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cement seal the producing zone, the operator has to provide a way for oil and gas to
get from the formation and into the well. Usually, the operator hires the services of a
completion rig, which is a relatively small portable rig whose crews perform the final
operations required to bring the well into production (fig.179).
One important task is to perforate the well. A special gun shoots several
relatively small holes in the casing. It makes them in the side of the casing opposite
the producing zone. These holes, or perforations (fig. 180), pierce the casing or liner
and the cement around the casing or liner. The perforations go through the casing and
the cement and a short distance into the producing formation. Formation fluids,
which include oil and gas, flow through these perforations and into the well.
The most common perforating gun uses shaped charges, similar to those used in
armor-piercing shells. Several high-speed, high-pressure jets of gas penetrate the steel
casing, the cement, and the formation next to the cement. A perforating specialist
installs the charges in the special gun and lowers it—usually on wireline, rather than
drill pipe—into the well to the desired depth. The depth can be determined by
running a collar locator log, which identifies the depth of each casing collar. By
comparing the log with the overall number and length of the casing joints, the
operator can accurately determine the depth. Once at the desired depth, the
perforating specialist fires the gun to set off the charges (fig. 181). After the gun
makes the perforations, the perforating specialist retrieves it.

Running tubing and installing the Christmas tree

After the well is perforated, oil and gas can flow into the casing or liner. Usually,
however, the operator does not produce the well by allowing hydrocarbons to flow up
the casing or liner. Instead, the completion rig crew places small-diameter pipe called
"tubing" inside the cased well. In fact, the operator sometimes runs tubing into the
well before perforating it. In such cases, the perforating gun is lowered through the
tubing to the required depth.
Tubing that meets API specifications has an outside diameter that ranges from
1.050 inches (26.7 millimeters) to 4 ½ inches (114.3 millimeters). Seven sizes
between the two extremes are also available. As it does with casing, the crew
commonly uses couplings to join tubing, although an integral joint tubing is available
that allows the crew to make up joints without using couplings.
Manufacturers also supply coiled tubing. Coiled tubing is a continuous length—
it does not have joints—of flexible steel pipe that comes rolled on a large reel.
Operators have completed wells over 20,000 feet (6,000 meters) deep with coiled
tubing.
Special equipment placed at the top of the well allows crew members to insert,
or inject, the tubing into the well as they unwind it from the reel (fig. 182). The main
advantage of coiled tubing is that crew members do not have to connect several
single joints of tubing when installing the string. Consequently, coiled tubing takes

considerably less time to run.
Whether using jointed or coiled tubing, the operator usually produces a well
through a tubing string rather than through the casing for several reasons. For one

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