Oil and gas production handbook An introduction to oil ... - ABB Group

well to add multiple completions to drain the formation most
effectively. When it is necessary to separate different completions,
hard rubber “packing” instruments are used to maintain separation.
• Drainhole completions are a form of horizontal or slanted drilling.
This type of completion consists of drilling out horizontally into the
formation from a vertical well, essentially providing a drain for the
hydrocarbons to run down into the well. These completions are more
commonly associated with oil wells than with natural gas wells.
3.5 Wellhead
Wellheads can involve dry or subsea completion. Dry completion means that
the well is onshore or on the topside structure on an offshore installation.
Subsea wellheads are located underwater on a special sea bed template.
The wellhead has equipment
mounted at the opening of the well to
regulate and monitor the extraction of
hydrocarbons from the underground
formation. This also prevents oil or
natural gas leaking out of the well,
and prevents blow-outs due to high
pressure formations. Formations that
are under high pressure typically
require wellheads that can withstand
a great deal of upward pressure from
the escaping gases and liquids.
These must be able to withstand
pressures of up to 140 MPa (1,400
Bar). The wellhead consists of three
components: the casing head, the
tubing head, and the “Christmas tree.”
Photo: Vetco Gray
A typical Christmas tree, composed of
a master gate valve, a pressure
gauge, a wing valve, a swab valve
and a choke is shown above. The
Christmas tree may also have a number of check valves. The functions of
these devices are explained below. Ill: Vetco Gray
At the bottom we find the casing head and casing hangers.
The casing is screwed, bolted or welded to the hanger. Several valves and
plugs are normally fitted to give access to the casing. This permits the casing
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