Wellbore instability of long open hole interval in the drilling operation of Tahe oilfield is prominent in recent years. The structures, physical and chemical properties of typical shale samples from targeted zone were characterized firstly. The wellbore instability of long open hole interval of Tahe oilfield was analyzed with the combination of shale sample characterization and field operation. Meanwhile, the strategies of drilling fluid technology were proposed. Two water-based drilling fluid formulas were optimized in laboratory according to the characteristics of up and bottom formation of Tahe oilfield. Polyamine polymer drilling fluid system was chosen for the up formation, it can resistant 10%NaCl, 5%bentonite, 1%CaCl₂, 1%CaSO₄ and the shale recovery was above 75%. Polyamine polysulfonate drilling fluid was chosen for the bottom formation. It can resistant 5%NaCl, 5%bentonite, 1% CaCl₂, 1% CaSO₄ and the shale recovery was above 85%. The optimized two formulas in laboratory were applied in the field respectively. The application results indicated that polyamine polymer drilling fluid could effectively suppress the shale hydration and swell in the up formation, and the average hole diameter enlargement rate in test well was 4.47%. Polyamine polysulfonate drilling fluid could effectively prevent the caving and flaking, and the average hole diameter enlargement rate in test well was 9.42%. The wellbore stability was improved, and the comprehensive benefits were improved.

Wall building properties of highdensity drilling fluid weighted with manganese tetroxide and traditional weighting materials（barite, iron ore powder）in different proportion were studied. Experimental results show that highdensity drilling fluid weighted with the compound of manganese tetroxide and traditional weighting materials were synthesized in the optimum proportion, the effect of small size and spherical manganese tetroxide particles filled into large size barite or iron ore particles was more obviously than one single weighting materials. When highdensity drilling fluid was weighted with manganese tetroxide with barite and iron ore respectively according to the quality ratio of 1∶4 and 2∶3, filtration and permeability of cake was lower.