Page 292 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 268

[ ] Well Programming
2.7.4

Cement job objectives for the conductor. Most conductors from
nonfloating rigs piledrive the conductor in. For a cemented conductor
pipe, the main criteria will be supporting the weight of the pipe and
diverter (if used) and minimizing losses. The drill mud will usually be
viscosified water/bentonite (spud mud) or seawater with viscous slugs.
An inner string technique will be used and cement slurry may be
mixed and pumped until cement is seen at surface. No spacers will be
needed—just water ahead.
On a subsea well where slurry is to be pumped until the ROV
detects returns, the slurry can sometimes be hard to see. A few hand-
fuls of Mica LCM mixed in with the first part of the slurry or spacer can
be more easily seen when it returns to the seabed and this is more effec-
tive than the dyes usually used. Both can be added if desired.
Cement slurry should not be too dense since losses may be easily
induced. It is common to displace extended lead slurry and finish
around the shoe with heavier tail slurry. This saves on cementing costs,
puts strength where it is needed at the shoe, and gives ample support
to the casing higher up as well as reducing bottom hole pressures. A
compressive strength of 500 psi will be adequate to support the casing.
Cement job objectives for surface casing. This string is again usu-
ally cemented to surface. This cement job is not often complex, though
a fairly large volume of slurry may be needed since annular capacities
tend to be big.
Shoe strength will be important to maintain well control while
drilling ahead. A competent shoe is probably the most important prop-
erty of the final job. On land rigs the surface casing may penetrate
water sources, which need to be protected from produced fluids.
Cement job objectives for intermediate casings. The deepest
intermediate job tends to be the largest volume of slurry pumped in
one attempt on the well. If the shoe is set deep, with cement high up
(say into the previous casing), check that the well temperatures at TOC
and shoe are close enough for one slurry design. A multistage cement
job may be necessary if this cannot be done with one slurry. As a rule
of thumb, the BHST at TOC should not be less than BHCT. Slurry
designed for BHCT will then have a satisfactory setting time at the top.
A long section of heavy slurry may give enough pressure differen-
tial down low in the string (with mud inside after displacement) to col-
lapse the casing and it is also possible for casing to float when the

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