Page 424 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 400

[ ] Practical Wellsite Operations
3.5.4

must be known. If this cannot be done then look at the actual efficien-
cy obtained on the last casing job (if the plug were bumped) and use
that figure.
The plug dropping head will ideally contain both bottom and top
plugs and will be able to drop each without having to open up the
cement head. Once the 120% casing contents has been circulated, keep
on circulating while the cementer starts mixing cement slurry and test-
ing the cement line up to the cement head. Switch to the spacers, pump
the spacers, then drop the bottom plug and start displacing the slurry.
Do not pump the cement faster than you can accurately mix it; it is
quite common for slurry in a recirculating mixer to vary by up to 2 ppg
on the designed weight if care is not taken. This either produces slur-
ry with far too much free water or more viscous slurry, both of which
compromise the job.
The spacer volume and displacement speed should ideally provide
a contact time of at least 10 minutes with the formation to allow the
spacer to work properly.

3.5.4. Cement Displacement

During the cement job a pressure chart should be recorded and
annotated so that the progress of the job can be inspected afterwards
in the event of problems. A Dowell PACR or similar printout is
acceptable.
Two opposing requirements are high displacement rate to attain
turbulent flow versus a lower rate to avoid imposing high annular pres-
sures on the open hole. The best displacement of mud with the spacer
and cement is when the fluids are in turbulent flow. In some cases, the
speed at which this occurs gives unacceptably high annular pressures
due to the circulating pressure drop in the annulus. This may lead to
losses that will in turn compromise the cement job.
With deeper casings and liners, a simulation program should be
used to calculate flow regimes and pressures throughout the job. This
service is available from all the major cementing companies.
Displacement of mud during the cement job is improved if casing
can be rotated or reciprocated. It is not certain exactly why this is so;
two contributing factors may be:

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