A wellhead stack is a set of stacked pressure-containing spools and hangers, and the names of the main components — casing head, tubing head, hanger — get used constantly without always being explained. Here’s what each one actually does, and how they fit together.
The basic idea: a wellhead is stacked, not built as one piece
Every casing string run into a well — conductor, surface, intermediate, production — needs to be supported at surface and sealed off from the strings around it. Rather than one part doing all of that, a wellhead is built as a stack of components, each landing on and sealing against the one below, with the stack growing as each new casing string is run and cemented. By the time the well is producing, what started as a single conductor casing head has grown into a full multi-bowl stack with the production tree on top.
Casing head
The casing head is the base of the stack — the first pressure-containing spool, welded or threaded onto the surface casing once it’s run and cemented. It provides the foundation the rest of the wellhead stack lands on, a bowl profile that supports the next casing hanger, and outlets for early well-control equipment. Everything above it depends on the casing head being correctly rated for the loads the full, eventual stack will impose — which is why it’s sized for the final well design, not just the string it’s landed on at the time.
Casing hangers
A casing hanger sits inside the bowl of the casing head (or the spool above it) and does two jobs at once: it supports the full weight of the casing string hanging below it, and it provides the sealing surface that isolates that string’s annulus from the one above. Each new string run into the well gets its own hanger, landed in its own spool, which is why the wellhead grows one section taller with every casing string — conductor, surface, intermediate, and production hangers stacking upward in the sequence the strings were run.
Tubing head
Once the production casing is landed, the tubing head sits on top of it — the transition point between the casing stack below and the production tree above. It supports the tubing hanger (which carries the production tubing string and seals off the casing-tubing annulus, the same way a casing hanger seals off a casing annulus) and provides the outlets used for annulus monitoring and access once the well is producing. It’s the last component in the “casing” half of the stack before the equipment shifts to controlling produced flow rather than just supporting and sealing strings.
Adapters and spools
Adapters and spools connect components with different flange sizes, pressure ratings, or bolt patterns — stepping from a lower-pressure surface casing head up to a higher-pressure tubing head, for instance, or connecting a wellhead built to one manufacturer’s dimensions to a BOP stack or tree built to another’s. They’re not just spacers: each one is a pressure-containing component in its own right and has to be rated for the full stack pressure and any structural load passing through it, not just the connection it happens to sit between.
Why the sequence matters for design
Because each component lands on and depends on the one below it, wellhead design has to work from the top of the stack down to the conductor, even though it gets installed from the bottom up: the final tree weight, the number of casing strings, their sizes and pressure ratings, and the loads from wellhead growth during production all have to be known before the casing head at the base is ever sized. Get that sequence wrong early and every component above it inherits the error.
We design casing and tubing heads, hangers, adapters, and complete wellhead stacks to API 6A. See our Design & Engineering services, browse more terms in our Glossary, or talk to an engineer about a wellhead design.