Wellbore stability model for argillaceous limestone - claystone thin interbeds under synergistic effect of fluctuating pressure -hydration

The effect of hydration and fluctuating pressure in drifting conditions presents challenges to wellbore stability, impacting cost savings and safety in drilling operations. This study investigates the stability of thin mudstone-limestone and claystone interlayers in the East Baghdad oil field, introducing strength damage variables influenced by hydration and fluctuating pressure. Utilizing damage mechanics, elasticity, and joint strength theories, and accounting for matrix and weak plane failures, drilling fluid hydration reactions, and fluctuating pressures, a wellbore stability model is established. Key parameters such as wellbore trajectory, weak plane quantity, hydration time on collapse pressure, and tripping speed are examined, assessing stability under combined hydration and pressure effects. The results suggest optimizing wellbore trajectory, particularly the inclination angle, can reduce collapse pressure and increase fracture pressure, thus enhancing operational safety. Weak planes raise collapse pressure, reduce fracture pressure, and limit safe drilling directions, heightening wellbore instability and tripping challenges. Prolonged formation exposure to drilling fluids should be minimized, and fluid density optimized to widen the safe density window. Controlling tripping speed and monitoring wellbore pressure are critical to mitigating instability risks. Field validation confirms the model's accuracy, aligning predictive outcomes with real conditions and enhancing safe drilling fluid density and tripping speed guidance.