Page 223 - Drilling Technology in Nontechnical Language

214 Drilling Technology in Nontechnical Language Second Edition

Cement slurry rheology is very complex and depends on many factors,
such as the following:
▪ Ratio of solids (cement powder, bentonite, etc.) to water.
▪ Sizes and shapes of the solids present in the slurry.
▪ How much energy was used to mix the slurry (affects the
distribution of particles and the speed of chemical reactions).
▪ Flow regime (laminar, turbulent, transitional).

▪ Time—the cement rheology continuously changes as chemical
reactions take place.

▪ Temperature and pressure—the rheology changes as the cement
moves down the well.
Even though tremendous efforts have been made by the industry to
completely characterize and explain cement slurry rheology, this work is
not yet complete. As with mud rheology, the best model currently available
to describe most cement slurry rheologies in the field is the Herschel-

Bulkley model.

Chemical additives

All characteristics of the cement can be modified by adding chemicals
to the slurry. Some of these additives have already been mentioned,
such as bentonite or powdered coal as extenders, barite as a weighting
additive, retarders (to slow down the setting speed of the slurry), and
accelerators (to make the slurry set faster). Other additives available
include the following:

▪ Defoamers. Prevents the slurry from foaming while mixing.
▪ Dispersants. Help to distribute the solid particles present in
the slurry.

▪ Fluid loss. Controls loss of filtrate into permeable formations.
▪ Lost circulation material. Inert solid materials to plug off pore
spaces at the formation face so as to prevent the loss of whole
slurry to the formation.
The use of additives allows one cement type (API class G) to be used
for many different wells and in different applications.

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