Though the mechanical studies of wellbore stability in the water-sensitive shale formations have been conducted for many years, there was almost no successful example of application in drilling operations. The main reason for this is that the in-situ mechanical properties, strength and hydrated stress of shales can not be accurately determined using traditional procedures.

The project focuses on studying the shale strength in different water contents and the changes of hydrated stress in different in-situ stress, hydraulic drop and classification of shales. The shale samples were obtained from S3 group in Oagang Oilfield. On the basis of plenty of multidisciplinary teamwork, the procedures for the comprehensive evaluation of the various properties of the shales, such as the in-situ strength, pore pressure, Yuang's modulus are given by virtue of a series of laboratory test data, log data and drilling information. In addition, a model that can be used to calculate the borehole collapse pressure, fracture pressure and to determine the suitable density of drilling mud is developed according to the theory of elastic mechanics in porous medium and the theory of partial molar free energy. The model is superior to the models provided in the past in the accuracy of prediction because its input parameters were synthetically identified with test data, log data and drilling information, and both the influences of drilling muds on the shale properties and hydrated stress were taken into account.

The model has been successfully applied to the drilling operations of the horizontal wells in Dagang Oilfield, located in east China. It is confirmed that the optimum densities of the drilling muds predicted with the model are practical, indicated by the quite stable boreholes with very low hole enlargements (less than 10%) in shale formations for the horizontal wells.

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