Abstract: Intermediate casings are under severe wear in the drilling duration of mega-extended-reach wells.A quantitative model was established for the prediction of the residual wall thickness of intermediate casing.According to the energy principle,the wear volume on the inner wall of casing is proportion to the work that is done at the same position by the rotation of drill string.The function of the wear volume of intermediate casing was established,it is related to well path,drill string,penetration rate,rotational speed,the section length drilled,and wear coefficient.Under the assumption that the shape of wear was in crescent shape,the relationship between casing wear depth and volume was established.The dada obtain from a really drilled mega-extended-reach well were used to verify the model established above,the results show that this model can be used to predict the wear depth in the wall of intermediate casing.The difficulty in the application of this model exists in that the wear coefficient was hard to obtain accurately.According to the results obtained above,the guide lines for avoiding casing wear problem are outlined.