As more and more complex wells are being drilled, an important problem to be addressed relates to casing, liner, and tubing wear. Casing wear is one of the continuing challenges faced by the industry, and an accurate estimation of downhole wear remains a paradox. Several casing wear estimation techniques were applied to actual drilling situations in the past; however, the results of those efforts have not always provided the necessary level of accuracy of wear prediction. The effects of improper estimation of casing wear may not be felt owing to the general overdesign of the casings by largely applying excessive casing-wear safety factors. This paper addresses these concerns by providing a comprehensive solution to this long-standing casing-wear puzzle.

The widely used conventional soft-string model is not applicable for all well paths for casing-wear modeling; it assumes that the drillstring continuously follows the exact wellbore curvature and cannot predict the different contact locations of the drillpipe with the inner casing wall. Consequently, a new modeling technique has been developed by applying the stiff-string model for casing-wear analysis. The stiff-string model calculates more accurate contact loads by accounting for the bending stiffness of the string and helps to estimate the contact position of the drillstring at any given casing depth. These contact points are used to model the development of multiple wear groove locations at any casing depth cross-section by accounting for the varying contact positions as various operations are performed through the casing. Estimates of multiple groove positions at each cross-section reduce the overestimation of casing wear, because the wear is now distributed across different grooves, thus providing a more realistic casing-wear estimate.

This modeling approach was validated by using ultrasonic logs from complex directional wells that are the most susceptible to wear. Detailed operational steps and parameters for each well were modeled to predict the development of multiple wear grooves for each casing section. These models were compared with the 360° cross-sectional distribution of remaining wall thickness from image-based ultrasonic logs. The estimated groove depths and positions correlated with the peaks observed from wear logs that showed the worst wear locations. The results obtained from this exhaustive analysis are promising to establish a new, more thorough means of validating casing-wear predictions. By applying a new comprehensive modeling approach using stiff-string analysis to estimate multiple wear grooves, this study has helped to reduce long-unsolved casing-wear uncertainties. A novel method for validating the estimated groove positions using the full 360° spectrum of caliper or ultrasonic logs has been presented. Accurate wear prediction is important for well integrity and optimized well designs.

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