Page 291 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 267

Cementing Program [ ]
2.7.4

nel which could allow communication behind the casing later).
Sometimes even if the hole is clean at the last trip out, continued cav-
ing may lead to solid beds before the casing reaches bottom.
The possible solutions to this include:

Designing a mud system to minimize wellbore instability (refer to
the notes discussed in “Drilling fluid” in Section 2.9.1).
Using a casing circulating packer and carefully washing down each
joint once the shoe reaches any unstable zones. Be careful not to
induce losses or fracture the formations since ECD can be high due
to the narrow annulus.
Pumping turbulent spacers to try to disturb cutting beds.

2.7.4. Cementing Design for Casings and Liners

To correctly specify the cement properties, define what the job is to
achieve. The most important objective is to have sufficient slurry (and
spacer) density to control downhole pressures at all points of the dis-
placement cycle. Further objectives may include minimum ECD (relat-
ed to density and rheology) to ensure that a weak zone is not fractured,
compatibility with formation fluids and formations, isolation of weak
or permeable zones, protection of water sources, and low fluid loss to
minimize filtrate invasion in sensitive zones, etc.
Having defined the objectives for each cement job, the cementing
contractor should propose slurry designs to meet those objectives.
Various slurry objectives and design properties that will be relevant to
cementing casings and liners are discussed below.
The cost of cementing during the well will be a significant propor-
tion of the total well cost. The slurry cost per barrel and the cost of
each additive should be shown. If expensive additives are used, check
to see if a cheaper additive is available that will still meet the objectives.
One example is where gas-blocking additives are used only for fluid
loss; cheaper fluid loss additives should be available if no gas-bearing
permeable zone is present.
It is worth checking through the proposed cementing programs
twice; the first time to check that all the parameters (volumes, depths,
temperatures, etc.) are per program, and the second to compare the
detailed proposals with the specific job objectives.

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