Lost circulation and wellbore failure are common problems in the petroleum industry and they increase drilling costs dramatically. Casing drilling in depleted zones helps reduce drilling costs and problems related to lost circulation and wellbore failure. Thus, casing drilling is an important technology to minimize or eliminate conventional drilling problems in depleted zones. This thesis is focused on a study of smear effect in casing drilling in depleted formations. It is based on information about casing drilling and a commercial computer software ABAQUS. The smearing mechanism of drilling solids into the wellbore wall and the effects of parameters that affect the stress distribution around the wellbore wall are studied. Moreover, multiple wellbore cracks are studied to determine their effects on hoop stress distribution and all the results are given at the results chapter of this study. All the discussions about the changing parameters are given in results section. In conclusion, the smear effect in casing drilling can significantly improve hoop stresses around the wellbore and lost circulation problems can be minimized by using casing as a drilling string. These models can be used as a basic tool to understand smear effects in casing drilling in depleted formations. / text