PVT Studies - The Corrosion-Metallurgical Aspects of Sucker Rods and their Oil Well Service Performance

The mechanisms of corrosion and corrosion fatigue damages to sucker rods are examined from a fundamental electrochemical viewpoint, and the relationships of sucker rod microstructures to these damaging effects are defined. The common corrosive agents in crude oil production and storage are discussed as to types of attacks that they cause. The variations of fatigue strength with alloy content, surface preparation, and stress ranges are analyzed. Finally, conclusions are drawn as to how microstructures largely influence and control sucker rod performance. INTRODUCTION Greater than 90 per cent of the crude oil produced by artificial lift methods is produced through sucker rod pumping systems. In the USA each year more than $14 million is spent for sucker rods in new and old pumping installations. It is important to make this investment a wise one. Essentially, the sucker rod system is a long series of steel rods, operating under alternating stress loadings and under mechanical vibrations, which connect the surface power source with the subsurface pump. These rods often operate in a variety of corrosive media, including water, brines, carbon dioxide, hydrogen sul-fide, and air. Basically, this paper is concerned with the problem of the behavior of certain steels in various corrosive media under high and dynamic stress patterns. The main objective of this paper is to show how, once the stress effects and the corrosive environments are defined, one can still realize structural control and beneficial influence upon corrosion and corrosion fatigue. FUNDAMENTAL DEFINITIONS AND ANALYSIS In the corrosion and stress behavior of this solid state mixture of iron and iron carbide called steel, one must analyze upon what major factors does its service life depend and of what, exactly, is steel composed. The former question can be referred to two external factors and two internal factors. The two major external factors are: 1. the nature of the stress patterns — size and range of stresses present 2. the nature of the corrosion — erosion environment — the corrodent and/or wear involved The two fundamental internal factors that govern the strength and useful life of a steel both in normal duty and in heavy-duty corrosion service are: 1. the nature of the steel volume properties 2. the nature of the steel surface properties For the latter question normal steel is defined as a solid mixture of iron and iron carbide particles, generally with certain accidental and intentional elements present. (The amount of iron carbide present is determined by the carbon content of the steel.) These additional elements generally dissolve into either the iron (ferrite) or the iron carbide (cementite) components.