Improve cement slurry mixability (lower Ty), Allows reduced water slurries (max. 18 ppg), Improve the efficiency of fluid loss control additives.	Retarding effect – longer thickening time. Modifies early and final compressive strength. Supports fluid-loss control (promotes deflocculation). Free water/sedimentation (allowing mixability at the surface might lead to over-dispersion).
Lost-circulation materials	Reduce or eliminate loss of slurry to formation.	Higher slurry viscosity. Lower early and final compressive strength. Careful design if intended in primary cementing (float equipment, liners).
Sodium chloride> 10%	Enhanced bonding across salt and shale formations.	Shorter thickening time (<10% BWOW) Longer thickening time (18% BWOW). Lower slurry viscosity Higher early compressive strength.
Silica flour/sand	Prevent strength retrogression above 230°F (110°C).	Higher slurry yield changes thickening time. Higher slurry viscosity.
Antifoam agent	Decreases air entrance (foam formation) during slurry mixing.	No major effect on other slurry properties.
Antisettling agent	Reduces free water, sedimentation and slurry instability.	Longer thickening time. Higher slurry viscosity. Lowers compressive strength.
Latex Styrene-butadiene	Gas-migration control. Enhanced bonding. Reduced slurry dehydration. Lowers fluid loss.	Changes thickening time. Modifies rheology Incompatibility with some retarders, accelerators, bentonite. Might need a stabilizer, particularly at higher temperature (prevent flocculation).

Further cement slurry design considerations

Select Additives following the below considerations.

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 than 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 by 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.

Disclaimer: The cement slurry design article is general information. Any recommendation on this website is only informative and intends to promote discussion and improve knowledge. Any implementation in actual cement slurries is subject to the approval of the relevant technical authority within the interested organizations and companies.

Cheers,

L. Diaz