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Intermediate casings in the build sections are subject to severe wear in extended-reach drilling.This paper presents a new method for predicting the depth of a wear groove on the intermediate casing.According to energy principle and dynamic accumulation of casing wear by tool joints,a model is established to calculate the wear area on the inner wall of the casing.The relationship functions between the wear groove depth and area are obtained based on the geometry relationship between the drillstring and the wear section and the assumption that the casing wear groove is crescent-shaped.The change of casing wear groove depth versus drilling footage under different-sized drillstrings is also discussed.A mechanical model is proposed for predicting casing wear location,which is based on the well trajectory and drillstring movement.The casing wear groove depth of a planned well is predicted with inversion of the casing wear factor from the drilled well and necessarily revised to improve the prediction accuracy for differences between the drilled well and the planned well.The method for predicting casing wear in extended-reach drilling is verified through actual case study.The effect of drillstring size on casing wear should be taken into account in casing wear prediction.