АНАЛИЗ ОТКАЗОВ СОЕДИНЕНИЙ ВСАСЫВАЮЩИХ ШТОКОВ В НЕФТЯНЫХ СКВАЖИНАХ

УДК 669

ABSTRACT

This study presents a comprehensive failure analysis of sucker rod couplings used in beam-pumped oil wells. These components operate under severe operating conditions involving cyclic mechanical loading, frictional interaction, and aggressive corrosive environments, which significantly increase their susceptibility to fatigue cracking, wear, and corrosion-related degradation. The research combines visual inspection, metallographic examination, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), hardness evaluation, and mechanical testing methods to identify the root causes and mechanisms of failure. The obtained results indicate that the majority of failures initiate at stress concentration regions, particularly at thread roots, where repeated loading promotes crack nucleation and propagation. Furthermore, improper assembly conditions, rod misalignment, inadequate lubrication, and exposure to corrosive well fluids accelerate material deterioration and reduce operational reliability. Based on the experimental findings, several engineering recommendations are proposed, including improved material selection, optimized thread geometry, protective surface treatments, and enhanced maintenance practices to increase the service life, durability, and operational reliability of sucker rod pumping systems in oil production applications.

АННОТАЦИЯ

Данное исследование представляет комплексный анализ разрушений муфт насосных штанг, применяемых в штанговых глубинно-насосных установках нефтяных скважин. Эти элементы работают в тяжелых эксплуатационных условиях, включающих циклические механические нагрузки, фрикционное взаимодействие и воздействие агрессивных коррозионных сред, что значительно повышает их склонность к усталостному растрескиванию, износу и коррозионному разрушению. В работе использованы методы визуального контроля, металлографического анализа, сканирующей электронной микроскопии (SEM), энергодисперсионной спектроскопии (EDS), измерения твердости и механических испытаний для определения основных причин и механизмов разрушения. Полученные результаты показывают, что большинство повреждений зарождается в зонах концентрации напряжений, особенно в корнях резьбы, где повторяющиеся нагрузки способствуют возникновению и распространению трещин. Кроме того, неправильная сборка, несоосность штанг, недостаточная смазка и воздействие коррозионно-активных пластовых сред ускоряют деградацию материала и снижают эксплуатационную надежность. На основе проведенных исследований предложены инженерные решения, включающие улучшение выбора материалов, оптимизацию геометрии резьбы, применение защитных поверхностных покрытий и совершенствование методов технического обслуживания для повышения долговечности и надежности штанговых насосных систем в нефтедобыче.

Keywords: Sucker rod coupling, failure analysis, fatigue cracking, corrosion, wear mechanisms, oil well engineering, artificial lift systems

Ключевые слова: Соединительные муфты для насосно-компрессорных штанг, анализ отказов, усталостное растрескивание, коррозия, механизмы износа, нефтепромысловое проектирование, системы искусственного подъема нефти.

Introduction

Oil production from mature fields increasingly relies on artificial lift systems, among which beam pumping remains one of the most widely used methods due to its operational simplicity, reliability, and cost-effectiveness. In such systems, sucker rods play a critical role by transmitting mechanical energy from the surface pumping unit to the downhole pump. These rods are connected by couplings, which serve as essential load-bearing components ensuring the continuity and integrity of the rod string [1-3].

Sucker rod couplings operate under severe service conditions characterized by cyclic tensile and compressive stresses, torsional loads, frictional contact, and exposure to aggressive downhole environments containing carbon dioxide (CO₂), hydrogen sulfide (H₂S), and saline formation water. These combined mechanical and chemical effects make couplings highly susceptible to various degradation mechanisms, including fatigue cracking, wear, corrosion, and hydrogen embrittlement. As a result, coupling failures represent one of the most frequent causes of rod string breakdowns in oil wells [4-6].

Methodology

The failure analysis of sucker rod couplings was performed using visual inspection, metallographic analysis, SEM/EDS examination, and numerical stress modeling. Initially, damaged coupling samples taken from oil wells were visually examined to identify fracture zones, corrosion traces, and wear defects. Metallographic analysis was conducted to evaluate the microstructural condition of the material and detect possible defects related to crack initiation. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to investigate fracture morphology and determine the chemical composition of corrosion products. In addition, finite element analysis (FEA) was applied to determine stress concentration regions in the threaded connections under cyclic loading conditions. The obtained results enabled the identification of the main mechanisms responsible for coupling failures.

Main part

Sucker rod couplings operate in oil wells under conditions of highly variable loads and aggressive environments; therefore, a comprehensive investigation of their failure mechanisms is of critical importance from an engineering perspective.

Figure 1. Visual representation of typical failures observed in sucker rod couplings in oil wells

These components are primarily manufactured from alloy steels and, although they attain high mechanical properties through heat treatment, they undergo degradation during service due to simultaneous exposure to both mechanical and chemical effects.

The analysis of sucker rod coupling failures requires the integrated application of various experimental and analytical methods, enabling the precise identification of the root causes of failure [7-9].

Figure 2. Morphological analysis of the fracture mechanism: (a–b) crack initiation zone; (c–d) crack propagation zone.

The conducted analyses indicate that the majority of failures occurring in the couplings are directly associated with the microstructural characteristics of the material and the stress distribution arising under service conditions.

Fatigue cracks represent the most prevalent failure mechanism in sucker rod couplings, typically forming as a result of stress concentration at the thread root.

Figure 3. Numerical modeling of stress concentration in a threaded connection: determination of maximum stress regions in a sucker rod coupling.

These cracks initially originate at the microscopic level and subsequently propagate under the influence of cyclic loading, ultimately leading to sudden fracture at the final stage [8].

Wear and abrasive effects, on the other hand, primarily arise from the contact between the rod and the tubing and are accompanied by material loss at the surface.

This process becomes particularly pronounced under conditions of insufficient lubrication or improper mechanical alignment [9].

Corrosion processes constitute one of the principal factors reducing the service life of the couplings, as the chemical environment progressively degrades and weakens the material’s structural integrity [10].

In particular, the presence of CO₂ and H₂S gases accelerates the corrosion process and promotes the development of critical mechanisms such as hydrogen embrittlement [11,12].

Errors made during installation and operation also play a significant role in the occurrence of failures, leading to the generation of additional stresses.

Thus, the failure of sucker rod couplings is not attributable to a single cause, but rather constitutes a complex process arising from the interaction of mechanical, metallurgical, and operational factors.

Conclusion

This study demonstrates that failures of sucker rod couplings used in oil wells exhibit a complex nature, arising from the combined effects of various mechanical, metallurgical, and operational factors. The conducted analyses have established that the primary cause of these failures is fatigue cracking, which predominantly initiates in regions of high stress concentration, particularly at the thread roots.

At the same time, corrosion processes and hydrogen embrittlement weaken the mechanical properties of the material, accelerating crack propagation and thereby increasing the risk of sudden fracture. In addition, errors during installation and misalignment within the system generate additional stresses, further intensifying the failure process. The findings of the study indicate that, in order to enhance the reliability of sucker rod couplings, material selection, surface treatment technologies, structural design, and operating conditions must be considered in an integrated manner. In particular, the application of advanced surface strengthening methods, proper installation techniques, and effective control of operating conditions play a critical role in extending the service life of the couplings.

Thus, the implementation of the proposed approaches can contribute to improving equipment reliability in oil production operations, reducing unplanned downtime, and increasing overall production efficiency.

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