Wellhead

For classical wellheads for water wells, see puteal.

A wellhead is a general term used to describe the component at the surface of an oil well that provides the structural and pressure containing interface for the drilling and production equipment.

The primary purpose of a wellhead is to provide the suspension point and pressure seals for the casing strings that run from the bottom of the hole sections to the surface pressure control equipment.

Whilst drilling the oil well the surface pressure control is provided by a Blowout preventer or 'BOP'. If the pressure is not contained during drilling operations by the casings, wellhead and BOP, then a well blowout can occur.

Once the well has been drilled, a completion is placed in the well that provides an interface with the reservoir rock and the tubular conduit for the well fluids. The surface pressure control is provided by a christmas tree which is installed on top of the wellhead, and has isolation valves and choke equipment to control the flow of well fluids during production.

Wellheads are typically cemented in place during drilling operations and form an integral structure of the well. In exploration wells which are abandoned, the wellhead may be recovered for refurbishment and re-use.

Where wellheads are located on the production oil platforms/onshore they are called a surface wellhead, and if located beneath the water then are referred to as a subsea wellhead or mudline wellhead.

Functions

A wellhead serves numerous functions; some of these are:

1. Provide a means of casing suspension. (Casing is the permanently installed pipe used to line the well hole for pressure containment, collapse prevention during the drilling phase)
2. Provides a means of tubing suspension (Tubing is removable pipe installed in the well through which well fluids pass)
3. Provides a means of pressure sealing and isolation between casing at surface when many casing strings are used.
4. Provides access to annuli between the different casing/tubing strings.
5. Provides a means of attaching a blowout preventer during drilling
6. Provides a means of attaching a christmas tree for well control during production, injection, or other operations
7. Provides a reliable means of well access
8. Provides a means of attaching a well pump

Components

The primary components of a wellhead system are:

• casing head
• casing spools
• casing hangers
• packoffs (isolation) seals
• bowl protectors / wear bushings
• test plugs
• mudline suspension systems
• tubing heads
• tubing hangers
• tubing head adapters

Design Specification

The oil industry specifications for wellhead systems (materials, dimensions, test procedures and pressure ratings etc.) are :

1. API 6A Specification for Wellhead and Christmas Tree Equipment
2. ISO 10423 Wellhead and Christmas Tree Equipment

See also

• Pumpjack
• American Petroleum Institute
• List of oilfield service companies
• Drilling rig (petroleum)

External links

• Schlumberger Oilfield Glossary: wellhead