Abstract: To address the challenges of poor wellbore cleaning and leakage risks in the reddish-brown mudstone formations of Oilfield A in Bohai, an environmentally friendly synthetic-based drilling fluid system with excellent plugging performance and strong inhibition (BIODRILL-S synthetic-based drilling fluid) has been developed. Field application results show that this drilling fluid exhibits strong inhibition, with only 9.9% wellbore enlargement rate of the applied wells. It has good lubricity, enabling smooth tripping without initial friction even in extended-reach wells. Additionally, it features high static and dynamic shear force, with simple maintenance of rheological properties. For extended-reach wells, the increase in ECD (equivalent circulation density) compared to the drilling fluid density is only about 0.12 g/cm³, indicating a certain anti-leakage effect.This drilling fluid has been successfully applied in many oilfields in the Bohai area, achieving favorable results in all cases. It provides effective support for the continuous green and stable production of Bohai Oilfield and the development of complex lithologic formations.