“Wellhead” and “Christmas tree” get used interchangeably in casual conversation often enough that it’s worth clearing up — they’re two distinct pieces of equipment, built to somewhat different (though related) standards, doing two different jobs, and understanding the split matters if you’re specifying, reviewing drawings, or troubleshooting equipment.
The wellhead: structure and pressure containment
The wellhead is the stack of housings, spools, and hangers installed as the well is drilled and completed: the casing head welded to the first casing string, casing spools for each subsequent string, and the tubing head spool on top that lands the tubing hanger. Each component suspends and seals its respective string, and provides side outlets for annulus access. The wellhead is, structurally, what the entire well is built on — it’s installed in stages during drilling, not all at once.
The Christmas tree: flow control
The Christmas tree is installed after the well is completed and ready to produce. It bolts onto the top of the tubing head spool and provides the valves needed to control production: master valve, swab valve, wing valves, and typically a choke. Unlike the wellhead, the tree is a single assembly installed as one unit, and in some workover scenarios it can be removed and replaced independently of the wellhead underneath it.
A simple way to keep them straight
Think of the wellhead as the foundation and the tree as what sits on top of that foundation once the building is finished. If a well is being drilled or a casing string is being run, you’re working with the wellhead. If the well is completed and producing, and you’re talking about controlling or metering flow, you’re talking about the tree. Both are typically governed under API 6A, but they’re distinct products with distinct part numbers, distinct pressure/temperature/material class qualifications, and distinct maintenance histories.
Where the confusion actually causes problems
Mixing the two up matters most in procurement and documentation — ordering “a wellhead” when a Christmas tree replacement was actually needed (or vice versa) wastes time and, in an emergency intervention, wastes time you don’t have. It also matters for re-qualification scope: an aging wellhead housing and an aging tree on the same well can be on completely different inspection and replacement schedules, since they were installed at different times and see different service conditions.
For a closer look at tree types specifically, see What Is a Christmas Tree in Oil and Gas? and Wellhead Equipment 101 for the housing and hanger components themselves.
Need design, analysis, or re-qualification support for wellhead or tree equipment? See our Design & Engineering services, or talk to an engineer.