Strain-Based Design of Tubulars for Extreme-Service Wells
- Jaroslaw Nowinka (Noetic Engineering Inc.) | Trent M.V. Kaiser (Noetic Engineering Inc.) | Bruce Lepper (Shell Canada Resources Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2008
- Document Type
- Journal Paper
- 353 - 360
- 2008. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 1.14 Casing and Cementing, 4.1.9 Heavy Oil Upgrading, 4.3.4 Scale, 2 Well completion, 1.6 Drilling Operations, 1.14.2 Casing Material Selection, 1.14.1 Casing Design, 1.2.1 Wellbore integrity, 4.2 Pipelines, Flowlines and Risers, 4.1.5 Processing Equipment, 5.4.6 Thermal Methods, 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating, 1.2.2 Geomechanics
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This paper presents a framework for a strain-based design of tubular strings in extreme-temperature or high pressure/ high temperature (HP/HT) wells. The relevant concepts are illustrated by examples from analytical and experimental investigation of a casing material considered for use in thermally stimulated wells operated by Shell Canada Limited (Shell) in western Canada. Much of this framework is also relevant to other applications where deformation-driven loading mechanisms are present.
Strain-based design uses material capacity beyond its elastic range to overcome a number of economic and technical hurdles encountered in conventional load-based designs. It has been used successfully in field applications where plastic deformations occur (e.g., thermal wells and pipelines). However, current industry standards for material selection have their origins in load-based design. More sophisticated material-characterization tools are required for strain-based designs, in which post-yield material properties govern much of the system response.
This paper describes the application of strain-based concepts to the design of casing strings under combined loading where some load components are deformation controlled. The paper emphasizes the need to address strain localization, high-strain cyclic plastic loading, strain-rate-dependent strength, and associated stress-relaxation effects.
Strain-based design is most effective if relevant and reliable post-yield material properties are available. Experimental investigation of a candidate material considered for Shell Canada's thermal wells consisted of a series of custom-designed coupon-scale tests. The tests were conducted to acquire data describing the post-yield material response to monotonic and cyclic loading at temperatures ranging from 20 to 350°C.
Conclusions of this paper summarize findings of the executed material-evaluation program, outline options to minimize strain-localization impacts, and provide recommendations for strain-based designs of well-completion tubulars. Following these recommendations should result in higher reliability and more cost-effective wells in completion programs using strain-based strategies for designs of extreme service wells.
Introduction--Strain-Based Design Considerations
Conventional elastic load-based well design is based on limiting all loads on the casing system to be within the initial yield strength of all components of the well structure. In contrast, an elastic/plastic strain-based design assumes that a certain portion of the structure will yield. Furthermore, the strain-based approach assumes the post-yield deformation to be an independent loading parameter that will govern the distribution of stresses throughout the structure. Under combined loading, one or more load components may be deformation-driven, while other deformation components will depend on applied loads and instantaneous post-yield material properties (for example, axial load in a pipe may be deformation-driven, but radial and hoop deformation will depend also on internal pressure). Designs for such combined-loading scenarios are sometimes referred to as hybrid designs.
Many thermal-well applications incorporate loads that bring the casing system into large-scale post-yield deformation, at least in the initial heating phase of the operation. Where cyclic loading is anticipated, a common design practice has been to limit the amount of plastic deformation in the load cycles following the initial heating. Minimizing that plastic deformation would provide a rationale for employing the load-based design strategy.
The conventional design has great appeal because of an extensive experience base, specifications, and design codes that are available. For example, connection specifications are load-based; material properties used for design are expressed in terms of stress; and stress-corrosion testing procedures incorporate stress loads that are severe, relative to stress limits. Challenges presented by thermal-well conditions include stresses that are higher than those used in standard testing procedures [e.g., sulfide-stress-cracking (SSC) tests according to the National Association of Corrosion Engineers specifications]. Such challenges are often met with development of application-specific test procedures, which are based on the standard procedures but incorporate updated stresses consistent with those expected in the application. Nonetheless, the testing is still based on stress conditions.
When field conditions and detailed designs are considered, it becomes apparent that elastic conditions are not likely to be maintained everywhere in the system after the initial heating phase. Furthermore, detailed material characterization of casing materials under cyclic loading reveals attributes that are not consistent with the assumptions used in conventional cyclic plasticity. Connections present regions where stress and strain concentrations are known to occur, both in the thread roots and across the wall thickness where primary load transfer occurs in the threads. Differences in pipe geometry and strength among various joints of the casing string may lead to strain localizations, causing incremental plastic deformation of the pipe in each loading cycle. Geomechanical loading also has been observed in many applications, where significant permanent plastic deformation is imposed on the casing system. All of these facets point to cyclic plastic deformation that is not accounted for currently in the design process.
In spite of the observation that most thermal wells endure cyclic deformations beyond the elastic-design limits, the rate of failure in such wells is remarkably low. Corrosion failures are infrequent in field applications, even where laboratory tests simulating those field conditions demonstrate a significant risk of stress-corrosion failure. Material failure caused by cyclic plastic hardening in the field is not common, even though localization is recognized to occur and to produce strains beyond the design assumptions.
It can be said, therefore, that field experience has already demonstrated that wells can be operated under conditions that produce cyclic plastic strain and that current load-based design strategies essentially limit the amount of such plastic strain to a modest amount and to limited regions of the well. However, the extent of such strains and the parameters controlling them have not been studied in sufficient detail to determine how much plastic deformation can be tolerated and how it can be harnessed. Therefore, a comprehensive strain-based design method is not yet available because many of the associated design parameters required to support such a method are not characterized or controlled. This represents a substantial opportunity for future design refinement.
Nonetheless, many strain-based design concepts can be applied in modern well-structure design. These concepts can be used to avoid implementation pitfalls, optimize material selections, and provide tools for forensic investigations of the few failures that do occur. Descriptions of current technology status and possible incorporation of strain-based designs in HP/HT and thermal-recovery wells can be found in literature (Hahn et al. 2005; Skeels et al. 2002; Wooley et al. 1977; Maruyama et al. 1990; Schwall et al. 1996; Slack et al. 2000; Dall'Acqua et al. 2005). This paper presents examples of how the strain-based design concepts are employed in a challenging thermal recovery field in western Canada.
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