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Severe borehole collapse has occurred during extended reach drillingsin shale formations of Dongping Block, Qinghai Oil-field. An extreme difficulty is to choose mud weight lower bounds properly. Previous isotropic wellbore stability model does not consider the heterogeneities in transversely isotropic porous media like shale, thus fails to predict mud weight accurately. To solve the problem, a new wellbore stability modelis proposed; and an optimized nano-type drilling fluid is designed.whichcan be applied fordrilling operation.The wellbore stability modelis developed through four steps: 1. The far-field stressesare transformed from an in-situ stress coordinate system to a borehole coordinate system; 2. An analytical solution for a near-borehole stress distribution in an elastic anisotropic formation under a transformed in-situ stresses regime is used, based on the complex-variable elasticity theory and the Biot's consolidation theory; 3. The around-borehole stressesare transformed from the borehole coordinate system to a weak-plane coordinate system; 4. An improved Mohr-Coulomb failure criterion is used to identify rock failure mode (along weak plane or through rock matrix) and to calculate the mud weight lower bound.In addition, laboratory experimensts have demonstrated the ability for water-soluble hybrid nanoparticles to plug shale matrix and micro cracks inside the shale, which helps to prevent chemical erosion of fluid to the rock. By sealing natural fractures with nanoparticles and optimizing mud weight using the new wellbore stability model, the borehole instability is effectively controlled. A procedure is also developed for wellbore stability analyses based on a comprehensive consideration of the formation property, the mud chemical property and the mud weight. This approach was applied to 12 deviated pilot wells drilled through shale formation in Dongping Block, where high pore pressure and natural fracture systems prevail: the mud weight is increased from previous 1.5g/cm 3 to the present 1.85g/cm 3 . It is observed that the drilling time and the drilling complexities have been reduced by up to 52.4% and 62.4%, respectively, with significant cost savings.

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