Float Collars Information
Float collars are a cementing component installed near the bottom of the casing string. Their primary functions are to serve as a landing spot for the cement plug during primary well cementing and to prevent fluid flow-back during well cementing and case running operations.
Float collars are a short section of casing with tool joints on both ends. They incorporate an integral backpressure valve which is most often a spring-loaded ball valve or flapper-type. The valve prevents flow back of cement slurry during cementing and also helps "float" the casing string during case running operations.
They mate with the casing string one to three sections above the guide shoe, a device that directs the casing string through the wellbore on descent. Learn more about guide shoes by visiting GlobalSpec's area specification guide for guide shoes (here).
During case running operations the casing string is inserted into an open wellbore. The derrick supports the weight of the top portion of the casing string. As additional sections of casing are connected and fed into the wellbore, the back-pressure valve in the float collar displaces reservoir fluids that would otherwise flow up through the bore of the casing string. This helps control the rate of descent and supports a portion of the load.
Cementing operations seal and secure the top portion of the casing string, referred to as the conductor casing, to an open wellbore. A cementing unit incorporating large slurry pumps displaces cement slurry down through the bore of the casing string and back up through the annulus (the void between the casing string and the open well bore).
The float collar serves two important functions during cementing:
They act as a landing post for cement plugs. Typically two cement plugs, top and bottom, are fed through the casing string before and after the cement slurry. The bottom plug mates with the float collar, then a spike in pressure bursts a diaphragm in the bottom plug and the cement slurry begins to fill in the annulus. After the cement slurry has been pumped through the wellbore the top plug mates with the seat of the bottom plug. The surface operator has positive indication that the slurry has been properly displaced when fluid pressure rises and pumping commences.
The float collar then prevents reverse flow of cement slurry through the casing bore. The displacement mud used to drive the top cement plug down through the wellbore and push remaining slurry up through the annulus is not as dense as the cement slurry. The integral back pressure valve in the float collar is required in order to prevent flow back of cement slurry up through the casing string.
API RP 10F -- Evaluating the performance of cementing float equipment
API RP 65 -- Cementing shallow water flow zones in deep water wells