Abstract: The Well Biye HF 1 is the first shale oil horizontal well,and the horizontal section was 1000.00m drilled in shale formation in third spud by well planning.The water-based polymer drilling fluid was used in the directional section.Serious unstable hole occurred near the horizontal hole when mudstone and shale were drilled.In order to prevent shale hydration,dispersion,wall collapse and stick that easily take place in long horizontal section,oil-based drilling fluid was tested in horizontal section of this well.In consideration of the flash point,toxicity and aromatic hydrocarbon content,the No.5 white oil was chosen as the continuous phase of the drilling fluid.In accordance with the requirements of the activity,the concentration of 28.0 g/L of CaCl2 aqueous solution was chosen as the aqueous phase.By optimizing the primary and auxiliary emulsifiers,their dosage and the amount of wetting agent,organic clay,filtrate reducer,shear strengthening agent and plugging agent were determined,ultimately establishing the formulation of the oil-based drilling fluid.Evaluation tests on basic performance and the rate of expansion,recovery,ability against cuttings contamination and plugging capacity were carried out,which demonstrated a satisfactory result.It was used in horizontal section in third spud for only 6.5 days to reach the length of 1,044.00 m,application shows that the oil-based drilling fluid has such advantages as strong plugging capability,stable de-emulsification performance under high voltage,low fluid loss,good rheology and strong cuttings carrying capability.