The purpose of a wellhead is to:
· Support the weight of the downhole tubing.
· Safely contain the maximum surface pressure to which the well will be exposed.
· Provide a way to control the flow of well fluids.
· Permit access into the well for servicing.
· Provide a way to 'kill' the well (close it down), when necessary.
A wellhead is normally in two parts:
· the wellhead it selves,
· a collection of valves called the Christmas tree.
|Typical Wellhead Assembly|
The wellhead is built up as the well is drilled. First there is the surface casing, then the
intermediate casing strings. Lastly, there is the production casing which goes down through
the oil producing formation.
The production tubing strings are then run inside the production casing strings. The
production tubing strings are hung either in tension (stretched) or in compression. This is
calculated from the expected well flowing conditions.
When the tubing becomes hot it expands and becomes longer. When the tubing becomes
cold, during well stimulation operations, it contracts and becomes shorter. When there is
pressure in the tubing it balloons and becomes shorter.
All this movement of the tubing strings has to be compensated for at the design stage of the completion.
|Casing Spool Assembly|
|Tubing Spool Assembly|
|Christmas Tree and Wellhead|
Christmas trees can be made from components bolted together (unitized) or from a solid
block of steel.
Most of the Christmas trees are for dual completion, both onshore and offshore. Remember,
'dual completion' means taking oil from two different oil producing zones or formations. It is
more economical to produce oil from two formations through one well than to drill single
wells into each formation.
Solid block trees are used on high pressure and corrosive wells. The smaller the number of joints, the less places there are for the Christmas tree to leak.
|Single‑Completion Unitised Christmas Tree|
|Christmas Tree Assembly|
Non‑rising stem gate valves are the main type of valve used on Christmas trees and
Needle valves are used for pressure gauge points.
Each valve on the Christmas tree and wellhead is there for a specific application.
Recall why gate valves are NOT to be used for throttling or controlling flow. Gate valves are ONLY to be used to stop or start flow.
|Single Completion Solid Block Christmas Tree|
Lower Master Valve
This valve is normally in the full open position. It is the 'back‑up' for the valves higher in the Christmas tree. The lower master valve can be closed when other valves higher in the Christmas tree leak and need to be serviced or removed.
Upper Master Valve
This valve is used to open and close the well. It can be manual or it can have an actuator
fixed to it that allows opening of the valve by hydraulic or pneumatic pressure on a piston in
Large springs in the actuator will close the valve when hydraulic or pneumatic pressure is
removed or fails for any reason.
Swab or Crown Valve
This valve is at the top of the Christmas tree and allows vertical access into the well for
servicing the well. This valve is normally fully closed except when service work on the well is
There can be one or two wing valves. The wing valve is used to open or close the flow from the well to the production stream. It can be manual or it can be fitted with an actuator. The actuator works the same way as the actuator on the upper master valve.
When fitted with an actuator, the wing valve can also be called a 'Surface Safety Valve'.
|Casing‑Head‑Housing 13 5/8" 3000 Slip‑on|
Casing head housing HF 13‑5/8" x 3000 by 13‑3/8" slip
on, two 2" fern. outlet.
Gasket, self‑sealing ring RX‑57
Stud, 1‑3/8" x 10‑3/4"
Nut, hexagon, 1‑3/8" dia
Plug, bull, 2" LP x 3‑3/4"
Nipple 2" x W' LP
Valve, Cameron 'F' model 'C'
gate 2‑1/16" x 3000 WP 2" LP x 2" LP
Bull plug, 2" LP x MW' long tapped 1/2" NPT
Valve, needle, angle 1/T' x 10000 WP male x female
Slip and seal assy. CA 12 x 9‑5/W casing.
|Dual‑completion Solid Block Christmas Tree|
Each of the casing spools in the wellhead assembly have one or more gate valves attached to side
outlets on the spools.
The most important of these valves is the one which is attached to the spool that supports the
production casing. This valve gives access to the annulus between the outside of the production
tubing strings and the inside of the production casing.
The names given to these wellhead valves varies from company to company. The more commonly used names are the sizes of the 7 inch, 9 5/8 inch, 13 3/8 inch, etc.
|Tubing Hanger Assembly, 10 inch Nominal|
WELLHEAD PRESSURE GAUGE POINTS
Pressure gauge points are located at various points on the wellhead.
The one located in the production tubing/production casing annulus has the following functions:
· It is used to monitor annulus pressure.
· It indicates if a leak occurs from tubing to annulus or across packer(s) (flow) between
production tubing and production casing.
The one located on the top of the Christmas tree (Above the Christmas tree cap) has the following
· It is used to monitor well closed‑in pressure and well flowing pressure (with swab or
crown valve open).
· It indicates pressure changes due to changing well reservoir conditions or to changes in
back pressure conditions (plugged choke, washed‑out choke, etc.).
The one located in the flow line (downstream of the choke) has the following functions:
It is used to monitor flowing pressure after the choke.
It indicates pressure changes due to choke plugging or wash‑out, rupture of production flow lines,
OPENING A WELL TO PRODUCTION
This is known as 'bringing a well onstream.'
There are certain basic steps that must be done in sequence, (on after the other), when opening a
well to flow to the production stream. These step‑by‑step actions are taken for the following reasons:
· Good production control.
· To protect surface equipment.
· To protect downhole equipment.
· To prevent damage to the producing formation.
· To protect the reservoir.
Note: There will be small variations to these steps if the wells are located onshore in the desert or offshore on platforms or well towers, but the basic steps are the same.
We will assume that all the valves on the wellhead are closed.
1. Check that all drain and purge valves are closed.
2. Isolate surface safety valve pilots.
3. Open lower master valve.
4. Open upper master valve.
5. Open wing valve (manual).
The status of the wellhead now is that the swab (or crown) valve is closed. The
surface safety valve (second wing valve ) is also closed. Continue as follows.
6. Slowly open downhole safety valve to equalising position. Monitor wellhead pressure
to ensure that the pressure across the downhole safety valve equalises.
7. Fully open downhole safety valve fully when no further increase in wellhead pressure
8. With the adjustable choke set to correct size or choke bean of correct size in position,
open the surface safety valve (second wing valve) slowly to allow flow into the
9. Once wellhead flowing pressure is stabilised, reset surface safety valve pilots to
It is a very good procedure to count the number of turns taken to open or close the non‑rising stem gate valves. You can then be certain if the valves are fully open or fully closed.
Valves should always be opened slowly to prevent pressure surge downstream of the valve.
Pressure surges, (water hammer), can produce forces that can exceed the working pressure of piping and possibly cause ruptures/leaks in the lines.