Abstract: Highly variable micro-fractures that demonstrate differing widths and irregular shapes are universally present in shale,so the borehole is easily subjected to instability.To solve this problem,a technology that combines the optimization of solid particle size and multiple collaborative plugging was proposed.In the study,the solid particle-size of drilling fluid itself and the particle size of the temporary blocking agent were optimized,three plugging materials(i.e.,latex asphalt,nanoemulsion and aluminum polymer) were combined,and consequently multiple collaborative plugging of micro-fractures in shale was achieved.Grain-size distribution optimization,filter cake shear strength coefficient and HTHP dynamic filtration experiments were carried out.The results showed that API filtration of drilling fluid decreased from 4.8 mL to 3.2 mL,the shear strength coefficient of HTHP filter cake was 0.74, and the dynamic filtration of the rock sample declined significantly.The technology of all solid particle size optimization combined with multiple collaborative plugging was applied successfully at many wells in Bohai Bay Oilfield and Shengli Oilfield.The HTHP filtration of optimized micro-fracture plugging drilling fluids significantly declinedand a long section of oil shale was penetrated without downhole failures.Laboratory tests and field applications showed that the technology of all solid particle size optimization combined with multiple collaborative plugging could significantly enhance borehole stability in shale formations and maybe used as the technological reference for the safe and quick drilling of horizontal wells in same type of formation.