Page 284 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 260

[ ] Well Programming
Well
Programming
2.7.1
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water moving up. This problem gets worse as inclination increases; in
a horizontal bore it will form a perfect communication channel on top
of the cement. During cementing, samples are taken to show when the
slurry has thickened. Often these form a pool of water on top, showing
that the slurry was mixed with excessive free water. This is a sign of a
less than optimum job.
When using recirculating mixers in the field, cement slurry densi-
ties can vary quite a lot. This can lead to parts of the slurry having sig-
nificant free water. The slurry should not be mixed and pumped faster
than is possible to closely control density. Check which type of equip-
ment is available on the rig for mixing the slurry. If care is not taken on
a manual recirculating mixer, cement density can vary by ±2 ppg!
The latest generation of field-mixing equipment has micro-
processor-controlled mixing, leading to much more homogenous
slurries than are possible with manual systems. If possible, batch mix
critical slurries such as horizontal wells, tail slurries, and small-vol-
ume slurries.
Thickening time. The thickening time of slurry is tested as part of
the slurry design. Procedures for testing are given by API specification
10, which measures the time it takes for the slurry to reach a consis-
tency of 100 Bearden units (Bc) at downhole temperature and pressure.
The Bc is a dimensionless value that cannot be directly converted to
oilfield viscosity units such as poises. While the test measures the time
to reach 100 Bc, it is generally accepted that the limit of pumpability is
reached at 70 Bc. The test lab can be asked to provide both values.
The thickening time generally should be enough to displace the
cement and circulate it back out if problems occur. Since it is measured
at downhole temperature and pressure, the time taken for batch mix-
ing on surface will not count minute for minute against thickening
time. In critical jobs when the slurry has to be batch mixed but thick-
ening time needs to be as low as possible, the test can be run by mix-
ing the slurry, agitating it for approximately 30 minutes at surface tem-
perature/atmospheric pressure, before applying downhole conditions.
Compressive strength. Compressive strength measurements do
not give the actual point of failure of the placed cement. The measure-
ment is used more for comparison purposes, but the tested compres-
sive strength can be related to the application. API specification 10
defines the procedure.

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