Casing and tubing

 

Casing and tubing string are the main parts of the well construction.All wells drilled for the purpose of oil or gas production （or injection materials into underground formations）must be cased with marerial 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

 

Casing strings

 

There are six basic types of casing strings:

 ● Conductor Casing

 ● Surface Casing

 ● Intermediate Casing 

 ● Production Casing

 ● Liner

 ● Liner tieback casing

 

Conductor casing

 

Contactor casing is the first string set below the structrai casing .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 dive or a blowout prevention(BOP)stack may be installed onto this string.When cemented,this shring is typically cemnented 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.

 

Casing a Well

 

The casing is fabricated in sections, or joints, that are usually about 40 feet long and screwed together to form longer lengths of casing, called casing strings. Each end of the casing joint has male threads that are protected by cap called a thread protector until the casings are ready to be jointed. Then, a collar or coupling, composed of a short cylindrical steel pipe that is slightly larger in diameter than the joints and also has female threads, is used to connect the two male joint ends. A thread compound is used on the two ends to ensure a tight seal.

 

Casing is run from the rig floor, connected one joint at a time by casing elevators on the traveling block and stabbed into the previous casing string that has been inserted into the well. Hanging above the drill floor, casing tongs screw each casing joint to the casing string. Casing is run into the well and officially landed when the weight of the casing string is transferred to the casing hangers, which are located at the top of the well and use slips or threads to suspend the casing in the well.