Study on the influence of time-varying characteristics of mud cake on the safe density window of drilling fluid

During overbalanced drilling, the presence of mud cake can effectively reduce the pressure of fluid in the well on the wellbore. However, the existing wellbore stability models mostly ignore the pressure loss caused by mud cake, which leads to the deviation of the safe density window of drilling fluid. Therefore, a time-dependent model of physical parameters of mud cake is introduced in this study. By coupling this model with seepage theory, the changes of pore pressure and water saturation near the wellbore during the dynamic generation of mud cake are analyzed, and then the wellbore instability model is reconstructed by considering the double influence of mud cake on the stress around the wellbore and the mechanical parameters of rock. The results show that with the continuous generation of mud cake, the effective pressure difference of shaft wall gradually decreases from the initial liquid column pressure and tends to be stable, and the deterioration degree of rock mechanical parameters decreases. After 30 h of drilling, the collapse pressure of the wellbore decreased from 1.42 g/cm³ to 1.33 g/cm³, while the fracture pressure increased from 1.71 g/cm³ to 1.87 g/cm³. The wellbore instability model considering mud cake proves that the drilling fluid system that is easy to form high-quality mud cake can expand the safe density window and achieve safe and efficient drilling with reduced density and increased drilling speed. This study provides a theoretical basis for determining the accurate safe density window of drilling fluid.