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thing, the crew does not cement a tubing string in the well. Accordingly, when a joint
of tubing fails, as it almost inevitably will over the life of a well, the operator can
easily replace the failed joint or joints or, in the case of coiled tubing, remove and
repair or replace the failed area. Since casing is cemented, it is very difficult to
replace.
For another thing, tubing allows the operator to control the well's production by
placing special tools and devices in or on the tubing string. These devices allow the
operator to produce the well efficiently. In some cases, the operator can produce the
well only by using a tubing string. Casing does not provide a place to install any tools
or devices that may be required for production. In addition, the operator installs
safety valves in the tubing string. These valves automatically stop the flow of fluids
from the well if damage occurs at the surface.
Finally, tubing protects the casing from the corrosive and erosive effects of
produced fluids. Over the life of a well, reservoir fluids tend to corrode metals with
which they are in contact. By producing fluids through the tubing, which the operator
can easily replace, the casing, which is not so easy to repair or replace, is preserved.
Crew members usually run tubing into the well with a sealing device called a
"packer." They install the packer on the tubing string and place it at a depth slightly
above the casing perforations. The end of the tubing is left open or is perforated and
extends to a point opposite the perforations in the casing. The packer expands and
grips the wall of the production casing or liner. When expanded, the packer seals the
annular space between the tubing and the casing above the perforations. The
produced fluids flow through the perforations and into the tubing string. The packer
prevents them from entering the annular space, where they could eventually corrode
the casing.
After the crew runs the tubing string, the operator has a crew install a collection
of fittings and valves called a Christmas tree (fig. 183) on top of the well. Tubing
hangs from the tree so the well's production flows from the tubing and into the tree.
Valves on the Christmas tree allow the operator to control the amount of production
or to shut in the well completely to stop it from producing. They also allow the
operator to direct the flow of production through various surface lines as required. In

addition, a special safety valve on the tree automatically shuts in the well if the tree is
damaged. This automatic shut-in valve prevents reservoir fluids from flowing onto
the surface if damage occurs. Usually, once the crew installs the Christmas tree, the
well is complete.
Acidizing
Hydrocarbons sometimes exist in a formation but cannot flow readily into the
well because the formation has very low permeability. If the formation reacts
favorably to acid, acidizing may improve flow. An acidizing service company can
pump anywhere from 50 to thousands of gallons (or litres) of acid down the well's
tubing. The acid, to which the acidizing company adds a chemical to prevent it from

corroding the tubing, enters the perforations and contacts the formation. Continued
pumping forces the acid into the formation, where it etches channels. These channels
provide a way for the formation hydrocarbons to enter the well through the

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