Wellbore collapsing and formation leak off in drilling are all resulted from wellbore instability. So, researches on wellbore instability mechanism and its critical conditions can provide theoretical foundation on which is based to prevent wellbore instability in drilling. The paper introduces catastrophic theory for the first time to analyze wellbore instability mechanism and provide a new calculating method of wellbore instability thresholds in drilling. Taking the ground stress and the fluid seepage into account, the paper developes the potential energy function of rock around wellbore on the basis of the elastic-plastic theory. For the potential energy function, the catastrophic theory is applied here to analyze the mechanism of wellbore instability in drilling and obtain the safe range of mud density. Also, the paper analyzes the effect of some sensitive parameters. From sensitivity analysis, it indicates that the mud density results in wellbore instability chiefly, the trend of wellbore instability speeds up with the decrease of plastic zone permeability.

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