Geopolymer Publications
April 11, 2024
Geopolymer Publications
April 11, 2024

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The main role of dispersants is to control the rheological properties of cement. Dispersants reduce interactions between cement particles, reducing the plastic viscosity, yield stress and gel strength of the slurry.

As dispersants reduce interactions between cement grains, their addition may lead to development of free fluid and/or cause slurry sedimentation. Depending on conditions there is sometimes only a small concentration window where optimum performance can be obtained. In these situations, the application of biopolymer anti-settling agents can be used to mitigate the negative effects of too much dispersion.

A well dispersed, non-gelling cement slurry is necessary for optimum gas migration control as discussed for latex-based gas migration control additives in SPE 17629.
Good dispersion also has a positive effect on fluid loss control. The figure shows the effect of dispersant concentration on fluid loss control of different density slurries with a fixed concentration of a fluid loss control additive. The graph is generated from data in Table 3.16 of Well Cementing, 2nd edition.

Note that there was no indication that the slurries were stable (no sedimentation) at all dispersant concentrations. Sedimentation may lead to lower fluid loss due to a higher concentration of particles near the filter screen.

Mechanisms of action

The main classes of dispersants act via electrostatic or steric repulsion. The most widely used, and the earliest to be developed, are those that act via electrostatic repulsion. These are typically low molecular weight anionic polymers containing sulphonate groups. The dispersants adsorb onto the surface of the cement grains via the interaction of the sulphonate groups with calcium ions. The adsorbed dispersant creates a negative charge around the cement grains causing them to repel one another. Some of the higher molecular weight components may also provide some steric dispersion.

Dispersants that act via steric repulsion are a relatively recent development from the construction industry. These products are low molecular weight comb polymers that have a backbone containing groups (such as carboxylate and sulphonate) that can adsorb on the surfaces of cement grains. The side chains are typically short polyethylene oxide chains that have a high affinity for water. The side chains extend into the water, preventing the cement grains from contacting each other. The performance of these additives can be tailored for specific applications: the length of the backbone and side chains, the number of side chains for a given length of backbone and the chemistry of the backbone and side chains can be varied.

Further reading