Page 218 - Drilling Technology in Nontechnical Language

Chapter 9 – CASING AND CEMENTING 209

state the chemical and physical attributes of the cement. Over time, one
class has proven to be the most useful when its properties are chemically
modified during mixing. This is API class G cement. It is universally

available around the world, and there is a vast amount of experience in
using it.
An amount of class G cement powder will require a certain volume
of water to hydrate it and make it pumpable. To completely hydrate the
cement, 22% water by weight of cement (BWOC) is needed, but this would
not make a pumpable slurry. To achieve a pumpable slurry, 44% water
BWOC is used, and the “extra” 22% water is held within the set cement
matrix. Excess water (above 44% BWOC) will be left as free water after
the slurry sets. The point at which the correct amount of water is used to
make a pumpable slurry with no free water is known as neat cement.
For API class G cement, the water requirement is 4.96 US gallons
for each 94 lb sack, and the resulting slurry weight is 15.8 pounds per
gallon (ppg). Free water for normal slurries should be no more than 0.5%
of the slurry volume, with 0% for a slurry designed for high-angle or
horizontal wells.

Cement Design

Density

Neat class G cement can be modified to suit particular requirements of
the well. The most important slurry property is density. As noted above,
neat cement slurry weighs 15.8 ppg, which equals a density gradient of
0.822 psi/ft. Normally, casings are cemented with two different slurry
densities—a light “lead” slurry and a neat “tail” slurry. This is done for
two main reasons:
1. Hydrostatic pressure. A long column of neat cement slurry
might cause formations downhole to break down due to the
high pressure.
2. Cost. The light slurry does not require as much cement powder
and additives and so is cheaper.

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