Page 224 - Drilling Technology in Nontechnical Language

Chapter 9 – CASING AND CEMENTING 215

Cementing casing in massive salt formations

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 as the salt moves. The potential
failure modes include the following:

1. Point loading of the casing due to uneven salt closure. The casing
can collapse with much less force than would be the case for
even loading.
2. Collapse due to overburden pressure being transmitted
horizontally 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 the cement is not salt saturated. If the
cement sheath degrades, uneven loading may occur, leading to
eventual collapse.
The contributing factors to these failures include the following:
▪ Salt creep may cause hole closure. This occurs faster in bigger
holes and is proportional to hole diameter; a 16" hole will reduce
in diameter twice as fast as an 8" hole.

▪ Lowered hydrostatic pressures will increase the rate of creep.

▪ Salt flow may occur due to directional field stresses.
▪ Leaching (dissolving) of the salt by mud and cement may lead to
overgauge hole and slurry chemistry alteration.
▪ Ionic diffusion of salts into a nonsaturated slurry may occur 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 annulus and to
prevent long-term degradation due to ion diffusion. There are several
things that can be specified in the drilling program to maximize the chance

of success.

_Devereux_Book.indb 215 1/16/12 2:11 PM