Page 296 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 272

[ ] Well Programming
2.7.4

3. Use suitable drilling fluids to minimize leaching out the salt. Large
washouts will lead to the normal problems of mud removal and
these will lead to an incomplete cement sheath. However, using oil
or salt-saturated water muds can cause problems because the hole
will close in while drilling. A bi-center bit to drill a slightly over-
gauge hole may be needed.
4. Increased mud densities will reduce the rate of creep.
5. Low salt slurries have been used successfully in the Gulf of
Mexico and other areas. These give a fast development of high
compressive strengths. These slurries will avoid problems against
other formations due to osmosis as mentioned in #2. However,
washouts are still likely to occur and long-term ionic diffusion
may be a problem later.

Clearly, cementing against massive salts is a complex problem if the
well is to meet its long-term objectives. The success of this cement job
starts when drilling through the salt (minimizing leached washouts).
Good planning, expert involvement, and attention to every detail
including slurry and spacer design, rig equipment, downhole casing
configuration, and cement job supervision/quality control are vital.
Cementing against permeable, gas-bearing formations. Slurry
design in this case will try to minimize the flow of gas into the setting
cement. In order to understand the design requirements, it is first nec-
essary to understand what happens as the cement slurry sets.
Cement after placement exhibits complex non-Newtonian fluid
behavior. It has a yield point, plastic viscosity, and gel strength. It trans-
mits full hydrostatic pressure from its own density and from the fluids
and pressures above it.
With time, gel strength develops and the cement enters a transition
2
state at the gel strength of around 21 lbs/100 ft . The cement solids
start to form chemical and electrostatic bonds and gradually the hydro-
static gradient of the slurry decreases to that of the mix water. At this
stage, the slurry is similar to a porous formation with mix water in the
connected pores. Calcium silicate hydrates form as setting continues
and this leads to a reduction in the bulk volume of the slurry because
the products of the chemical reactions have less volume than the total
volumes of the reactants. Portland cement shrinks up to about 5%
depending on the exact constituents of the cement. The pore pressure

272