This paper presents the findings of a comprehensive structural analysis in which the influence of thermal transient pressure behavior on the trapped annuli in an injection well in a Brazilian pre-salt field was assessed, mainly motivated by physical evidence of a well failure. The study focus on a transient heat flow in radial direction during a well failure investigation, and its impact in tubular design safety factor under a given casing design methodology.
During the investigation of this well integrity failure, a thermal analysis was performed considering the well construction history, but standard simulations using a world-class commercial software was not enough to explain the failure. Thus, a modified thermal analysis for casing and tubing was made in order to evaluate the design safety factor during each operation. This modified thermal analysis consists in splitting each operation in short time steps, in order to capture the short transient behavior. It was found that, during short transient time, the collapse stresses reached higher values than predicted in the previous standard steady-state modeling. Such result is basically related to the transient effect caused by radial heat flow.
Based on theoretical studies and comparing them to downhole P&T sensors in confined annuli, a correlation was stablished and showed the importance of this type of analysis. In certain scenarios, where the confined annuli are subjected to progressive and non-proportional cooling down effect between the casing layers, a sudden pressure drop may occur in the internal side of the casing, without reaching the same pressure drop on the external side, which can lead to a dramatic external differential pressure for a given string.
In wells with multiple confined annuli, such as in ultra-deepwater projects, this type of analysis represents a greater challenge. The results obtained so far have shown that the permanent and transient radial heat flow cannot be neglected in some scenarios and, therefore, open a new frontier for well design, especially when the tubing and multiple casing trapped annuli are subjected to rapid transient cool down.