Mechanical and Microstructural Changes of Fine Grained C-Mn Steel Tubular Undergoing Down-Hole Cold Expansion Process

Abstract

As easily recoverable hydrocarbon resources are depleting, the oil and gas industry focuses more on producing oil and gas from ultra-deep, tight and scattered pockets of reserves. However, these recoveries are not only difficult and expensive, but also require the development of new technologies and materials that can meet stringent requirements regarding operation in sub-surface environment. The emergence of expandable tubular in the late 1990s has opened a new avenue for oil and gas wells design and remediation processes. However, these tubular go through large expansion in diameter at kilometers depths in onshore and offshore wells. This alters the post expansion mechanical and microstructural properties of the tubular that may lead to premature failure during operation. The idea of understanding such variations revolve around complex mechanisms occurring at micro level including multiphase microstructure, grains sizes and morphology, and crystallographic orientations. Initial grains morphology and distribution of phases, and the subsequent changes due to the expansion process lead to significant variations in material properties at macro level. Optical micrographs showed that the expandable tubular material is composed of fine grained microstructure of ferrite phase with some traces of martensite and plate-like structures. Induced martensite results from the phase transformation of metastable austenite induced by thermomechanical processing applied during the manufacturing stage. A reasonable presence of martensite phase in the tubular material enhances its structural integrity, collapse and burst strengths, as well as provides a safeguard against possible mechanical failures such as buckling. On the other hand, the ferrite phase is a soft phase and its presence improves the formability of the tubular resulting in higher expansion ratio. It was also observed that the grains size is affected by the tubular expansion. The presence of elongated grains in the microstructure is due to the excessive deformation as well as the crystallographic reorientation of grains due to the course of tubular expansion. However, no strong texture has been found in the expanded tubular material, which may be attributed to the complex nature of loadings induced during the expansion process. In order to understand the influence of tubular expansion process on mechanical properties of tubular, samples from un-expanded and expanded sections of the tubular (expanded at 16%, 20% and 24% of the tubular original inner diameter) are investigated using standard mechanical testing procedures. Mechanical testing results revealed an increase in yield strength, ultimate tensile strength and hardness, whereas ductility and impact toughness tend to decrease. Fracture surface analysis of fractured tensile samples has also been done using scanning electron microscope (SEM). At lower expansion ratio, fracture surface micrographs revealed a predominantly ductile nature of failure with clusters of fine microscopic dimples intermingled with voids. However, at higher expansion ratio, the test specimens revealed a mixed mode of failure with both brittle and ductile features.