中文摘要：

目前在计算油气井壁坍塌压力时，通常采用摩尔一库仑准则，该准则没有考虑中间应力对岩石破坏过程的影响，计算结果比较保守，限制了低密度钻井液发展。针对这一情况，考虑岩石破坏过程中的损伤，基于统计损伤原理，对统一强度准则进行了修正，并利用修正后的强度准则建立了新的井壁坍塌压力模型。结果表明，修正后的统一强度准则能更好地模拟岩石强度，利用该准则建立的井壁坍塌模型计算出的坍塌压力比传统的摩尔一库仑准则低，准确率更高。

英文摘要：

At present, the Molar-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.