Study of the Wellbore Collapse Area in a Fractured Shale Formation from a Dagang Oilfield in China

Abstract

Shale reservoirs in the Dagang Oilfield are characterized by developed bedding and microfractures, leading to prominent wellbore instability issues during drilling. Existing models remain insufficient in characterizing the competitive “matrix-multiple weak planes” failure mechanism in such formations. Consequently, this study employs the ubiquitous joint model and Mohr–Coulomb criterion to establish a coupled numerical model that comprehensively considers bedding orientation, in situ stress difference, and the time-dependent hydration effects of drilling fluid. The results indicate that wellbore instability exhibits a dual failure mechanism: matrix shear failure is concentrated in symmetrical areas on both sides of the borehole, whereas weak-plane slippage occurs at four specific azimuths. The morphology of the collapse zone evolves dynamically with the bedding dip angle, transitioning from a symmetrical ellipse to an asymmetrical quadrilateral and undergoing rotation; this effect diminishes when the dip angle approaches 0° or 90°. The in situ stress difference controls the dominant direction of collapse, whereas hydration simultaneously weakens both matrix and weak-plane strength, causing the volume of the failure zone to expand significantly during the initial period (0–5 days). This research reveals the dynamic evolution patterns of wellbore collapse under multifactor coupling, providing a quantitative basis for optimizing well trajectories and safe drilling fluid density windows for deep shale oil in this region.