In the oil and gas industry, a Christmas tree is referred to as a series of valve & spool assembly fitted on top of the well. A Christmas tree is installed on top of the last casing spool on a surface well or the high pressure wellhead housing for a subsea well. Figure 1 demonstrates the diagram of a Christmas tree and wellhead of a surface wellhead. The Christmas part is located at the top part (a blue box) and the wellhead part is the lower section (a red box). Many people get confused about what a Christmas tree and a wellhead are and many times they think of them as the same thing.
Its functions are as follows;
- Allow reservoir fluid to flow from the well to the surface safely in a controlled manner.
- Allow safe access to the wellbore in order to perform well intervention procedures.
- Allow injections as water or gas injection.
- Provide access to hydraulic line for a surface control sub surface safety valve (SCSSSV)
- Provide electrical interface for instrumentation and electrical equipment for electrical submersible pump (ESP)
In this section, it will describe about a surface Christmas tree (Dry Tree) which is referred to as any Christmas tree used above water level. A Christmas Tree consists of a series of valves and the components are shown and described below;
A master valve is located above the tubing hanger and its function is to allow the well to flow or shut the well in. Typically, there are two master valves. One is called a lower master valve and another is an upper master valve. Two valves are often used because they provide redundancy. If one master valve cannot function properly, another valve can perform the function. Figure 2 and Figure 3 shows a simple diagram of a single and two master valves, respectively.
T type fitting (T-Block)
T type fitting (T-Block) allows diversion of flow stream from vertical to a horizontal flow line.
Wing Valve (Flowing Wing)
A wing valve is located on the side of a Christmas tree and it is used to control or isolate production from the well into surface facilities. Depending on each design of a Christmas tree, it can be equipped with one or two wing valves. Some operators require two production wing valves, one as a main production and another one as a backup (Figure 5). In many cases, one wing valve is used for production and another wing valve is used as a kill wing valve (Figure 6).
Choke is the smallest restriction in a Christmas tree, and its function is to control the production rate of a well. It is also use to control sand production in some cases. A choke restricts areas for production flow through a bean or an orifice inserted into a choke body. The smaller diameter of the beam results in the lower the production rate. Two types of chokes are 1) positive choke with interchangeable beans 2) adjustable choke which allows adjusting the choke size easily.
On a Christmas tree, a swab valve is the topmost valve providing vertical access to the well for well intervention operations conducted by wireline, slickline, coiled tubing or a snubbing unit.
T-Cap and Pressure Gauge
T-Cap is a flange located on top of the swab valve which allows a wireline lubricator or a coiled tubing/ snubbing unit BOP to connect to a well in order to perform well intervention programs. A pressure gauge is used to monitor the pressure of the well. Nowadays, most of the operators often use electronic gauges so pressure and/or temperature data can be transmitted via an electronic system for better well monitoring.
Unitized Christmas Tree
A unitized Christmas tree is an integrated Christmas tree which consists of a lower and upper master valve, and a swab valve in one body. This will allow an operator to save operational time for installation.
Jr. Adam T. Bourgoyne, 1986. Applied Drilling Engineering (Spe Textbook Series, Vol 2). Edition. Society of Petroleum Engineers.
J.J. Azar, 2007. Drilling Engineering. Edition. PennWell Corp.
The Australian Drilling, 1997. Drilling: The Manual of Methods, Applications, and Management. 4 Edition. CRC Press.
Steve Devereux, 1999. Drilling Technology in Nontechnical Language. Edition. Pennwell Pub.