A float collar (Figure 6) is positioned 1 or 2 joints above the guide shoe. It acts as a seat for the cement plugs used in the pumping and displacement of the cement slurry. This means that at the end of the cement job there will be some cement left in the casing between the float collar and the guide shoe which must be drilled out.
The float collar also contains a non-return valve so that the cement slurry cannot flow back up the casing. This is necessary because the cement slurry in the annulusgenerally has a higher density than the displacing fluid in the casing, therefore a U-tube effect is created when the cement is in position and the pumps are stopped.Sometimes the guide shoe also has a non-return valve as an extra precaution. It is essential that the non-return valves are effective in holding back the cement slurry.
The use of a non-return valve means that as the casing is being run into the borehole the fluid in the hole cannot enter the casing from below. This creates a buoyancy effect which can be reduced by filling up the casing from the surface at regular intervals while the casing is being run (every 5 – 20 joints). This filling up process increases the running in time and can be avoided by the use of automatic or differential fill up devices fitted to the float collar or shoe. These devices allow a controlled amount of fluid to enter the casing at the bottom of the string. The ports through which the fluid enters are blocked off before the cement job begins. The use of a differential fill-up device also reduces the effect of surge pressures on the formation.