Page 295 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 271

2.7.4
Cementing Program [ ]

The contributing factors to these failures include:

1. Salt creep causing hole closure. This occurs faster in bigger hole
and is proportional to hole diameter; a 16 in hole will reduce in
diameter twice as fast as an 8 in hole. Lowered hydrostatic pres-
sures will increase the rate of creep.
2. Salt flow due to directional field stresses.
3. Leaching by mud and cement leading to overgauge hole and slurry
chemistry alteration.
4. Ionic diffusion of salts into a nonsaturated slurry after setting.
Magnesium is particularly detrimental.

The essential objectives are to cement throughout the whole salt
body interval, to ensure that good cement completely fills the annu-
lus, and to prevent long-term degradation due to ionic diffusion.
Several things can be specified in the drilling program to maximize the
chance of success:

1. Use a salt-saturated slurry. If the slurry is unsaturated at downhole
temperature, substantial quantities of salt can be leached out by the
slurry. This will give overgauge hole and significantly affect thick-
ening time, rheology, and compressive strength. Supersaturating
slurry may involve heating the mixwater to dissolve more salt. At
these saturations, special additives (especially dispersants and fluid
loss) are needed.
Saturated KCl slurries give higher compressive strengths faster
than saturated NaCl slurries. Setting time is important (see #2).
Salt-saturated slurries can cause problems against other forma-
tions. If exposed long term to unsaturated formation water, osmot-
ic forces will leach salt out of the cement slurry that can lead to
cement failure. This may or may not be a problem, depending on
what formations are exposed and where.
2. Use fast setting times. Once cement gels and hydrostatic pressure
is lost, salt creep rate will increase substantially. With long setting
times, the salt could creep in enough to touch the casing. Since salt
does not creep uniformly, the resulting point loading on the casing
will quickly collapse or deform it. Even the strongest casing cannot
resist such point loadings.

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