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Chapter 9 – CASING AND CEMENTING 213

at any particular depth, two working temperatures will be relevant to
cementing operations: circulating and static.
A temperature log run on wireline some hours after finishing circulating

will give the static temperature at the bottom of the well (BHST). The
circulating temperature at depth (BHCT) can be calculated by reference to

API Specification 10, which contains temperature tables. It is also possible
to measure this temperature directly during circulating with logging while
drilling tools in the BHA. Of these two temperatures, BHST is relevant
to investigating cement stability and compressive strength development
with time. Bottomhole circulating temperature is used when calculating
pumpable time.
As a rule of thumb, the static temperature at the depth of top of cement
should not be less than BHCT used in slurry design. If it is significantly

less, it may take an unacceptable length of time to cure. In this case, extra
testing should be done at the actual TOC static temperature to see if the
cement characteristics are still acceptable.

For deep, hot wells (BHST > 110°C [230°F]), the long-term stability of

Portland cement requires the addition of silica flour, usually 35% BWOC. If

silica flour is not added, the strength of the set cement will slowly decrease
with time.

Rheology

Rheology was covered in some depth in chapter 7 when discussing
drilling fluids. The cement slurry rheology is very important because this

will affect downhole pressures while pumping cement around the casing
and up the annulus. It will also affect mud displacement, mixability,
pumpability, and free fall of the slurry down the casing. (When cement
slurry is pumped into the casing, the total hydrostatic pressure of the fluids

inside the casing is greater than the fluid in the annulus because cement
slurry is denser than mud. This leads to a condition whereby the cement
will continue to fall down the casing, even if pumping is stopped. With a
large cement job, it is possible for this U-tubing effect to cause the cement
to fall faster than the pumps can fill the casing behind it, causing a partial

vacuum inside the casing. This condition is called free fall.)

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