Christmas tree (oil well)

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In petroleum and natural gas extraction, a christmas tree is an assembly of valves, spools and fittings for an oil well, named for its resemblance to a decorated tree. The term "Christmas Tree" has now become something of a misnomer as complexity has increased. Especially in subsea applications the resemblance no longer exists given the frame and support systems into which the main valve block is integrated. It is also more usual now to determine either "Subsea Tree" or "Surface Tree".

The function of a christmas tree is to both prevent the release of oil or gas from an oil well into the environment and also to direct and control the flow of formation fluids from the well. When the well is ready to produce oil or gas, valves are opened and the release of the formation fluids is allowed through a pipeline leading to a refinery, or to a platform or to a storage vessel (known as a Floating Storage (and) Offloading Vessel or FPSO).

It may also be used to control the injection of gas or water in application on a none-producing well in order to sustain "producer" volumes.

On producing wells injection of chemicals or alcohols or oil distillates to solve production problems (such as blockages) may be used. Functionality may be extended further by using the control module on a subsea tree to monitor, measure and react to sensor inputs on the tree or even down the well bore. Crucial to safety the tree will probably also control the downhole safety valve.

Christmas trees are used on both subsea (current technical limits are up to around 2000 to 2500 metres) and surface wellheads and both are available in a wide range of sizes and configurations, such as low- or high-pressure capacity and single- or multiple-completion capacity or horizontal or vertical in their primary valve bore axis.