OCTG (Oil Country Tubular Goods) is the name commonly applied to identify the pipes used in the oil and gas industry. These pipes can serve different purposes in
the wells, such as producing or injecting oil, gas, steam and water. Depending on their application, they can be classified as casing or tubing. Casing preserves the
integrity of the well during its entire life, while tubing is used either to inject or to produce fluids.
To select the appropriate pipe, that is, the Outside Diameter (OD), the Weight and the Grade for each application, it is important to understand its purpose. If we
want to drill a well with a depth of 15,000 ft. We need to drill by sections and after having drilled each section, we must isolate and protect them because the
raw sides of the well cannot support themselves. We do this by using casing. Casing also allows us to protect the well stream from outside contaminants, as
well as any fresh water reservoirs from the oil or gas that is being produced. 

Casing can be divided into different types:
• Conductor Casing: this is the first casing step. It is used to circulate the drilling fluid up to the surface without eroding the upper sediments below the rig foundations. Generally this is a very short string, normally consisting of 1 to 3 joints.
• Surface Casing: this string prevents the cave-in of unconsolidated sediments near the surface and fundamentally it protects the shallow freshwater sands
from contamination. It supports the subsequent casing strings and also provides primary pressure control, as it is the support for the Blowout preventer (BOP).
This is a valve that allows us to control the well in the event of a sudden influx of gas.
• Intermediate Casing: It seals off troublesome formations or transition zones which could cause drilling problems, such as weak formations which lead to a
loss of circulation, sloughing shales, high pressure zones, depleted zones and salt formations, which tend to collapse the well, and so on. 

• Production Casing: this is set in the productive interval of the well. The production casing is always set facing the tubing. In this way, it must be designed to withstand all the fluid properties (such as pressure, temperature and corrosiveness). This casing protects the environment in the event of failure or damage to the tubing during the production stage, as well as allowing the production tubing to be replaced or repaired during the entire life cycle of the well.
• Drilling Liner: this is used for the same purpose as intermediate casing. It does not face the drilling tubing. 

• Production Liner: this is set in front of the productive interval of the well. It faces the tubing.
• Tie Back: this is a string connected from the top of a liner up to the surface of the well. Tie backs are used when a large and heavy casing string needs to be run from the surface to the bottom of the well, and due to its weight, it cannot be hanged by the rig. In this particular case, this casing string is divided in two shorter and lighter strings: a liner at the bottom and a tie back at the upper part, and both of them are run separately. All of the pipes must be identified, and the most important international standards
for pipe manufacturing are the API 5CT and the ISO 11960 standards, ‘Petroleum and natural gas industries — Steel pipes for use as casing or tubing for wells’. A tube is identified by its outside diameter expressed in inches, its weight expressed in pounds per foot, its steel grade and its connection.