Cyclic Mechanical and Fatigue Properties of OCTG Materials
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 97775, "Cyclic Mechanical and Fatigue Properties for OCTG Materials," by T.M.V. Kaiser, SPE, and V.Y.B. Yung, SPE, Noetic Engineering Inc., and R.M. Bacon, Imperial Oil Resources, prepared for the 2005 SPE International Thermal Operations and Heavy Oil Symposium, Calgary, 1-3 November.
Much of the design for well structures subjected to high-amplitude cyclic loading is based on material assumptions that extrapolate strength properties from uniaxial, monotonic tests to conditions where cyclic, multiaxial stresses are imposed. The full-length paper presents results from cyclic testing on common oil-country tubular goods (OCTG) materials and shows the difference between physical behavior measured under cyclic-loading conditions and theoretical behavior extrapolated by numerical modeling. Modeling theories for plastic deformation are discussed, including their limitations and relevance in a cyclic-loading environment.

Introduction
Most thermal-recovery oil wells in western Canada operate by use of cyclic steam stimulation (CSS) or steam-assisted gravity drainage. Both methods impose thermal cycles on well structures, particularly the intermediate casing. Thermal expansion is constrained by the formation and cement, producing loads that exceed tubular yield strength when the well is heated. Localization mechanisms amplify strain magnitude, imposing additional plastic-fatigue loads at discrete locations in the well.
Thermal-well casing designs have evolved through more than 30 years of operating experience, and much of the computer modeling that describes casing performance is based on uniaxial material properties, extrapolated to multidimensional and cyclic behavior through engineering models.
Cyclic material-properties data are very sparse, particularly in the temperature range common in thermal-recovery wells. Furthermore, plastic-fatigue-life information for materials commonly used in well construction is difficult to obtain. Such information, however, is required to make reliable predictions of certain deformation mechanisms and associated fatigue life for wells exposed to cyclic, thermally imposed deformations.
A test program for characterizing cyclic material properties was undertaken to evaluate both cyclic mechanical properties and low-cycle fatigue life. Test results demonstrate consistency indicative of reliable material characterization that can be applied in analysis models and component life assessments. The observed cyclic behavior also demonstrates material characteristics significantly different from those predicted through engineering models using uniaxial, monotonic material properties for input. This has important implications for selection of steels used in thermal-well designs and for implementing techniques to mitigate casing-deformation effects.

Background
Post-Yield Plasticity. Much of the work to characterize metal post-yield behavior was developed to support structural analysis under conditions where the load exceeds that required to yield some components. Material properties usually are determined from uniaxial coupon tests that give elastic modulus, yield strength, and post-yield strengthening or hardening. Structural modeling, whether analytic or numeric, requires that behavior measured in one dimension be extended into a multidimensional stress state. Furthermore, plastic behavior is load-path dependent (i.e., the stress state is not defined uniquely by the strain, but rather depends on the history of plastic deformation).