Page 225 - Drilling Technology in Nontechnical Language

216 Drilling Technology in Nontechnical Language Second Edition

▪ 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 an overgauge hole

and will significantly affect thickening time, rheology, and
compressive strength. Supersaturating a slurry may involve
heating the mix water to dissolve more salt. At these saturations,

special additives (especially dispersants and fluid loss)
are needed.
▪ Use a saturated KCl slurry. Saturated KCl slurries give higher
compressive strengths faster than saturated NaCl slurries. Setting
time is important (see the next point).
▪ Be aware of potential problems with other formations. Salt-
saturated slurries can cause problems against other formations. If
exposed long-term to unsaturated formation water, osmotic forces
will leach salt out of the cement slurry, which can lead to cement
failure. This may or may not be a problem, depending on what
formations are exposed where.
▪ Use fast setting times. Once cement starts to set, it holds
hydrostatic pressure from above. This reduces pressure on the
salt. As pressure is lost, salt creep rate will increase substantially.
With long setting times, the salt could creep in enough to touch
the casing. As 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.

▪ Use suitable drilling fluids to minimize leaching out the salt.

Large washouts will lead to the normal problems of mud removal,
and this will lead to an incomplete cement sheath. However,
using oil- or salt-saturated water muds can give problems, as the
hole will close in while drilling.
▪ Increase mud densities. Increased mud density will reduce the
rate of creep.
Low-salt slurries have been used successfully in the Gulf of Mexico
and other areas. These aim to give fast development of high compressive
strengths. These slurries will avoid problems against other formations due
to osmosis as mentioned above. However, washouts are still likely, and
long-term ion diffusion may be a problem later.

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