The sealing performance of the premium connection casing joint is closely related to its own structure, such as the form of metal sealing structure, the size of sealing interference, the form of auxiliary sealing structure, etc.
As we all know, the casing string is a high-pressure vessel with a length of thousands of meters or even thousands of meters, which is connected by one piece of oil casing through thread. High connection strength and good sealing performance are the basic requirements for connecting thread. In the traditional design of string, the strength of casing is usually designed, but the critical sealing pressure of thread is not checked, so the sealing problem of joint is seldom considered. However, with the continuous development of oil drilling and production technology, especially with the development of natural gas wells and high-pressure oil and gas wells, more and more attention has been paid to the sealing of casing joints.
The sealing performance of different screw thread is different. In the past, API round thread and eccentric trapezoid thread were mainly used in oil casing. Because of its structural design, API thread has serious defects in sealing performance, so it is not suitable for use in wells with severe conditions. It forces people to develop and use the premium connection with higher tightness.
The good sealing performance of premium connection depends on its ingenious structure design and precise processing technology. For API threaded joints, whether circular or trapezoidal, even if the machining tolerance of the joint is 0, there will be a certain fit clearance between the internal and external threads after the joint is tightened. The clearance is spiral, which can connect the inner and outer space of the casing to form the leakage channel of the fluid. Only by the filling of thread grease can API thread play a sealing role, but the thread grease is easy to volatilize and deteriorate, especially when the downhole temperature is higher, the volatilization and metamorphism are faster, which significantly reduces the role of plugging the leakage channel. When the pressure of the fluid in the joint, especially the gas, is higher When high, API joints are prone to leakage accidents. Premium connection breaks through the design framework of API thread, and the sealing function is not only borne by the thread, but also designed a special metal sealing structure. Generally speaking, the premium connection has multiple sealing structures such as metal radial seal, shoulder seal, thread seal, and some have elastic sealing structures, which greatly improves the sealing resistance and reliability.