Classification and use of oil casing

Classification and use of oil casing

According to the function, oil casing is divided into: surface casing, technical casing and oil layer casing.

1. Surface casing

1. Used to isolate the soft, easy to collapse, easy to leak formations and water layers that are not very fixed on the upper part;

2. Install the wellhead installation to control the blowout;

3. Support the partial weight of the technical casing and oil layer casing.

The depth of the surface casing depends on the specific situation, usually tens of meters to hundreds of meters or deeper (30-1500m). The cement return height outside the pipe usually returns to the air. When drilling a high-pressure gas well, if the upper rock formation is loose and broken, in order to prevent the high-pressure airflow from escaping into the air, the surface casing needs to be properly lowered. If the surface casing needs to be deeper, when the first drilling time is long, you should consider lowering a layer of conduit before lowering the surface casing. Its function is to isolate the surface, prevent the wellhead from collapsing, and form a drilling fluid circulation channel for long-term drilling. The depth of the casing is generally 20-30 meters, and the cement outside the pipe returns to the air. The casing is generally made of spiral pipe or straight seam pipe

2. Technical casing

1. Used to isolate complex formations that are difficult to control with drilling fluid, serious leakage layers, and oil, gas, and water layers with large pressure differences, so as to prevent the expansion of the wellbore;

2. In directional wells with large well inclination, the technical casing is lowered in the inclination section to facilitate the safe drilling of the directional well.

3. It provides conditions for the installation, blowout prevention, leakage prevention and suspension of tail pipes of well control equipment, and also has a protective effect on the oil layer casing.

The technical casing does not have to be lowered. The complex conditions under the well can be controlled by adopting high-quality drilling fluid, accelerating drilling speed, strengthening drilling and other measures, and strive to not lower or lower the technical casing. The lowering depth of the technical casing depends on the complex formation to be isolated. The cement return height should reach more than 100 meters of the formation to be isolated. For high-pressure gas wells, in order to better prevent leakage, cement is often returned to the air.

3. Oil layer casing

It is used to separate the target layer from other layers; to separate the oil, gas and water layers with different pressures, to establish an oil and gas channel in the well to ensure long-term production. The depth of the oil layer casing is determined by the depth of the target layer and the completion method. The cement slurry of the oil layer casing is generally returned to the uppermost oil and gas layer more than 100 meters. For high-pressure wells, the cement slurry should be returned to the ground, which is conducive to reinforcing the casing and enhancing the sealing of the oil casing thread, so that it can withstand a larger shut-in pressure.