Page 294 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 270

[ ] Well Programming
2.7.4

In addition, maintaining well control during cementing is important as
with the previous jobs.
Cement job objectives for liner cementing. One of the most diffi-
cult aspects of cementing liners is the small cement volume needing to
be placed in a narrow annulus. However, several things can be done to
improve matters:

1. Batch mixing the slurry will produce a more homogenous slurry
and a better job.
2. Under-reaming below the previous shoe gives better annular clear-
ance. (Ideally, hole diameter should be 3 in greater than
casing/liner diameter.) This was done in the North Sea where a 5 in
liner was normally set in a 6 in hole. Under-reaming the 6 in hole
to 8 1 /2 in avoided problems experienced while running liner on the
earlier wells (none of the liners had reached bottom in the 6 in hole
but they all did in the under-reamed hole) and cement logs were
much better in the larger hole. A PDC under-reamer did the job in
one fast run so a lot of time was not taken.
3. Liner rotation during cementing will improve mud removal.
4. Pump a good spacer volume ahead and as much cement excess
as necessary.
5. Check that the mud or formation fluid does not contain chemicals,
which may cause flash setting of cement—such as calcium chloride.

The cement job objectives for a production liner will be similar to
those described for production casings.
Cementing against massive salts. When cementing in massive
salts, the cement forms an essential part of the casing string integrity.
Inadequate cement here will make shearing, distortion, or failure of the
casing possible if the salt moves. The potential failure modes include:
1. Point loading of the casing due to uneven salt closure. The cas-
ing can collapse with much less force than would be the case for
even loading.
2. Collapse due to overburden pressure being transmitted by mobile salt.
3. Shearing of the casing due to directional salt flow.
4. Corrosion of the casing, particularly if magnesium salts are present.
5. Long-term degradation of the cement sheath by ionic diffusion into
the cement, if it is not salt saturated. If the cement sheath degrades,
uneven loading may occur leading to eventual collapse.

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