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Framework for Defining Thermal Cycle Severity and Assessing Impacts on Thermal Well Intermediate Casing System Integrity


 Temperature fluctuations that occur during the service life of thermal enhanced oil recovery (EOR) wells are one of the primary design considerations for intermediate (production) casing systems from a long-term well integrity perspective. Restraint from the cement, formation, and surrounding well structure as the well is operated creates thermally-induced mechanical strain cycles that may impact various facets of casing integrity performance. A minor temperature reduction that occurs after the well reaches operating temperature will generally have less impact than a larger cycle that results in greater variations in mechanical strain and associated loading conditions. A more rigorous understanding of these effects will be beneficial, particularly as different sections of the well can be expected to go through different degrees of thermal cycling. Once the impact of these thermal cycles and their spatial variations on casing integrity performance is better understood, wells can be designed, constructed, operated and abandoned accordingly, reducing the associated integrity risks.
 From a casing system design perspective, anticipating the impact of thermal cycles in light of expected operating conditions and associated service interruptions in applications such as steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS) provides a means for enhanced confidence in design and well operation choices, and for reconciling long-term field performance with expectations. This paper describes an engineering framework for characterizing the impact of temperature excursions in thermal production casing given key structural integrity and connection sealability performance indicators, based on fundamentals of the mechanical behavior of this complex casing system and associated field practices.
 The approach described herein is founded on the premise that thermal cycles of varying magnitudes in thermal EOR applications will affect key casing system performance indicators in different ways and to different degrees. For instance, the temperature reduction required to yield the casing body in tension during a shutdown will typically be much larger than the temperature reduction required to cause local yielding in the thread roots of the premium connections typically used for these wells. This methodology is intended to provide guidance for operators’ casing and connection evaluations considering anticipated well operating conditions and to support the development of integrity monitoring procedures that can target specific damage mechanisms arising from those conditions.
 Novel elements introduced in this paper include the suggested systematic engineering approach to assessing the significance of thermal cycles on a variety of production casing system performance considerations, based on typical field and operational practices and available measurements. The authors also identify sample approaches that could be used to assess the impacts of thermal cycles on a variety of casing system performance criteria. The use of the framework should next be demonstrated using sample distributed temperature data; future developments could build on this approach and could be adapted to other applications.