The casing spool represents the upper resting place of a casing string.
The spool forms part of the wellhead assembly, specifically it facilitates the casing string resting there, its load being distributed to and supported by the ground. The casing hanger sits within the casing spool’s bowl, with it resting on the load shoulder.
Casing spools are usually flanged on either end, the sizes of these flanges need not be the same. The spools are stacked and bolted together vertically allowing for several casing strings to be held and supported concentrically with smaller strings being held higher in smaller spools.
The lowest portion of the spool, is a secondary seal area for the previous casing string, these seals will sit above the previous strings’ casing hanger, further sealing the annulus and isolating the annulus of the previous string from that of the current. As a result of the geometry of the spool only the string it is designed for or smaller will pass beyond the load shoulder. Above this will sit the casing hanger, thus isolating the current annulus from the next or simply terminating the annulus altogether preventing fluid from communicating with the bore.
Below the casing hanger however we have in most cases ports, that may be either threaded for a direct connection of pipe or valve or port, or we have a flanged connection to facilitate the attachment of a flanged connection.
In the case where the casing spool terminates the wellhead assembly, then the spool is open through the bore of the previous casing string, with the two ports allowing access to the fluid flow through the bore. If a string is seated in the spool the ports will have access to the casing annulus, depending on the well configuration or completion strategy this may be an asset or part of the design. In most, if not all, conventional cases the annulus pressure should be zero, as this will be cemented with no fluid flow possible.
If you have a completion that uses tubing, then the last spool will be a tubing spool, this is identical to the casing spool in geometry and features. In this case though the bore will only facilitate the passing/running of tubing. In most conventional completions that use tubing there is no cementing of the annulus, thus the ports on the tubing spool would have access to the annulus and thus permit fluid flow. Depending on the completion strategy, their will be a pressure reading on the annulus.
Above the tubing spool, is a tubing head, which crosses over from the top flange to the connection required to attach the production tree and valve assembly.
