Abstract: Based on the investigations to the geology characteristics of condensate oil and gas reservoirs in the buried hill of Qianmiqiao, and analysis to the completed wells in this area, studies were carried out on the drill-in fluid used for under-balanced drilling in the small clearance holes. For example, study on such performances as density, viscosity, shale inhibition, high-temperature tolerance, contamination resistance of the drilling fluid used in the small clearance of the Ordovician system; study on formation protection in the small clearance of the Ordovician system; study on drill pipe corrosion in drilling caused by the sour materials in the Ordovician system. Study results showed that when drilling fluid density of lower than 1.0g/cm3 is demanded, water-in-oil drilling fluid should be used; when plugging the lost circulation zones, the composite plugging agent and unidirectional pressure sealing agent should be adopted. Basic zinc-carbonate and corrosion inhibitor can be used for protecting drill pipe form corrosion. These study achievements have been used in over ten wells, with good results obtained.