Key roles and types of oil well casing

Oil well casing, an indispensable steel pipe in oil wells, shoulders the important mission of establishing a pressure-resistant and sealed zone from the oil and gas reservoir to the surface. Casing is divided into Seamless Casing and ERW Casing according to the production process. Its role is extensive and critical, covering the following seven aspects:

Casing pipe plays an important role in pressure resistance and sealing in oil wells. It can not only effectively prevent well collapse and excessive erosion of the well wall by drilling fluid, but also protect the freshwater layer and low-pressure layer from pollution. At the same time, the casing can also regulate the pressure in the well, maintain the pressure balance in the well by adjusting the amount of oil and gas entering the well, provide convenience for the installation of oil production equipment, and provide the necessary channels for the flow of oil and gas.

Important role of casing:
Casing not only plays an important role in preventing well collapse and drilling fluid erosion, but also can effectively regulate the pressure in the well and protect the freshwater layer and low-pressure layer from pollution. At the same time, it provides a channel for the flow of oil and gas.

Casing pipe types:
During the drilling of oil and gas wells, multiple types of casing pipe are usually used to meet different drilling needs. Typical oil and gas wells require at least three types of casing: conduit, surface casing, and reservoir casing. In addition, depending on the formation conditions that may be encountered, it may also be necessary to run an intermediate technical casing. In some cases, a tail pipe or double-layer technical casing is also used. Each layer of casing has its specific purpose and is essential for the smooth drilling and completion of the oil well.

Functions and applications of conduits:
Conduits play a vital role in the drilling of oil and gas wells. It mainly serves as a circulation channel to guide the drilling fluid to the mud pool, thereby effectively preventing well collapse. In this process, the conduit also undertakes the important task of diversion and blowout prevention, especially in the presence of shallow gas. In addition, it can also provide protection for the subsequent casing from corrosion. In areas where the ground support capacity is insufficient, the conduit can also share some of the weight of the wellhead equipment.

Application of intermediate casing:
The core function of the intermediate casing string, sometimes called "protective casing" or "salt casing", is to protect the wellbore. During the deepening of the oil well, it mainly seals the fragile formation, prevents drilling fluid contamination, and paves the way for deeper drilling. When heavy mud is required, the intermediate casing can resist the pressure of the well wall, and its lowering depth needs to ensure that the formation can withstand the expected mud weight.

The role of production casing:
In the drilling process of oil and gas wells, it is a crucial link to lower the production casing or reservoir casing. It not only isolates the oil-producing layer from unnecessary liquids and other formations drilled through, but also provides protection for the oil pipe and downhole production tools. The reservoir casing needs to have high strength and stability because it usually bears the highest pressure in the oil well.

Design advantages of tail pipe:
The tail pipe, as a shortened casing string, has a unique design that extends upward from the bottom of the well, beyond the bottom of the intermediate casing or the reservoir casing. The application of tail pipe is mainly based on its economy, short construction time and reduced risk of casing string. However, it is worth noting that the potential leakage problem at the liner seal may cause considerable trouble.

Consideration of casing size:
The size of the first layer of casing lowered into the well will be restricted by the size of the final production casing. Once the diameter of the oil layer casing is determined, the diameter of the intermediate casing and the surface casing and the corresponding drill bit size are also determined. The casing diameter size here determines the maximum drill bit and tool size that can pass through the pipe.

Influencing factors and selection:

The casing size is restricted by the production casing, and its selection must comprehensively consider the cost, economic benefits and matching of drilling equipment. At the same time, when multiple layers of intermediate casing need to be lowered, the diameter of the surface casing also needs to be increased accordingly to ensure that the size of the oil layer casing meets the requirements. In the case of weighing the cost and economic benefits, this is usually a worthwhile investment.

Read more: Oil tubing VS. Casing pipe or Casing pipe VS. Drill pipe