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Cement slurry dispersants
Dispersants, also known as friction reducers, are used extensively in cement slurries to improve the rheological properties that relate to the flow behavior of the slurry.
Advantages/uses of dispersants
Dispersants are used primarily to lower the frictional pressures of cement slurries while they are being pumped into the well. Converting frictional pressure of a slurry, during pumping, reduces the pumping rate necessary to obtain turbulent flow for specific well conditions, reduces surface pumping pressures and horsepower required to pump the cement into the well, and reduces pressures exerted on weak formations, possibly preventing circulation losses.
Another advantage of dispersants is that they provide slurries with high solids-to-water ratios that have good rheological properties. This factor has been used in designing high-density slurries up to approximately 17 lbm/gal without the need for a weighting additive. The concept can also be used to design low-density slurries in which the high-solids contents include lightweight extenders.
Dispersants have been extensively studied. It is generally agreed that the dispersants minimize or prevent flocculation of cement particles, because the dispersant adsorbs onto the hydration cement particle, causing the particle surfaces to be negatively charged and repel each other. Water that otherwise would have been entrained in the flocculated system also becomes available to further lubricate the slurry.
Types of dispersants
Polyunsulfonated napthalene (PNS)
This is the most common dispersant. It is available as a calcium and/or sodium salt, and can be obtained in both solid and liquid form. The commercial liquid form typically has a solids content of approximately 40%.
The benefit of using PNS is that improved rheological properties can be obtained, and slurries can be pumped with reduced frictional pressures. PNS can also allow higher solids-to-water ratio slurries to be designed with improved properties.
These acids, such as citric acid, may be used as the primary dispersant in freshwater slurries at higher temperatures (BHCT ≥ 200°F). This is typically advantageous with cements that have a high free alkali (> 0.75%) content to offset their retarding properties. Citric acid is also used as a dispersant in salt- and seawater cement slurries. The concentration of use is limited by the temperature and thickening time desired, although concentrations of 0.5 to 1.0% BWOC are usually sufficient.