Oil casing is divided according to the function: surface casing, technical casing, and oil layer casing.

Surface casing：

1. To seal the soft, collapsing, and leaky strata and water layer in the unstable upper part;

2. Install the wellhead device and control the blowout;

3. Part weight of supporting technical casing and oil layer casing.

The lower depth of the surface casing, depending on the specific situation, is generally dozens of meters to several hundred meters or deeper (30~1500m). The cement returned outside the pipe is usually returned to the ground. When drilling a high-pressure gas well, if the upper rock layer is loose and broken, it is necessary to prevent the high-pressure airflow out of the ground. If the surface casing needs to be deeper, and the first drilling time is longer, the next casing should be considered before the lower surface casing. Its function is to block the surface and prevent the wellhead from collapsing, forming a drilling fluid circulation channel, in order to facilitate long-term drilling. The depth of the catheter is generally 20-30 meters, and the cement outside the pipe returns to the ground. Cathecatheter is usually a spiral or straight tube.

Intermediate casing:

1. Use to seal off the complex strata difficult to control of the drilling fluid, the serious leakage layer, and the oil, gas, and water layer with great pressure difference, to prevent the expansion of the good diameter;

2. In the directional well with a large inclined, the technical casing is put into the inclined section to facilitate the safe drilling of the directional well.

3. Provide conditions for the installation, spray prevention, leakage prevention, and hanging tailpipe of the good control equipment, and also have a protective role for the oil layer casing. The technical casing does not have to be lowered. It can be a complex situation in the underground controlled by adopting high-quality drilling fluid, accelerating drilling speed, and strengthening drilling, so as to win no or less technical casing. The lower entry depth of the technical casing is determined according to the complex stratum to be sealed. The cement return height should reach the stratum of more than 100 meters to be sealed. For high-pressure gas Wells, in order to better prevent air leakage, the cement is often returned to the ground.

Production casing:

Use to separate the target layer from other layers; separate the oil, gas, and water layer of different pressures, and establish an oil and gas channel in the well to ensure long-term production.

The lower depth of the oil layer casing determines the depth of the destination layer and the completion method. The cement slurry of the oil layer casing generally returns to the top oil and gas layer of more than 100 meters. For high-pressure Wells, the cement slurry should return to the ground to reinforce the casing and enhance the sealing of the casing wire buckle, so that it can withstand large well-closing pressure.

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