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.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
29 August 2005
- Meeting paper published:
1 November 2005
- Revised manuscript received:
23 April 2008
- Manuscript approved:
10 May 2008
- Version of record:
15 December 2008
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