A wellhead is the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment.
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The primary purpose of a wellhead is to provide the suspension point and pressure seals for the casing strings that run from the bottom of the hole sections to the surface pressure control equipment.[1]
While drilling the oil well, surface pressure control is provided by a blowout preventer (BOP). If the pressure is not contained during drilling operations by the column of drilling fluid, casings, wellhead, and BOP, a well blowout could occur.
When the well has been drilled, it is completed to provide an interface with the reservoir rock and a tubular conduit for the well fluids. The surface pressure control is provided by a Christmas tree, which is installed on top of the wellhead, with isolation valves and choke equipment to control the flow of well fluids during production.
Wellheads are typically welded onto the first string of casing, which has been cemented in place during drilling operations, to form an integral structure of the well. In exploration wells that are later abandoned, the wellhead may be recovered for refurbishment and re-use.
Offshore, where a wellhead is located on the production platform it is called a surface wellhead, and if located beneath the water then it is referred to as a subsea wellhead or mudline wellhead.[2][3][4][5]
The primary components of a wellhead system are:
A wellhead serves numerous functions, some of which are:
The oil industry specifications for wellhead systems (materials, dimensions, test procedures and pressure ratings etc.) are :
In general well heads are five nominal ratings of wellheads: 2, 3, 5, 10 and 15 (×) psi working pressure. They have an operating temperature range of −50 to +250 degrees Fahrenheit. They are used in conjunction with ring type seal gaskets.
In general the yield strength of the materials range from to psi.
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After the digging of oil or gas well is finished, a set of wellhead equipments that can control and regulate oil and gas production is required:
(1) Suspending the weight of all tubing supports in the well;
(2) Seal the annular space between tubing and casing;
(3) Control and regulate the production of oil well;
(4) Ensure all underground operations, such as major repair and minor repair;
(5) It can be usd to record oil pressure and casing pressure data, measure pressure, paraffin removal and other daily production management.
Oil and gas well wellhead equipments from professional oil well equipment manufacturers can be divided into three types: flowing well wellhead equipments, pumping well wellhead equipments and gas well wellhead equipments.
The same principle of well tubing suspension goes for the three kinds of wellhead equipments. Today, oilfields generally adopt the jacking valve suspension method, that is, a jacking valve is installed on the casing four-way, and there is a tapered channel in the jacking valve. The tubing hanger is also a cone.
With a sealing ring, the tubing suspension sits firmly in the seat of the jacking valve under the pull of the weight of the entire wellbore tubing. The function of the jack wire is to prevent the tubing string from being pushed out when the pressure in the well is too high.
During workover operations, after killing the well, remove the wellhead equipment above the casing spool firstly, then loosen the jack wire and take out the tubing before starting to construct according to the design. After the construction is completed, the tubing is seated in the tubing suspension with the jacking valve tightened, and the wellhead device is installed. So far, the construction is completed.