Hole preparation in cementing

The predominant cause of cementing failure appears to be channels of gelled drilling fluid remaining in the annulus after the cement is in place. If drilling-fluid channels are eliminated, any number of cementing compositions will provide an effective seal. Proper hole preparation is the key to success.

Flow in an eccentric annulus

In evaluating factors that affect the displacement of drilling fluid, it is necessary to consider the flow pattern in an eccentric annulus (i.e., where the pipe is closer to one side of the hole than the other). Flow velocity in an eccentric annulus is not uniform, and the highest velocity occurs in the side of the hole with the largest clearance.

If the casing is close to the wall of the hole, it may not be possible to pump the cement at a rate high enough to develop uniform flow throughout the entire annulus (Fig. 1). To reduce the chances for eccentricity, centralizers should be used to maintain the pipe in the center of the annulus.

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  Fig. 1—Effect of eccentricity on cement flow in the annulus.

Standoff

100% standoff

This shows a hole with casing that is exactly centralized in the hole. The shaded areas are the cement, and it shows the cement level is the same on both sides of the casing.

75% standoff

This shows a hole with the casing decentralized to a 75%; standoff, and it shows that as you decentralize the casing the flow is higher up the wide side of the hole compared to the narrow side.

50% standoff

This is the same as the 75%; standoff, only more pronounced with the cement height.

Additional considerations

In preparing the hole for cementing, the following factors should also be considered:

• Circulability treatment
• Spacers and flushers
• Contact time
• Sweep pill design and analysis.

See also

Cementing operations

PEH:Cementing