Page 288 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 264

[ ] Well Programming
2.7.3

mud and cement and to aid cleaning formation and casing surfaces.
2. Rheology must be correct. Turbulent flow is very beneficial in
removing gelled mud and any solids left in the wellbore.
Sometimes a turbulent spacer is used to disturb cuttings beds fol-
lowed by a higher viscosity spacer to sweep up the solids.
3. Volumes of washes and spacers should ideally be enough to give at
least 10 minutes of contact time at the planned displacement rate.
4. Alternating high and low viscosity spacers can be used in high-
angle holes where solids will have to be removed.
5. One technique that is not so common nowadays is to place a scav-
enger slurry ahead of the main slurry. This is just a thin cement
slurry, which may be too thin to actually set up in any reasonable
time, but it will help prepare the wellbore for the cement behind.
6. If expensive mud is in the hole that could be stored and reused, it
would be worthwhile to program enough spacers so that all the
mud is displaced out.

Removal of mud and mud solids and water wetting the formation
and casing surfaces are essential in creating a good bond with the slurry.

2.7.3. Factors for Ensuring a Good Cement Job

There are two main keys to getting a good cement job. The first is fully
displacing mud from the intended interval to be cemented and the second
is having a properly formulated, homogenous slurry placed correctly.
Mud removal. Many interrelated factors come into play when con-
sidering mud removal. Following are important considerations:

1. Casing centralization. Centralization must be assured if cementing
outside a casing (as opposed to setting a cement plug). Fluids will
flow preferentially around the largest space if the casing is cement-
ed eccentrically, and fluids in the narrowest part of the annulus
may not move at all. API recommends a minimum standoff of 67%
(where 100% is perfectly centralized). Standoff percentage is
obtained by dividing the shortest distance between the casing and
the wall by the average radial clearance. (See Fig. 2-9.)

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