Abstract: [Objective] Aiming at the frequent wellbore collapse problems encountered in the drilling of highly deviated and horizontal wells in the hard brittle oil shale formation of the Shahejie Formation in the Jiyang Depression, this study clarifies the main controlling factors affecting wellbore stability and establishes a multi-field coupled evaluation and prediction method suitable for this formation. [Method] X-ray diffraction, scanning electron microscopy and CT scanning were used to analyze the mineral composition and micro-fracture characteristics of core samples. Petrophysical and mechanical parameters of the rock were determined by porosity-permeability tests and triaxial mechanical experiments. Wellbore stability evaluation was carried out by combining the effective stress field and seepage field around the wellbore, considering the coupling of fracture mechanical weak plane effect, drilling fluid pressure penetration and wellbore trajectory. [Results] The results show that the oil shale is dominated by quartz and clay minerals, and the clay minerals are mainly non-expansive illite, so water-based drilling fluids have little influence on rock mechanical properties. The rock mass is developed with multiple sets of micro-fractures with a width of about 2–3 μm, and is characterized by low porosity, low permeability and dense lithology. The compressive strength of homogeneous core samples is 183.5 MPa, while that of fractured core samples decreases to 37.2–62.5 MPa. After 12 days of immersion in drilling fluid, the strength reduction ranges from 3.87% to 20.51%. When drilling horizontal wells along the minimum horizontal principal stress direction, the collapse risk is high within the inclination angle range of 50°–90°, leading to wellbore instability. When drilling along the maximum horizontal principal stress direction, the high collapse pressure zone is concentrated at the inclination angle of 25°–45°. The drilling fluid pressure penetration effect increases the equivalent density of critical collapse pressure by 0.115–0.155 g/cm³. [Conclusion] The wellbore stability of the Shahejie Formation is jointly controlled by wellbore trajectory, mechanical weakness of fractures, and drilling fluid pressure penetration. Optimizing the wellbore trajectory along the maximum horizontal principal stress direction and enhancing the sealing performance of drilling fluid can effectively reduce the risk of wellbore collapse. [Significance] The research findings can provide guidance for achieving effective and safe drilling in oil shale formations.
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