What Is a Wellhead?

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What a Wellhead Does

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.

Main Wellhead Components

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.

Monitoring the Wellhead in Production

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.

Instrumentation Points Around a Wellhead

A producing wellhead offers a small set of measurement points that together tell most of the story. Each is a tap where a local gauge or a pressure transmitter can be mounted:

Measurement point	What it tells you
Tubing pressure	Producing pressure the well delivers against the surface equipment
A-annulus pressure (tubing by production casing)	Integrity of the tubing string and its packer or seals
Outer annulus pressures	Condition of the cemented casing barriers
Flowline pressure	Downstream restriction, hydrate or wax buildup, or a closed valve
Wellhead or flowline temperature	Presence of flow and freeze or hydrate risk

Which of these get continuous transmitters and which stay as local gauges is a site-specific economics call, but tubing, A-annulus, and flowline pressure are the usual first three, because between them they distinguish a well problem, a barrier problem, and a downstream problem.

Sustained Casing Pressure

Annulus pressure that returns after being bled down is called sustained casing pressure, and it is the signature of a communication path into the annulus - a leaking tubing connection, a failed seal assembly, or gas migrating past a cement sheath. It is distinguished from benign thermal pressure, which builds as a shut-in annulus warms and does not keep rebuilding once temperature stabilizes, by monitored bleed-down and build-up observations run under the operator's well integrity program.

Diagnosis of a pressured annulus, and any intervention on it, sits firmly with qualified well integrity personnel working to the site's procedures; the monitoring side's job is to make the behavior visible. Trending each annulus pressure continuously, rather than reading it on occasional site visits, is what turns a slow rebuild over weeks from an invisible drift into an obvious ramp that gets investigated early.

The Wellhead Under Artificial Lift

Most wells do not flow unassisted for their whole life, and the lift method changes what sits on and around the wellhead. A rod-pumped well adds a stuffing box at the top of the tree to seal around the reciprocating polished rod. A plunger lift well adds a lubricator and catcher assembly above the master valve to receive the arriving plunger. Gas lift injects supply gas into a casing annulus through a wing valve, which makes that annulus an active flow path rather than a static sealed space.

Each arrangement brings its own measurement and failure modes - a leaking stuffing box, a plunger that fails to arrive, injection pressure drifting from plan - which is why the instrumentation set at a wellhead tends to grow when a well moves onto lift. The broader families of lift and their surface equipment are covered in the artificial lift overview.

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