The main functions of oil casing are as follows: 1). Loose sealing, collapsing and leaking formation; 2). Sealing oil, gas and water layers to prevent collusion, thus forming oil and gas channels; 3). Installing wellhead to control oil and gas flow, so as to facilitate drilling and production. For the convenience of processing and transportation, the length of each oil casing is generally about 10 meters. When in use, hundreds or even hundreds of casings are connected together by threads to form a casing string.
The casing string is actually equivalent to a high-pressure vessel with a length of thousands of meters or even thousands of meters. On the one hand, it needs to bear a large tensile load due to its own weight, on the other hand, it needs to bear a large amount of internal and external pressure, in which the connection thread, namely the casing joint, is the weakest link in the whole casing string. It is the starting point of modeling and stress analysis to deeply understand the structure, material and stress characteristics of casing joint.
1. Structural characteristics
Generally speaking, the connecting thread of petroleum casing can be divided into two types: API thread, round thread (short round thread, long round thread), trapezoid thread and straight thread. The former two types of thread are mainly used, but the straight thread is rarely used. The other is non API thread, also known as special thread or high-quality thread.
2. Material characteristics
The sleeve and its joint are generally made of alloy steel, and the carbon content in the chemical composition is generally between 0.2% and 0.5%. The requirements for other components are specified in API Spec 5CT. The casing pipe body is divided into seamless pipe and welded pipe. Seamless pipe is a seamless forged steel pipe product, which is made of hot-processing steel. The electric welding pipe adopts resistance welding or electric induction method. The steel plate is welded by non filler metal to form a tubular product with a longitudinal weld. The coupling is seamless pipe.
The toughness of the steel used for petroleum casing is better, which makes the strain of the threaded connection part larger when it bears larger tensile load.
3. Stress characteristics
The main loads on the casing thread are: make-up torque, tensile load and internal pressure load.
In addition, casing joints sometimes bear external extrusion load, compression load, bending load, etc. In the thermal recovery well of heavy oil, the thermal stress of environment temperature can not be ignored. At the same time, casing often bears not a single load, but several loads at the same time.