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Factors Affecting the Quality of Casing String Threaded Connection

Date:2022-07-12    keywords: casing string threaded connection
Oil casing pipe is also called special oil pipe, which is mainly used for the drilling of oil and gas wells and the transportation of oil and gas. It includes oil drilling pipes, oil casings, and oil sucking pipes. Oil drill pipes are mainly used to connect drill collars and drill bits and transmit drilling power. The oil casing is mainly used to support the well wall during the drilling process and after the well completion, so as to ensure the drilling process and the normal operation of the entire oil well after the well completion. The oil pipe mainly transports the oil and gas at the bottom of the oil well to the surface.

Downhole casing string is the only and permanent channel for exploiting oil and gas resources, and plays a vital role in protecting the borehole wall and separating different pressure layers. Oil pipes are mainly used for the drilling of oil and gas wells and the transportation of oil and gas. Therefore, it is particularly important to ensure the quality of the downhole casing string and improve its service life.


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The threaded connection part is the weakest part of the casing string. The connection strength of the round threaded casing that meets the API standard is only 80% of the strength of the body, and the sealing performance of the thread is also easily damaged. 86% of the failure of the casing string Appears at threaded connections. Therefore, improving the connection quality of casing threads is the most critical factor to ensure the quality of casing strings.

Factors affecting the quality of casing string threaded connection:

The key to the influence of torque on the quality of threaded connection lies in the two aspects of threaded connection strength and thread sealing.

Oil pipelines are the lifeblood of keeping oil wells running. Due to different geological conditions, the downhole stress state is complex, and the comprehensive action of tension, compression, bending and torsional stress is applied to the pipe body, which puts forward higher requirements on the quality of the casing itself. Once the casing itself is damaged for some reason, it may lead to the reduction of production of the whole well, or even to be scrapped.

The effect of torque on the strength of threaded connections:

① For API standard threaded casing, if the thread torque is too large, a large circumferential stress will be generated between the threaded coupling and the pin, and the excessive circumferential stress will easily cause stress damage or strain cracks at the coupling. , thereby reducing the strength of the entire pipe string, and even causing breakage at the casing thread. According to the strength of the steel itself, petroleum casing can be divided into different steel grades, namely J55, K55, N80, L80, C90, T95, P110, Q125, V150, etc. The well conditions and well depths are different, and the steel grades used are also different. In a corrosive environment, the casing itself is also required to have corrosion resistance. In places with complex geological conditions, the casing is also required to have anti-collapse properties.

②If the tightening torque of the thread is too small, the axial load capacity of the thread will be directly reduced, resulting in a thread slip.

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