Cause analysis of failure of the hottest pair of d

2022-08-09
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Analysis on the causes of drill pipe failure

in the process of drilling, the failure of drill pipe at any part will cause serious consequences, and even scrap the well. There are about 500 or 600 drill pipe accidents in oil fields in China every year, resulting in huge economic losses. Importing drill pipes of various specifications every year will cost hundreds of millions of yuan of foreign exchange. With the continuous depletion of shallow resources, more and more deep and ultra deep wells will be drilled in the future, and the safety and reliability of drill pipes has become a very prominent problem

drill pipe failure is generally manifested as body fracture, puncture and leakage, drill pipe thread failure, etc. The reason is mainly caused by the following factors: the basic mechanical working conditions of the drill pipe during drilling, the assembly of drill tools and drilling technology, the regularity of the well diameter, eccentric wear, thread sealing grease, drilling fluid, drill pipe structure and materials, formation factors, well corrosion medium, etc. the interaction of the above factors leads to the failure of the drill pipe

basic mechanical working condition of drill pipe drill pipe works in a long and narrow wellbore filled with drilling fluid inside and outside, and usually bears loads such as pressure, bending, torsion and hydraulic pressure. If the stress on the drill pipe is less than 206.8 Newtons per square meter, the drill pipe will not produce fatigue cracks even after countless times of bending. When drilling, the drill pipe bears a great compound stress composed of bending, torsion and tensile stress, especially in the large displacement directional well and horizontal well, the torque is great, and the drill pipe will produce fatigue microcracks when it is far less than 1million bending times; After the microcrack is produced, it will continue to expand and extend. At this time, if the corrosive high-pressure drilling fluid enters the microcrack, it will accelerate the crack propagation, and eventually lead to the failure accident of drilling fluid penetrating the drill pipe. As a result of the piercing development, the effective cross-section of the drill pipe is continuously reduced, and the fracture occurs when the total length of the puncture plus crack exceeds its critical crack size. In addition to the downward movement of rotation, there are also various vibrations and whirls of drill pipes

as a rotating slender elastic rod, drill pipe has its natural vibration frequency, which is determined by the assembly of drilling tools. When the drill pipe rotates, it will also produce three forms of vibration: longitudinal, transverse and torsional. When their frequency is consistent with the natural frequency, it will produce resonance. The result of resonance will add an additional fatigue stress to the original drill pipe fatigue stress, accelerating the failure of drill pipe. The rotary bending fatigue of drill pipe can be reduced by using long-term screw drill pipe instead of rotary table drilling in directional wells and horizontal wells

if the longitudinal vibration frequency produced by the cone bit shaft is the same as the natural natural vibration frequency of the bit drill string system, there will be resonance, which will increase the amplitude of the bit, produce great impact load, and aggravate drill pipe fatigue. In addition, if the fitting joint used for the transition of drill pipes with different diameters or different types of buttons is not used properly, the thread of the fitting joint itself (mainly the male button) and the drill pipe thread connected with it may be broken

the irregular well diameter affects the well section with irregular well diameter or serious expansion, and the bending degree of the drill pipe increases accordingly. When the drill pipe rotates, the connecting thread is subject to alternating bending, and the standardized unitized design stress is implemented to accelerate the fatigue failure of the drill pipe. At the same time, the stress of the threaded connection is complicated, which intensifies the fatigue damage of the thread. There are three forms of rotation of downhole drill pipe with eccentric wear: rotation, revolution, and coexistence of rotation and revolution. Rotation causes uniform wear of drill pipe, revolution causes eccentric wear of drill pipe, and the strength of drill pipe decreases after wear

improper use or application of thread sealing grease, such as cleaning drill pipe threads with diesel oil, mixing drilling fluid with thread sealing grease without capping, impurities or diluting with engine oil, making it difficult for thread sealing grease to adhere, less or inadequate application of thread sealing grease, etc., resulting in ineffective sealing and lubrication during drill pipe thread connection and thread adhesion

drilling fluid the chemicals in the drilling fluid system currently used in oil fields, such as salt water drilling fluid, acid drilling fluid, iron ore powder weighting material system, will cause the corrosion of drill pipes

formation factors the formation composition is different, and the grinding degree of drill pipes, especially drill pipe couplings, is also different. The drillability of the formation is poor (grade), the heterogeneity of the formation is strong, and it is easy to choke and jump during drilling, which intensifies the longitudinal vibration of the drill pipe and increases the probability of fatigue failure of the drill pipe

drill pipe structure and materials the drill pipe body near the thickening in the drill pipe joint is the bending stress concentration area. When the drill pipe joint contacts the shaft wall that cannot form mud cake, the contact friction coefficient is large, and the friction is enough to stimulate reverse precession. In the reverse precession state, the frequency of bending stress alternating increases exponentially, so fatigue cracks are easy to form in this region. Thread is the weakest link on the whole drill string, and its service life is generally difficult to compete with the drill pipe body. When bearing the frequent action of complex alternating stress (fatigue), the penetration and leakage of male and female threads are common phenomena. The defect in the drill pipe material is the weak link in the drill pipe. In the service environment with complex force on the drill pipe, it is also easy to cause the drill pipe fracture accident

corrosive medium because the drilling fluid circulation system is not sealed, the oxygen in the atmosphere will be mixed with the drilling fluid into free oxygen through the mud pit, mud pump and other equipment during the circulation of the drilling fluid. When the mud contains a certain amount of dissolved oxygen, it will cause corrosion on the surface of the drill pipe H2S from the formation or due to the high-temperature decomposition of some sulfur-containing organic additives in the mud and the metabolism of sulfate reducing bacteria in the mud will also lead to hydrogen induced stress corrosion cracking of drill pipes. The iron sulfide produced by H2S corrosion forms a corrosion couple with steel, resulting in deep ulcerative corrosion pits on the surface of the drill pipe, which makes the drill pipe puncture and hole corrosion in a short time. CO2 immersed in drilling fluid makes it weakly acidic, which will also cause corrosion to drill pipes. In addition, when the drill pipe is stuck and injected with strong acid similar to HCl, it will also cause serious corrosion to the drill pipe. Other ions in drilling fluid, such as cl-, work together with H2S and CO2 to accelerate the corrosion of drill pipe surface

to sum up, there are many factors of drill pipe failure, and the failure of drill pipe is the result of the comprehensive action of various factors. For each well with sandwich material (referred to as organic-inorganic composite material for short) that points out the composite of organic and inorganic materials, the main influencing factors should be extracted and corresponding measures should be taken according to the failure analysis of drill pipe. At present, the coating branch of Anton orr Engineering Co., Ltd. is gradually rolling out the equipment accuracy grade: 0.5 according to the failure causes of drill pipes in various oil fields; A kind of functional coated drill pipe is used to solve the problem of drill pipe puncture and fracture failure caused by the interaction of corrosion and stress

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