ABSTRACT: The failure mechanism of hard brittle shale formation is one of the key research objects to ensure the safety development of unconventional oil and gas. In order to further study the wellbore instability of middle-deep hard brittle shale under the influence of various factors, the study of mineral composition, microstructure and mechanical tests was carried out. It is found that the middle-deep shale in Bohai bay basin not only has typical characteristics of hard brittle shale, but also has certain hydration characteristics. Based on the experiment results, wellbore stress distribution model and weak plane strength criterion have been used to establish the wellbore stability model, which considers the influence of weak plane and chemical effect. Investigation on influence factors of collapse pressure is conducted through the established model. The results show that the existence of weak plane makes the collapse pressure increase significantly. In particular, the distribution of collapse pressure is variable with the change of weak plane occurrence and the distribution mode is more complex due to weak plane. The effect of drilling fluid significantly narrows the safe mud density window and increases the risk of wellbore instability. The case analysis shows that under the influence of weak plane and chemical effect, this model can predict wellbore collapse pressure more comprehensively and accurately than previous methods, which is meaningful in drilling design of middle-deep hard brittle shale formation.

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