The wellhead is the surface termination of an oil or gas well - the pressure-containing assembly that sits at ground level (or on the seabed) where the drilled well meets the surface equipment. It anchors the casing strings, seals the annular spaces between them, and provides the foundation the Christmas tree bolts onto. This guide explains what a wellhead is, its main components, and how it fits into surface production.
Wellhead in one line: A wellhead is the pressure-containing assembly at the surface of an oil or gas well that supports the casing and tubing strings, seals the annuli between them, and provides the connection point for the Christmas tree and surface flow equipment.
As a well is drilled, successive strings of steel casing are run and cemented into place, each one hung and sealed at surface. The wellhead is the stack of forged components that hangs those strings, seals the annular spaces between them, and contains the pressure that can exist in each annulus. It gives the well structural integrity at surface and a controlled boundary between the reservoir and the atmosphere.
A wellhead is built to hold high pressure - ratings are commonly expressed in psi (for example 3,000, 5,000, 10,000, or 15,000 psi) per API Spec 6A. It provides access points, called annulus valves or wing valves, to monitor and bleed pressure from each casing annulus, which is important for well integrity and for spotting sustained casing pressure.
From the bottom up, a typical wellhead includes the casing head (or casing spool), which lands on the surface casing and supports the next string; one or more intermediate casing spools; and the tubing head (tubing spool), which supports the production tubing and seals the tubing-by-casing annulus. Casing and tubing hangers inside these spools carry the weight of the strings, and packoffs or seal assemblies isolate the annuli.
Above the tubing head sits the Christmas tree, the valve assembly that controls flow out of the well. The wellhead is the static, pressure-sealing foundation; the tree is the dynamic flow-control stack on top of it. Together they form the complete surface pressure barrier.
Once a well is on production, the pressures at the wellhead are key operating data. Tubing pressure, casing pressure, and flowline pressure are read by transmitters and fed to a control system so operators can watch for abnormal conditions - a rising annulus pressure that signals a failing seal, or a flowline pressure spike that indicates a downstream restriction.
In a distributed field, an RTU or flow computer at each wellsite reads these pressures and reports them to SCADA. A cloud-native platform such as Merobix polls those field devices over Modbus or DNP3 so wellhead pressures and states are visible remotely, without a trip to the location.
The wellhead is the pressure-sealing foundation at surface that hangs and seals the casing and tubing strings. The Christmas tree is the assembly of valves and fittings that bolts on top of the wellhead to control flow out of the well. In casual use people call the whole stack the wellhead, but strictly they are two parts.
A casing head (or casing spool) that lands on the surface casing, one or more intermediate casing spools, and a tubing head that supports the production tubing. Inside sit casing and tubing hangers and seal assemblies that isolate each annulus. Annulus and wing valves provide access to each space.
Tubing, casing, and flowline pressures reveal how the well is performing and whether its barriers are intact. Rising annulus pressure can signal a leaking seal or tubing failure, so operators monitor it continuously to protect well integrity and catch problems early.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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