Abstract: The L Gas Field in the Western South China Sea has a shallow depth and low fracture pressure as well as high mudstone content,which increases viscosity and shear force due to the high rheology of drilling fluid and mudstone mud-making,as well as frequent lost circulation caused by high ECD in horizontal well drilling.Hence,the rheological properties of drilling fluid were optimized by combining the core displacement test results with software-simulated ECD calculations.The method of high-concentration mudstone mud-making was used to optimize the inhibition performance of the drilling fluid,and the self-made high-pressure sand-filled pressure meter was used to evaluate the pressure bearing capacity of drilling fluid.When the EZFLOW drilling fluid had a viscosity of 15 000-30 000 mPa·s at low shear rate,and when the recovery rate of direct flow back permeability was higher than 85%,the ECD was less than the formation fracture pressure;the optimized drilling fluid demonstrated strong inhibitory and plugging properties,with an ability to resist 25% of in-situ mudstone contamination.Field application results showed that the apparent viscosity of the drilling fluid was insensitive to well depth change,the maximum ECD added value was only 0.07 g/cm3,no leakage occurred during drilling and gravel packing,and the gas production exceeded the allocated rate of 10×104m3/d,with the maximum rate of 16×104m3/d.The study results indicate that an optimized EZFLOW solid-free weak gel drilling fluid can solve the problem of formation break-down caused by low fracture pressure in ultra-shallow horizontal wells as well as reservoir protection.