The main cause for poor isolation after a cement job is the presence of mud channels in the cement sheath in the annulus. These channels of gelled mud exist because the mud in the annulus has not been displaced by the cement slurry. This can occur for many reasons. The main reason for this is poor centralisation of the casing in the borehole, during the cementing operation. When mud is being displaced from the annulus the cement will follow the least path of resistance. If the pipe is not properly centralised the highest resistance to flow occurs where the clearance is least. This is where mud channels are most likely to occur (Figure 18).

In addition, field tests have shown that for a good cement bond to develop the formation should be in contact with the cement slurry for a certain time period while the cement is being displaced. The recommended contact time (pump past time) is about 10 minutes for most cement jobs. To improve mud displacement and obtain a good cement bond the following practices are recommended:

• Use centralisers, especially at critical points in the casing string
• Move the casing during the cement job. In general, rotation is preferred to
reciprocation, since the latter may cause surging against the formation. A specially
designed swivel may be installed between the cementing head and the casing to
allow rotation. (Centralisers remain static and allow the casing to rotate within
them.)
• Before doing the cement job, condition the mud (low PV, low YP) to ensure
good flow properties, so that it can be easily displaced.
• Displace the spacer is in turbulent flow. This may not be practicable in large
diameter casing where the high pump rates and pressures may cause erosion or
formation breakdown.
• Use spacers to prevent mud contamination in the annulus.

chanelling

Institute of Petroleum Engineering, Heriot-Watt University

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