In the complicated but vital industry of drawing oil and gas out of the earth and getting it to the surface, people often get confused about the difference between a Christmas tree assembly and a wellhead. Although the terms are used interchangeably, they are not the same piece of equipment. Each has valves and related equipment that help it control and guide the flow of this precious resource.
WHICH IS WHICH?
A Christmas tree is an assembly of valves, spools and fittings used for oil, gas, water injection, water disposal, gas injection, condensate and other types of wells. It is named for its resemblance to a decorated tree at Christmas.
A wellhead must be present to utilize a Christmas tree and is used without a Christmas tree during drilling operations. The wellhead is the component at the surface of a well that provides the structural and pressure-containing interface for the drilling and production equipment. Producing surface wells that require pumps (pump jacks, nodding donkeys, etc.) frequently do not use any tree because no pressure containment is required.
The Christmas tree and the wellhead work together to bring oil and gas to the surface.
Tree complexity has increased over the last few decades. The trees are frequently manufactured from blocks of steel containing multiple valves rather than made from multiple flanged valves.
The primary function of a tree is to control the flow into or out of the well, usually oil or gas.
A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means (such as annulus vent), tree and well monitoring points (such as pressure, temperature, corrosion, erosion, sand detection, flow rate, flow composition, valve and choke position feedback, connection points for devices such as down-hole pressure and temperature transducer.)
WHAT PURPOSE DOES A TREE SERVE?
Trees function for these purposes:
- On producing wells, injecting chemicals or alcohols or oil distillates to prevent and or solve production problems (such as blockages).
- Controlling the injection of gas or water on a producing or non-producing well to sustain economic “production” volumes of gas from other wells in the area (in the field).
- The control system attached to the tree controls the downhole safety valve (surface controlled subsurface safety valves, downhole safety valve or subsurface safety valve) while the tree acts as an attachment and conduit means for the control system to the downhole safety valve.
As Figure 1 shows, there are five valves: the kill wing valve, swab valve, production wing valve, upper master valve and lower master valve. When the operator, well and facilities are ready to produce and receive oil or gas, valves are opened and the released formation fluids are allowed to flow into and through a pipeline. It is important to understand where these valves are located and what role they play in getting gas from the well bore to the customer.
The two lower valves are called the master valves (upper and lower, respectively) because they lie in the flow path, which well fluids must take to get to the surface.
The lower master valve will normally be manually operated, while the upper master valve is often hydraulically actuated.
Hydraulic tree wing valves are usually built to be fail-safe closed, meaning they require active hydraulic pressure to stay open.
The right-hand valve is often called the flow wing valve or production wing valve, because it is in the flow path the hydrocarbons take to production facilities.
The left-hand valve is often called the kill wing valve. It is primarily used for injection of fluids such as corrosion inhibitors or methanol to prevent hydrate formation.
The valve at the top is called the swab valve and lies in the path used for well interventions like wireline and coiled tubing.
The choke is the device, either stationary or adjustable, used to:
- Control the gas flow, also known as volume, or
- Create downstream pressure, also known as back pressure
Understanding the various parts of these assemblies can help oil and gas industry professionals see the differences between wellheads and Christmas trees.
JESSICA LEE is account specialist for Croft Production Systems (www.croftsystem.net, an exploration and production consulting company for the oil and gas industry. This column was excerpted from a blog entry for the company. More blog entries are at https://www.croftsystems.net/oil-gas-blog.