A tubing head is a mechanical fitting used on the wellhead structures of crude oil wells. It is generally located in the lower section of the wellhead above the casing hanger and casing spool. The head features a profiled inner surface designed to accept the tubing hanger which supports and seals the tubing conduit. The upper section of the wellhead assembly is known as the Christmas tree, and is attached to the top of the tubing head by means of a specially-designed head adapter. The head is usually provided with several ports for the inclusion of various valves and instruments.
A wellhead is the surface termination arrangement of a subterranean crude oil well. This assembly typically consists of two multi-part sections, namely the tubing head and casing hanger section, and the Christmas tree. The hanger section is designed to support the casing and tubing strings, to terminate the wellbore during the construction phase, and to support the Christmas tree during production. The Christmas tree is a complex arrangement of valves, spools, and instruments so named because of its characteristic cluttered, crucifix shape. Its function is to control and monitor the well and its output during the production phase.
The tubing head is one of the units which make up the lower section of the wellhead assembly. It is usually the top unit in the assembly onto which the Christmas tree mounts courtesy of a specially-designed adapter. Located within the profiled interior of the head unit is the tubing hanger. The function of these two units is to support and seal the tubing conduit, or string. This is the length of tubing which runs down into the well on the inside of the outer casing. The casing hanger and spool units are located below the tubing head and support the outer casing.
The tubing support function of the head is achieved by means of locking studs along the top of the unit and the profiled inner surface of the tubing hanger. The hanger ensures that the tubing is hydraulically isolated from the annulus, or space, between it and the outer casing of the wellbore. The tubing head is also equipped with several ports and fittings used to attach a series of valves and diagnostic instruments. Although the basic design of most tubing heads are similar, well specifics may dictate some design and construction departures with differing concentrations of certain chemicals and pressure levels requiring adjustments to the materials and features of the units.