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Oil casing pipe is divided into surface casing, technical casing, and production casing according to their functions.
Seal off soft and weak formations and water-bearing formations that are unstable, prone to collapse, and prone to leakages.
Control well blowouts by installing wellhead devices.
Support some of the weight of the technical casing and production casing.
The depth of the surface casing is determined by specific circumstances, generally ranging from several tens to several hundred meters or deeper (30-1500m). The cement outside the casing usually returns to the surface. When drilling high-pressure gas wells, if the upper rock formations are loose and broken, the surface casing needs to be properly set deeper to prevent high-pressure gas from flowing out of the ground.
If the surface oil casing pipe needs to be set deeper, before setting the casing, a conductor pipe should be set to seal the surface, prevent cave-ins, create a drilling fluid circulation channel, and facilitate long-term drilling. The conductor pipe is usually set about 20-30 meters deep and the cement outside the pipe returns to the surface. The conductor pipe can be made of spiral or straight-seam pipes.
Block complicated formations, significant loss zones, and oil, gas, and water layers with huge pressure differences that are difficult to control with drilling fluids, and prevent the wellbore from widening.
In directional wells with large inclinations, technical casing can be set below the build section for the safe drilling of directional wells.
Provide conditions for the installation of well control equipment, kick prevention, leak prevention, and hanging tail pipes, and also play a protective role for the production casing. Technical casing may not be needed and the complications at the wellbore can be controlled by using high-quality drilling fluids, accelerating drilling, and strengthening drilling methods, with the aim of little or no technical casing.
The depth of the technical oil casing pipe depends on the complexity of the formations that need to be blocked, and the cement returns should reach the layers that need to be sealed off at a depth of more than 100 meters. For high-pressure gas wells, the cement should be returned to the surface to improve the reinforcement of the casing and enhance the sealing of the casing threads to withstand greater shut-in pressures.
Separate the target layers from other layers.
Separate oil, gas, and water layers of different pressures and establish an oil/gas channel in the well to ensure long-term production.
The depth of the production oil casing pipe is determined by the depth of the target layer and the completion method. The cement slurry for the production casing is usually returned to the uppermost oil and gas layers at a depth of more than 100 meters. For high gas pressure wells, the cement slurry should be returned to the surface to reinforce the oil casing pipe and enhance the sealing of the casing threads to enable them to withstand larger shut-in pressures.
