Float Joint

A Float Joint is a full-sized length of casing placed at the bottom of the casing string that is usually left full of cement on the inside to ensure that good cement remains on the outside of the bottom of the casing. If cement were not left inside the casing in this manner, the risk of over displacing the cement (due to improper casing volume calculations, displacement mud volume measurements, or both) would be significantly higher. Hence, the well designer plans on a safety margin of cement left inside the casing to guarantee that the fluid left outside the casing is good-quality cement. A float collar is placed at the top of the float joint and a float shoe placed at the bottom to prevent reverse flow of cement back into the casing after placement. There can be one, two or three joints of casing used for this purpose. (Source: Schlumberger Oilfield Glossary)