Page 290 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 266

[ ] Well Programming
2.7.3

Accessories can be used on casings (turbolizers) that are designed
to deflect mud flow into enlarged areas. A turbolizer is like a spring
centralizer with fins added on.
4. Turbulent flow. Even if the mud and cement cannot be designed to
become turbulent at the possible flow rates, the spacers could be
thin enough to be turbulent. This would be the best flow regime for
displacing mud.
5. Chemical washes and spacers. Chemical washes and spacers help to
displace mud and mud solids and also prepare the formation and
casing for the slurry. Wash and spacer design is covered later.
6. Buoyancy. The slurry density should be greater than the mud den-
sity and the spacers should come in between.
7. Use of proper cement plugs and shoetrack. The bottom plug, dropped
ahead of the cement slurry, will scrape the film of mud off the
inside of the casing that would otherwise adhere to the steel. If a
bottom plug was not run, the top plug would wipe off the mud and
this would lead to contamination of the cement slurry in the worst
place—around the casing shoe. The shoetrack (usually two joints
between the float shoe and float collar) contains a volume of slur-
ry when the plug is bumped. This is to ensure that minor leakage
of mud past the top plug, or minor wiping of mud by the plug, con-
taminates cement which ends up in the shoetrack, so as not to
compromise the cement around the shoe.
8. Continuous pumping. After running to depth, casing is usually cir-
culated for at least 110% of the casing internal volume. This checks
that no foreign bodies in the casing will plug the floats before
cement is pumped. While circulating for this extended time, gelled
mud may be broken and moved. However, if pumping is stopped
for any period before cement is placed, mud may gel up again in
dead areas.
9. Pump a good excess volume of lead slurry. If the first part of the slur-
ry is contaminated, it does not matter if there is enough unconta-
minated slurry behind it.

Solids removal. In high-angle hole sections, especially if the hole
is overgauged, solids may have settled on the low side. These cause
problems running casing (solids pushed ahead by the casing can pack
off and stick the casing) and cementing (leaving an uncemented chan-

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