Page 282 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 258

[ ] Well Programming
2.7.1

slurry recipes (designed using samples of cement, mixwater, and
additives taken from the rig)

One of the most difficult and critical things to achieve is a good
primary cement job. Remedial cementing is difficult and expensive to
execute and in practice rarely achieves a satisfactory result. This means
that the primary cement jobs must be planned so that the objectives are
met the first time. A few dollars saved at the expense of job quality can
potentially cost millions in remedial work later.
Quality control procedures are vital—first, to ensure a good job
and second, to record all necessary data to evaluate the job afterwards.
Field quality control (QC) procedures are noted in Section 3.2.2,
“Quality Control.” The program should specify the QC procedures to
follow and flag any particular recording requirements.

2.7.1. Slurry Properties

Density. An amount of cement powder will require a certain vol-
ume of water to hydrate it and make it pumpable. Excess water will be
left as “free water” after the slurry sets. The point where the correct
amount is used to completely hydrate the cement with no free water
is known as “neat” cement. For API class G cement, the water require-
ment is 5 U.S. gallons for each 94 lb sack and the resulting slurry
weight is 15.8 pounds per gallon.
To modify the slurry density, additives such as bentonite are used
to soak up extra water (extenders), allowing a lighter slurry, or
weighting agents are used (barite) to increase the density. Neat slur-
ry will generally have a higher compressive strength than extended
slurry. There are other methods of modifying slurry density; for
instance, by using hollow glass microspheres in the slurry or foamed
cement for extremely light slurry (perhaps when cementing in zones
of total losses).
Usually a casing cement job will use two slurries: a light “lead”
slurry ahead and a denser (often neat) “tail” slurry around the shoe.
Using neat slurry for the whole job would increase hydrostatic and
circulating pressures in the wellbore and is less economical than
using an extended lead. If satisfactory shoe strength for drilling ahead

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