Casing string

OCTG casing pipe is a special oil pipe that has been used for a long time. The oil casing string is an integral structure of the oil well, which plays the role of strengthening the wellbore, protecting the wellbore and ensuring normal drilling.

Steel casing is the most widely used pipe material in the petroleum industry. It is threaded to form a casing string to work. The casing string mainly includes casing head, surface casing, technical casing, production casing, substructure, tail pipe, etc.

1. Structure of casing string

(1) Casing head

A device for hanging casing on the wellhead. According to the formal requirements, after each layer of casing is laid and cement is injected, the coupling joint should be removed and the casing head should be connected to the top of the casing to seal the two layers. The annular space between the casings hangs the second layer of casing string and bears part of its weight.

For example, standard wellhead equipment should include: a casing head connected to the surface casing, with a casing hanger connected to the technical casing; an oil layer casing head installed on the surface casing head, with a casing hanger attached to the surface casing head. The oil layer casing is connected to the hanger; the oil pipe head is installed on the oil layer casing head, and the oil pipe is connected to the oil pipe hanger.

According to different connection methods, casing heads are divided into two categories: threaded type and flange type. In recent years, foreign countries have produced automatic sealing casing heads, which are suitable for high-pressure gas wells.

(2) Surface casing

In order to prevent the collapse of the loose layer on the surface of the wellbore, the upper freshwater layer is isolated from the pollution of the drilling work and the casing installed with the wellhead blowout preventer device is installed. The depth of surface casing should be determined based on the depth of the loose formation and the rupture pressure gradient of the upper formation. The diameter of the surface casing is determined based on the number of casing layers and the diameter of the production casing.

(3)Technical casing

Technical casing, also called intermediate casing, is the casing run between the surface casing and the production casing due to the limitations of drilling technology. Technical casing can be one or several layers. There are many reasons why technical casing is forced to be run. For example, if the lower part of the drilling encounters high-pressure oil and gas layers, and the fracture pressure gradient of the upper formation is not enough to withstand the pressure gradient generated by heavy drilling fluid, technical casing needs to be lowered to seal the upper formation. Another example is when drilling into a thick loose sand layer or expansile shale, and the performance of the drilling fluid cannot be controlled, technical casing also needs to be run. The size of technical casing should be determined based on the diameter of the production casing and the number of casing layers.

(4)Production casing

Also called oil layer casing or gas production casing. The last layer of casing dropped in an oil (gas) well is to create a solid path for oil and gas from the bottom of the well to the wellhead, and to prevent oil and gas from leaking into other low-pressure formations. The production casing size is determined based on the output of the production layer. Usually two sizes are used: 5″ (127mm) and 6″ (152mm). In oil and gas wells with particularly large production, 8″ (203mm) casing is also used as production casing.

(5)Substructure

Substructure refers to the general name of the lower device accessories of the casing string. Different types of casing have different substructures. The substructure of technical casing and production casing generally consists of the following components from bottom to top: cement guide shoe (or pig iron guide shoe + casing shoe), floating hoop, support ring, casing Centralizer and mud cake scraper pin. The purpose of the casing lower device components is to smoothly run the casing into the well and improve the cementing quality.

(6) Tail pipe

The liner is also called the drilling liner. In a well that has been cased, only the open hole section of the casing is filled with cement, and the casing string whose casing apex does not extend to the wellhead is called the liner. Depending on the purpose of running the liner into the well, the liner can be divided into oil production liner, technical liner, protection liner and tie-back liner. The production liner is used as completion casing instead of production casing; the technical liner is used to deepen the technical casing; the protective liner is used to repair damaged or broken casing; the tie-back liner is to tie the lower liner back into the technical casing , covering the damaged casing.

2. Causes of damage to casing strings

Through research and investigation on domestic and foreign oil fields, the main forms of casing damage are: deformation, rupture, casing staggered breakage, corrosion fatigue leakage, etc. Analysis shows that the main causes of casing string damage are:

(1)Drilling engineering

Drilling factors mainly include drilling engineering design and on-site construction, such as unreasonable casing string strength design, excessive wellbore curvature (too small curvature radius), which causes excessive bending stress on the casing string downhole, etc.

(2) Downhole operation

Downhole operating factors mainly include excessive hollowing out of the well tube during oil production and oil testing, casing damage caused by acidification, fracturing, perforating, well workover, salvage and repeated well logging, unreasonable deep well gas lift measures, infill wells and injection The water injection pressure of the water well is too high, etc.

(3) Cementing engineering

The cementing engineering factors include three aspects: construction, cementing design and testing. For example, the way the casing is fixed at the wellhead is unreasonable, the quality of the cementing is unqualified, the completion method selected is not the best, and the casing is damaged during the running process. Mechanical wear and unreasonable design related to cement injection, etc., cause the sealed casing string to be subjected to a large external extrusion force, resulting in damage to the casing.

(4) Geological

Geological factors mainly include oil layer sand production, mud shale expansion, rock salt layer creep, release of formation tectonic stress, and loss of the original formation pressure balance in the in-situ stress field.

(5) Casing quality

The quality factors of casing mainly refer to the unqualified mechanical manufacturing defects of the casing when it leaves the factory, as well as the mechanical damage caused during the placement, transportation and downhole process of the casing. If casing with problematic casing quality is run into the well, the casing string will be damaged and fail under the combined action of various external forces such as internal pressure, axial tensile load, additional bending load, and external extrusion force.

(6) Corrosion fatigue

Corrosion includes corrosion of casing strings caused by scaling, bacteria, dissolved oxygen, high-salinity formation water, and chemicals. After the casing string is corroded, the strength decreases and easily causes fatigue damage to the casing.

(7) Water and gas injection

In the process of thermal recovery of heavy oil and high-pressure gas and water injection in adjustment wells, fault repair or dislocation, the hydration expansion of mudstone causes the casing to be subject to very large external pressure, causing casing damage.

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