OCTG Casing & tubing

Casing and tubing strings are the main parts of the well construction. All wells drilled for the purpose of oil or gas production (or injecting materials into underground formations) must be cased with material with sufficient strength and functionality.

Casing

Casing is the major structural component of a well. Casing is needed to:

Maintain borehole stability
Prevent contamination of water sands
Isolate water from producing formations
Control well pressures during drilling, production, and workover operations

Casing provides locations for the installation of:

Blowout preventers
Wellhead equipment
Production packers
Production tubing

The cost of casing is a major part of the overall well cost, so selection of casing size, grade, connectors, and setting depth is a primary engineering and economic consideration.

Casing strings

There are six basic types of casing strings:

Conductor Casing
Surface Casing
Intermediate Casing
Production Casing
Liner
Liner tieback casing

Conductor casing

Conductor casing is the first string set below the structural casing (i.e., drive pipe or marine conductor run to protect loose near-surface formations and to enable circulation of drilling fluid). The conductor isolates unconsolidated formations and water sands and protects against shallow gas. This is usually the string onto which the casing head is installed. A diverter or a blowout prevention (BOP) stack may be installed onto this string. When cemented, this string is typically cemented to the surface or to the mudline in offshore wells.

Surface casing

Surface casing is set to provide blowout protection, isolate water sands, and prevent lost circulation. It also often provides adequate shoe strength to drill into high-pressure transition zones. In deviated wells, the surface casing may cover the build section to prevent keyseating of the formation during deeper drilling. This string is typically cemented to the surface or to the mudline in offshore wells.

Intermediate casing

Intermediate casing is set to isolate:

Unstable hole sections
Lost-circulation zones
Low-pressure zones
Production zones

It is often set in the transition zone from normal to abnormal pressure. The casing cement top must isolate any hydrocarbon zones. Some wells require multiple intermediate strings. Some intermediate strings may also be production strings if a liner is run beneath them.

Production casing

Production casing is used to isolate production zones and contain formation pressures in the event of a tubing leak. It may also be exposed to:

Injection pressures from fracture jobs
Downcasing, gas lift
The injection of inhibitor oil

A good primary cement job is very critical for this string.

LINER

Liner is a casing string that does not extend back to the wellhead, but is hung from another casing string. Liners are used instead of full casing strings to: