Page 293 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 269

Cementing Program [ ]
2.7.4

cement is displaced if there is enough difference between slurry and
mud density. Check these when the slurry and spacer densities are
known. Inducing losses or fracturing formation is also possible, espe-
cially since ECDs will be high during displacement.
As with the surface casing, obtaining a competent shoe to drill
ahead may be the primary function of the set cement. The main reason
for a high top of cement would probably be to isolate troublesome or
permeable formations. Some regulatory bodies require abandoned
wells to have cement across all exposed formations, while others only
require cement against permeable zones.
Cement job objectives for intermediate casings: multistage
cementing. If possible, it is best to avoid cementing up into the pre-
vious casing to allow more options for sidetracking later. Before
deciding to perform a multistage cement job, consider whether it is
necessary to have TOC so high that a multistage cement job is called
for. One reason may be to isolate a higher permeable formation from
the well annulus.
A multistage job could also be used to isolate a potential kick zone
higher in the annulus while avoiding an excessively long cement col-
umn all the way from bottom. That is, the stage collar is set above the
primary job TOC and below the zone of interest, and a separate job is
performed. Therefore, there will be an uncemented interval between
the two jobs.
There are two main disadvantages to multistage cementing: the
stage collar will always be a weak point in the casing string (a poten-
tial leak point) and the equipment sometimes fails to function correct-
ly during the cement job. It also adds to the cost of the well.
Cement job objectives for production casings. The main objec-
tives for production casings (and liners) will be quite different from
those for the previous jobs. First, isolating the producing formations
without damaging them is vital for the whole production life of the
well. This requires good cement-formation and casing-cement bonding
and low fluid loss and filtrate-to-formation compatibility. Gas channel-
ing through the gelled (unset) cement must be prevented.
The cement must be able to withstand perforation charges without
shattering and be strong enough for any hydraulic fracturing, back-
surging, or other mechanical treatments. Isolation within the zone may
be important to prevent water or gas production along with oil.
Compressive strength above 2000 psi is recommended for perforating.

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