Abstract: During the drilling process of stratified shale oil reservoirs,if the wellbore stability is not well enough,it is prone to frequent incidents such as wellbore collapse and stuck pipe.A numerical model for the stress around the wellbore and a collapse zone prediction method in shale oil reservoir is developed to reveal the influence of bedding plane on wellbore collapse and enlargement of shale oil reservoir in accordance with MG-C strength criterion and multi-weak plane criterion.Also,the collapse and enlargement patterns for single and multiple weak planes under different stress mechanisms are calculated.The results show that the presence of multiple weak planes causes simultaneous damage to the matrix and weak planes,leading to more complex wellbore collapse patterns.While collapse expansion induced by weak planes is significantly higher than that of the matrix.Compared to the strike-slip stress mechanism,the collapse of wellbore under normal stress mechanisms shows a more significant change with stress ratio,and the collapse risk is higher.Under normal stress mechanisms,when the stress ratio exceeds 1.5,the wellbore expansion rate exceeds 100% and wellbore increases sharply.The research findings provide theoretical guidance for preventing wellbore collapse during drilling in shale oil reservoirs under different regional stress mechanisms.

Key words: shale oil, bedding, failure criteria, borehole wall, collapse and stuck pipe, weak plane