Abstract: Secondary damage to a casing often happen in casing repairing when using the expander because minimum repairable inner diameter for reduced casing is not specified.A program for determining the minimum inner diameters of a diameter reduced casing was made by explicit dynamic ANSYS/LS-DYNA finite element program based on the expander repair mechanism and casing repair technology.As an example for 139.7 mm casing of steel grade P110,the minimum repairable inner diameter was caculated and a chart of expander sizes versus reduced casing inner diameters was provided.The research showed during the remedial operation,the minimum casing repairable diameter depends on the yield strength of the casing and its required minimum run-through size.In order to obtain the required minimum run-through size,it is necessary to change the expander size for 3 to 4 times.Its application in Wen 95-86 Well of Zhongyuan Oilfield showed the calculated minimum repairable inner diameter could improve the success rate of reduced casing repairment.