In oil and gas extraction, a Christmas tree is an assembly of valves, spools, pressure gauges and chokes fitted to the wellhead of a completed well to control production. Christmas trees are available in a wide range of sizes and configurations. A Christmas tree can also be fitted to a gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells in the oil and gas industry. It was named for its resemblance to a decorated tree you would find during the holidays.
Christmas trees are used on both surface and subsea wells. It is common to identify the type of tree as either “subsea tree” or “surface tree.”
Please note that a Christmas tree is not the same equipment as the wellhead, it is a different separate piece of equipment, but are both installed on top of the well. A wellhead can be used without the Christmas tree during drilling operations. The Christmas tree is installed on top of the wellhead. As the well production flows up the tubing, it enters the Christmas tree. The oil and gas production can be controlled by opening or closing valves on the Christmas tree.
Therefore, the primary function of a Christmas tree is to control the flow of oil and gas out of the well. A tree may also be used to control the injection of gas or water into a non-producing well in order to enhance production rates of oil from other wells. When the well and facilities are ready to produce and receive oil or gas, tree valves are opened and the formation fluids are allowed to go through a flow line. This leads to a processing facility, storage depot and/or other pipeline eventually leading to a refinery or distribution center. Flow lines on subsea wells usually lead to a fixed or floating production platform or to a storage ship or barge, known as a floating storage offloading vessel (FSO), or floating processing unit (FPU), or floating production, storage and offloading vessel (FPSO).
“Oil and Gas Production in Nontechnical Language”. PennWell Publishing