Page 298 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 274

[ ] Well Programming
2.7.5

sures or thermal stresses can also weaken cement bonding to cas-
ing during the well life, providing possibilities for gas migration.
4. Additives can be used that reduce and/or eliminate slurry shrink-
age during setting.
5. Zero free water in the slurry is important, otherwise the slurry will
not be homogenous as lighter, unbound water migrates upwards.
6. Shorter setting times (up to the end of the transitional state) are
clearly beneficial. Longer setting times allow more opportunity for
gas to flow.

Another technique is to set an ECP above the gas zone, on the basis
that this mechanical barrier will form a limit to upward gas movement.
If it seals against the formation, then this should indeed stop migra-
tion. However, the slurry beneath the ECP will lose pore pressure
much faster than if the ECP was not used, which may lead to porous
cement below the ECP.
It is worth noting that extended cement with a high water content
can show permeabilities in the millidarcy range (up to 5 md) when set.
Over extended time, gas can flow through the set cement matrix,
which may lead to the slow development of annular pressure.
As explained above, effective cementing against gas zones requires
definition of objectives for the life cycle of the well, gathering of all rele-
vant information, expert input, careful planning, and proper execution.

2.7.5. Cementing Design for Cement Plugs and Squeezes

Programmed cement plugs are needed for jobs such as aban-
donment and kickoffs. Placement techniques are important and,
with careful planning, the chances of first-time success are general-
ly high. Plugs set in open hole require as much planning as would
be put into cementing casing; this is covered in “Kickoff plugs”
within this subsection.
The usual squeeze technique is a hesitation squeeze. Surface pressure
below fracture pressure is applied and the surface pressure is monitored.
As the pressure bleeds off, pump slowly into the well to restore surface
pressure. When no more bleed off is seen, maintain pressure until surface
samples are hard or earlier if decided to stop for any other reason.

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