Page 32 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

way deeper underground, or it may be more desirable in a horizontal
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 under water 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 (1400 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 here. The
Christmas tree may also have a number of check valves. The functions of
these devices are explained in the following paragraphs. Ill: Vetco Gray

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