Study on the Dynamic Evolution of Wellbore Stability in Thinly Interbedded Shale-Limestone-Clay Formations under Drifting Condition

ABSTRACT: This study investigates wellbore stability in thinly interbedded shale-limestone-clay formations under drifting conditions, providing insights for drilling operations in complex geological settings. Focusing on the East Baghdad Oilfield, the research identifies shale as weak planes and limestone as the matrix, examining their interactions through physical and chemical properties, microstructure, and mechanical testing. The study incorporates effects of drilling fluid hydration, fluid seepage, and fluctuating pressure from pipe movements into a dynamic wellbore stability model under multi-field coupling effects. Employing finite element numerical simulations, this model simulates the evolution of wellbore stability under different conditions. Results reveal that hydration and seepage significantly weaken the mechanical properties of clay layers, while the fluctuating pressure during pipe running amplifies instability risks, particularly in inclined and horizontal well sections. This study not only highlights key factors affecting wellbore stability such as pipe running speed, drilling fluid density, and well inclination but also suggests parameter optimizations to mitigate instability risks.