Page 297 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 273

2.7.4
Cementing Program [ ]

within the cement matrix now drops rapidly due to the low compress-
ibility of cement slurry. The transition state ends when the gel strength
is enough to prevent large gas bubbles from percolating upwards due
2
to buoyancy, at around 250 lbs/100 ft .
Gas can start to flow from the formation as soon as the pressure
imposed on the formation drops below formation pore pressure. This
may happen as the cement gradient drops to that of mixwater or, later,
when the bulk cement volume reduces. The gas can enter the cement
matrix and create channels through the cement, which cannot be
closed by cement hydration.
There are several contributing factors to gas flow during or after
cementing:

If the hydrostatic pressure falls below pore pressure during dis-
placement or before the cement is fully set.
If there exists a channel of mud within the slurry (e.g., poor mud
displacement or contaminated cement).
If a microannulus exists between formation cement or cement casing.
If gas movement into the slurry occurs, at best the cement will be
porous and at worst there will be a channel allowing gas migration
upwards to another zone or to surface. This could ultimately lead
to a blowout.

Slurry design for gas zones therefore targets the causes of the above
mechanisms.

1. Effective mud removal is an absolute prerequisite for a good cement
job. This was previously covered in detail.
2. Low fluid loss is one of the key elements in cementing gas-bearing
zones. If filtrate leaves the slurry for the formation, the resulting
volume loss will cause pressure drop in the slurry, which is likely
to allow gas flow. This is second in importance only to effective
mud removal.
3. A good cement bond between formation/cement and formation/
casing must be obtained and must not be broken. Pressure testing
casing after cement setting, or displacing the casing to a lighter
fluid after cement setting, may create a microannulus. Weak bonds
may provide a path for gas to break through. Injection/frac pres-

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