Abstract: At present, the Mohr-Coulomb criterion is usually used to calculate well casing collapse pressure in oil and gas wells. Since this criterion doesn’t take the impact of the intermediate stress in rock failure process into consideration, the calculated result is conservative and hence limits the development and application of low-density drilling fluid. For this situation, it is often advisable to apply the unified strength criterion. In this paper, the unified strength criterion is adopted and modified, based on the statistical damage theory and the damage condition during the process of rock failure. In addition, a new model of well casing collapse pressure calculation is established by using the modified unified strength criterion. It is indicated that the modified unified strength criterion is capable to simulate rock strength with higher accuracy. The calculated collapse pressure using the new model of well casing collapse pressure calculation is lower than that when applying conventional Mohr-Coulomb criterion and the accuracy is higher

Key words: damage mechanics, unified strength criterion, modified criteria, collapse pressure, well casing