Role of Microstructure in Sucker Rod String Failures in Oil Well Production
Sofiane Benhaddad and Glen Lee
(Submitted 12 December 2000; in revised form 20 February 2001)
Sucker-rod pumps are operating in very aggressive environments in oil well production. The combined effect of a corrosive environment and significant mechanical loads contribute to frequent cases of failure of the rod string during operation. Standards and recommendations have been developed to control and avoid those failures. This study presents various failure cases of sucker rods in different applications. The heat treatment of the steel material and the resulting microstructure are an important factor in the behavior of the sucker rod. A spheroidized microstructure presents a weaker resistance to corrosion affecting the rod life. Non-metallic inclusions are a pitting preferential site leading to fatigue crack initiation. Heterogenous microstructure as banded martensite and ferrite/pearlite decreases the ductility of the material affecting the fatigue propagation resistance.
Keywords: corrosion-fatigue, inclusions, pitting, steel microstructure, sucker rod 1. Introduction
The oil production wells in western Canada generally operate in aggressive environments, including corrosive elements, high velocity of the fluids, and considerable depth for completion. Determination and analysis of the necessary data are required for selection of materials for well components. Materials for the tubing, casing, and sucker rod string, as well as a suitable coating or chemical protection program are selected appropriately for each operating well. In such an aggressive environment, the most vulnerable downhole component is the sucker rod string due to its function.
Specifications and standards [1-4]have been written for the improvement of the performance and life expectation of sucker rods. The recommendations of these codes and standards are directed towards a better choice of material for good corrosion resistance and a better design of the rod string for an even distribution of stresses. The resulting recommendations are restrictive and may pay significant attention to economic factors. Therefore, many operators continue to use carbon or alloy steel components with an appropriate chemical inhibition program. Carbon steel is preferred over other materials due to its machinability, high strength, and low cost.
The use of carbon steel rod strings in aggressive environments, without properly considering the complex interactions between the material and its service environment, leads to failures. In this context, numerous rod string failures have been studied through metallurgical investigation that included macro-observation, chemistry, fractography, microscopy, and hardness testing. This paper summarizes several of the studied cases and divides the cases according to the main root cause of failure: mechanically induced failure and environmentally induced failure. 2. Mechanically Induced Failures
All sucker rod failure cases were related to the most commonly used materials: carbon or alloy steel.These rod strings were operating under one of three conditions: 1) in conjunction with a chemical corrosion inhibition program; 2) in conjunction with a protective coating; and 3) without any protection from the service environment. Cases are summarized along with the details of the failure analysis. 2.1 Role of the Microstructure
2.1.1 Observation. Three sucker rods broke at the pin end (Fig. 1) and produced the same microscopic rupture with identical features. The rods were fabricated from carbon or alloy steel. The observation of the surface of the rupture revealed three distinct zones (Fig. 2). The first zone, corresponding to the crack initiation, revealed pits and corrosion. The second zone was flat and smooth, indicating a macroscopically brittle fracture. The second zone surface was covered with beach marks, suggesting fatigue crack
propagation. The third and last zone was a shear lip. The third zone surface was rough and covered with dimples (Fig. 3), showing that the final rupture process was ductile overload. All these features indicated that corrosion-fatigue was the cause of failure. This was confirmed with the presence of fatigue striations on the surface of the second zone (Fig. 4). The pitting of the surface of the rod introduced a stress concentration at the “pit tips” and led to the initiation and propagation of fatigue cracks due to cyclic loads resulting from the operation of the pump. The aggressive down-hole environment may have accelerated the crack growth.
2.1.2 Microscopy. Microscopy observations were completed on sections removed from the rod through the surface of the rupture. The microstructure was ferrite and pearlite with numerous inclusions dispersed throughout the structure (Fig. 5). The inclusions were analyzed as MnS particles. The inclusions were elongated parallel to the rod length and contained numerous micro-cracks. Additionally, the inclusions provided preferential sites for pitting in the material (Fig. 6).
Several studies have highlighted the detrimental effect of inclusions in the steel [5-8] and shown that MnS particles create a micro-galvanic couple with the steel: MnS being the anode and the steel the cathode. Our observations were consistent with previous works that showed that the presence of MnS inclusions at the surface leads to pit formation in acid environments.
The pits formed at the surface of the rod provided local stress concentration that altered the resistance to fatigue initiation. The pits, coupled with the cycles created in the operation of the pump, have served to initiate fatigue. The crack is then propagated through the thickness of the rod.
In one of the failure cases studied, elongated bands in the microstructure were observed (Fig. 7). These bands corresponded to the rupture features of the fracture surface. Microhardness testing of the bands and the bulk of the structure showed a significant variance, with the hardness of the bulk being 293 HV and the band significantly harder at 411 HV. The high hardness of the bands is indicative of a brittle material. It corresponds to the location of inclusions and micro-cracks in the material. Additionally, the band areas show segregation of chrome, which leads to the higher hardness. The banding resulted from partial austenitizing, which caused the original banded ferrite and pearlite microstructure to transform to bands of very hard martensite and bands of ferrite/pearlite. The presence of the hard martensite bands combined with MnS inclusions to promote the initiation of fatigue cracks and lead to a decreased resistance to crack propagation.
A high number of inclusions in the steel is extremely detrimental and the use of higher quality steel is preferred. Changes in the fabrication process for the rods, including improved quality control, resulted in homogeneous microstructure with an even chemistry and hardness and decreased the tendency for failure.
Fig.1 Sucker rod broken at the pin end
Fig.2 Surface of the rupture of the sucker rod
Fig.3 Surface of the last stage of the rupture
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