Abstract: Caving pressure of WZ12-1N oilfield is high while the reservoir liquid has normal pressure gradient.Formation pressure coefficient is less than 1.05 g/cm3 and the drilling fluid used is bigger than 1.3 g/cm3.Two well completion systems of perforation completion and open hole completion were adopted.Because the drilling fluid pressure is higher than that of the reservoir fluid,the drilling fluid will intrude into formation deeply and quantitatively.This would cause reservoir contamination and damage as well as abnormally high flow-back pressure.Measures of properly protecting reservoir,optimizing and choosing appropriate drilling and completion fluid systems were formulated by studying the reservoir nature,sensitivity and special requirements of deviated holes in WZ12-1N oil field for drilling fluids.The goal of drilling fast and excellently was realized and the possible damage in the process of drilling was effectively alleviated.