Cyclic Mechanical and Fatigue Properties for Oil-Country-Tubular-Goods Materials

Summary This paper describes differences between actual material behavior and idealizations used for modeling purposes and discusses some of the implications for interpreting model predictions. Much of the design for well structures subjected to high-amplitude cyclic loading is based on material assumptions that extrapolate strength properties from uniaxial, tensile tests to conditions where multiaxial, cyclic stresses are imposed. This paper presents results from cyclic testing on a common oil-country-tubular-goods (OCTG) material and demonstrates differences between the physical behavior measured under cyclic loading conditions and theoretical behavior extrapolated by numerical modeling. Modeling theories for plastic deformation are discussed with their limitations and relevance in a cyclic-loading environment. The implications of these limitations for design choices in thermal wells also are discussed with example applications of cyclic material behavior and fatigue-life prediction. Material fatigue properties for the high-amplitude, low-cycle application of thermal operations have not been investigated in much depth previously, particularly for OCTG. Along with characterizing cyclic mechanical properties, the tests discussed here also assessed the low-cycle fatigue properties of the sample OCTG steel. The consistent fatigue measurements, combined with analysis results using representative cyclic mechanical properties, can provide a basis for estimating fatigue life. Depending on analysis-model assumptions, substantial variation in predicted fatigue life can occur; therefore, exact fatigue-life predictions are not anticipated. The primary value in such modeling is in evaluating the relative effectiveness of mitigation options for extending well life. Introduction Most thermal enhanced-oil-recovery (EOR) wells in western Canada operate using either the cyclic-steam-stimulation (CSS) or the steam-assisted-gravity-drainage (SAGD) method. In both methods, operational factors result in thermal cycles being imposed on the well structures, particularly in the intermediate casing (Placido et al. 1997). Thermal expansion is constrained by the formation and cement in CSS and SAGD wells, producing loads that exceed the yield strength of the tubulars when the well is heated. Localization mechanisms also might amplify the strain magnitude, imposing additional plastic fatigue load at discrete locations along the well structure. Thermal-well casing designs have evolved during more than 30 years of operating experience, and much of the computer modeling that describes casing performance is based on measured uniaxial tensile material properties that are extrapolated to multidimensional cyclic behavior through engineering models. Cyclic material-properties data are sparse, particularly in the temperature regime 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 the associated fatigue life for wells exposed to cyclic, thermally imposed loading. A test program for characterizing cyclic material properties was implemented to evaluate both cyclic mechanical properties and low-cycle fatigue life. Test-result consistency indicates a reliable material characterization that can be applied in constitutive analysis models and component-life assessments. The observed cyclic-stress-strain material behavior also demonstrates different characteristics from those predicted through engineering models using uniaxial monotonic material properties for input. This has important implications for thermal-well design and operations.