Bottom hole assembly

 Float Collars

A float collar is a one way valve placed at one or two joints above the shoe. The float collar
provides the same functions as a float shoe by preventing fluid back flow into the casing:
mud backflow during running in hole and cement slurry backflow after cement displacement.
The distance between the shoe and float collar is called Shoe Track. The float valve can either
be a ball type or a flapper, Figure below. Flapper type valves are normally used where a small
hydrostatic pressure difference is expected, providing a better seal than a ball type valve.

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Centralisers


A centralizer is a mechanical device attached to the outside of casing. The primary purpose of
this equipment is to center the casing in the hole and provide a uniform flow passage with
relatively equal frictional pressure losses surrounding the casing. Another major function of
centralizers in a deviated hole is to help prevent differential pipe sticking.

Cement Baskets

Cement baskets protect weak formations from excessivehydrostatic pressure exerted by the
weight of the cement column. They form a restriction against a downhole fluid motion by
reducing the flow area. They are normally installed on the casing string above weak
formations, but they are also used in stage cementing or in cementing the annulus from the
surface.

Cement Plugs

The main functions of cement plugs are, Figure below
• Separate mud from cement
• Wipe the casing from mud before cement is pumped and then wipe casing from th cement
film after the complete volume of cement is pumped.
• Prevent over-displacement of cement
• Give surface indication that cement placement is complete
• Allow the casing to be pressure tested
In effect, the cement plugs act as barriers between mud and cement providing physical separa -
tion between the two fluids. Bad cement jobs, especially around the casing shoe, result from
cement contaminated with mud.

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