The deep shale oil reservoir in the Junggar Basin of China is facing challenges due to elevated deviatory stress conditions and extensively developed fracture features, which increase the risk of wellbore breakout during drilling. This paper presents a poro-elastic dynamic based mechanical model to characterize the wellbore stress response and breakout features for deep shale oil reservoirs. Different from the conventional method, this proposed model includes the fluid-solid coupling and the effects of fluid and solid inertia. The Mohr-Coulomb failure criterion is applied to evaluate shale matrix failure and the weak-plane failure criterion is applied to evaluate the failure of shale bedding/fracture plane in this model. Rock mechanical properties and permeability anisotropy is also considered in this model. Then, a set of deep well parameters were taken as the examples to calculate the near-borehole stress field. The failure behaviors were also analyzed and compared with the pore-elastic static model.

The results show that compared with the poro-elastic static model that does not consider inertia, the poro-elastic dynamic model can reflect the dynamic response characteristics of pressure and stress around the wellbore after excavation. Static models tend to underestimate the severity of wellbore instability, while dynamic models are more accurate in predicting borehole breakout. The study on the factors affecting the wellbore breakout shows that Biot coefficient, in-situ stress difference, fracture parameters and bottomhole pressure have a great influence, while the elastic modulus and matrix permeability have minor influence on the wellbore breakout. This model offers insights into the dynamic mechanisms underlying wellbore instability in deep shale oil reservoirs, serving as a valuable tool for accurately predicting breakout pressure, optimizing drilling mud density, and ensuring wellbore stability.

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