Comprehensive FE modeling of worn casing collapse strength: Parametric analysis and assessment of a wear log case study

Casing systems for oil and gas wells consist of tubular elements installed along the depth, providing borehole stability and tightness. They must fulfill premises of structural integrity, especially in offshore environment, which imposes extreme pressure and temperature conditions to the well structure. Among the loads undergone by casing tubes, such as axial force, internal pressure, and external pressure, this latter stands out, leading to a failure mode known as collapse. Imperfections associated with casing tubular manufacturing processes, such as cross-sectional eccentricity and ovality, and the residual stress of the steel, are expected to influence the collapse mechanism. Moreover, the inner wall of the casing experiences wear, predominantly caused by intense contact with the drill string during the well construction. These imperfections can lead to a significant reduction in the collapse strength, especially in thin-walled tubes, whose failure mechanism is associated with geometric instability caused by high cross-sectional slenderness. This paper presents a 2-D finite element analysis of the collapse strength of worn tubulars with initial imperfections, via software Abaqus. The plane strain hypothesis is assumed, in a physically and geometrically nonlinear approach. The mechanical response of steel is described by the elastoplastic model with nonlinear hardening provided by ASME VIII - div. 2 (2019), and the Riks’ method is employed to evaluate the nonlinear behavior. The consideration of internal wear is made by removing material from the inner wall, as a groove shape. Model validation is performed using experimental results from the literature. A parametric study is carried out to assess the influence of slenderness, wear depth, steel grade, tool joint diameter, and initial imperfections (ovality and eccentricity) in the residual collapse strength. The results indicate the wear groove depth as the most influential variable for the reduction in collapse strength. Other parameters also play a role in the determination of collapse strength and may be relevant for casing integrity analysis. These conclusions are consistent with and converge with findings reported in previous studies in the literature. Finally, a case study of a critical region of a well, which data were obtained through an ultrasonic logging tool, is presented. Different types of inspection data and data application approaches (inner radius-based and thickness-based) are evaluated, with the latter proving to be more reliable due to its lower sensitivity to measurement noise. A severe derating of the collapse pressure is observed, with reductions ranging from approximately 14% to 50%, even though the maximum wear depth observed in this case study is limited to 13%. This variability highlights the importance of carefully interpreting inspection data, since at the time of inspection the cross-section may be significantly deformed due to in-situ subsurface well conditions, directly affecting casing integrity analysis. Moreover, the influence of modeling multiple wear grooves, as opposed to considering only the deepest one, is also observed and highlights the importance of working with real-world industry inspection data.