The increasing complexities of wellbore geometry imply an increasing potential of damage resulting from the casing-wear downhole. Much work has been done to quantify and estimate wear in casing; however, the results of such predictions have been mixed. While the locations of critical-wear areas along the casing string have been predicted fairly accurately, quantifying the actual amount of casing wear has been a magnitude off. A mathematical model that describes this casing wear in terms of the total wellbore energy has been developed and used to estimate the depth of the wear groove and the wear volume downhole. The wellbore energy provides a mathematical criterion to quantify the borehole quality and incorporates the parameters, borehole curvature, and the wellbore torsion. The casing wear observed downhole is also an integral function of these two parameters. Hence, a combined “wear-energy” model has been proposed to estimate the casing wear in curved sections of the wellbore that have the drill string lying on its low side. The fundamental assumption of this model is that the volume worn away from the casing wall is proportional to the work done by friction on its inner wall by the tool joints only. It also assumes that the primary mechanism for casing wear is the rotation of the drill string, and the wear caused during tripping is insignificant. The borehole torsion models of wellbore trajectory, namely spatial-arc, natural-curve, cylindrical-helix, and constant-tool face, have been incorporated separately to enhance the accuracy of estimating the wear volume downhole. The wear-energy model for a detailed analysis of a practical example using real-time well survey data will be presented. Wear zones along the wellbore have been identified using a mathematical criterion of the “contact zone parameter.” The wear-groove depths for each contact zone along with an equivalent average wear for the curved section of the wellbore have been estimated. The wear volumes predicted by the various curvature and torsion models of wellbore energy have been graphically studied. The wellbore torsion has been found to significantly impact the casing-wear downhole.