We usually refer to drill pipes, collars, square drill pipes, transfer joints, other drilling tools, tubing and casing as OCTG, as the name implies – the tubing into the oil well. In the past, OCTG were also called special oil pipes. In fact, special oil pipes also include pipelines. OCTG and pipelines are generally referred to as special oil pipes.
China’s annual demand for OCTG is about 1.2 million tons, with a cost of more than 10 billion yuan. Casing and tubing pipes account for about 90% of the total demand. The oil well string of an oil well is composed of drill string, tubing string and casing string. The oil well pipe is made up of taper threads connecting a single pipe, which is hundreds of meters shallow, thousands of meters deep and even tens of thousands of meters deep. Taper thread has the advantages of fast up-unloading speed, high connection strength and good sealing.
Pipe strings in different wellbore sections are subjected to long-term tension, compression, bending, internal pressure, external pressure and thermal cycling. Thread connection is the weakest link. More than 80% of failure accidents occur at the threaded connection. Therefore, the threads of oil well tubing should have two main characteristics:
Oil well pipe threads should have two main characteristics: (1) structural integrity, that is, the threads should have enough connection strength after meshing, so as not to damage the structure under external force; (2) sealing integrity, that is, to ensure that the string with hundreds of threaded connections can withstand the long-term effect of internal and external pressure difference under various stress conditions without leakage. Thread connection strength and sealing performance are two very important technical indicators of oil well pipe.
The purpose of thread inspection is to check out the unqualified pipe to prevent the unqualified pipe from going down the well and to minimize the occurrence of tubing failure accidents. The main failure modes of oil well pipe threads are: breakage of threads, gluing, slippage, leakage, etc. In dealing with accidents, there are hundreds of thousands, many millions, and even the whole well is scrapped, resulting in significant economic losses.
The OCTG plants have to go through many procedures, such as process inspection, sampling inspection before warehousing and so on. Each oilfield will also carry out acceptance check after arrival or send someone to the factory to supervise the production, or inspect before shipment before leaving the factory. Some oilfields will entrust a third-party inspection agency to supervise the production in the factory. In addition to thread parameters and proportional inspection of appearance, hydrostatic sealing performance tests of threaded joints are also carried out in some oilfields. Before the oil well pipe goes down into the well, the operator also needs to carry out the appearance inspection of the threads, such as damage, deformation, rust and so on. Path test is also needed for the pipe body.
The above inspection is to control the quality of threaded connection as much as possible. Of course, whether the operation is standard, whether the torque and the speed is appropriate, and whether the performance of thread grease meets the requirements also have a great impact on the quality of thread connection.
There are also some hidden quality hazards that can not be found by existing means and inspection methods, such as the problem of insufficient tightening torque of factory coupling machine. When a well team goes into the well, if it is equipped with a floating buckle of a torque meter, it can make up for this shortcoming. If the old method is used to look at the residual buckle, it is very likely that the tubing and casing can not be sealed or even slip off after they go down the well.