Wellhead and Christmas Tree
A wellhead is the component on the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment.
The main purpose of a wellhead is to provide the suspension point and pressure seals for the casing strings running from the bottom of the wellbore to the surface pressure control equipment.
During drilling of the oil well, the surface pressure is controlled by a blowout preventer (BOP). If the pressure during drilling is not controlled by the column of drilling fluid, the casings, the wellhead and the BOP, a blowout of the well may occur.
When the well is drilled, it is completed to establish an interface with the reservoir rock and a tubular channel for the well fluids. Surface pressure control is provided by a christmas tree installed on top of the wellhead, with isolation valves and choke devices to control the flow of well fluids during production.
Wellheads are typically welded to the first string of casing, cemented in place during drilling, to form an integral structure of the well. For exploration wells that are later abandoned, the wellhead can be reclaimed for refurbishment and reuse.
Offshore, a wellhead located on the production platform is called a surface well, and if it is underwater a subsea well or mudline well.
Design specification
The oil industry specifications for wellhead systems (materials, dimensions, test procedures and pressure ratings etc.) are:
API 6A, Specification for Wellhead and Christmas Tree Equipment
ISO 10423, Wellhead and Christmas Tree Equipment
What are Christmas tree valves?
Christmas tree valves are a system of valves, chokes, coils and meters that, not surprisingly, resemble a Christmas tree. It is important to note that Christmas tree valves are separate from wellheads and are the bridge between what happens below the well and what happens above the well. They are placed on top of wells after production has started to direct and control the product out of the well.
These valves also serve many other purposes, such as pressure relief, chemical injection, safety equipment monitoring, electrical interfaces for control systems and more. They are typically used on offshore oil platforms as subsea wells, as well as surface trees. This range of components is required for the safe extraction of oil, gas and other fuel resource(s) deep in the earth, providing a central connection point for all aspects of the well.
