Study On Prediction Of Wellbore Collapse Pressure In Formation Of Weak Planes Rock

Currently,the study on borehole stability in formation of weak planes rock such as shale and coal,has been concentrated on weak planes such as bedding planes?cleat faces and micro cracks.However,it's no longer apllicable to regard the elastic and strength of rock as isotropic.The prediction work of collapse pressure and in-situ stress need consider the rock as anisotropic.Based on the common expression concerning generalized Hooke's law,the constitutive theory of rock elastic anisotropy is recommended from the point of view of mechanics of anisotropic material.The approach for the determination of elastic constants of transversely isotropic rocks is summarized and the calculating method of in-situ stress for the normal ?orthotropic and anisotropic formation is proposed.As the Jaeger-Cook law and Mclamore law were analysised,concerning the results of rock mechanic experiments,this paper employing the shear failure strength criteria based on JaegerCook law to propose a rock strength model of weak plane rock in tri-axial stress state.This paper also analysis the changes of rock strength in this model.According to tensor transformation rule,the coordinate transformations of stress,strain,and the flexibility coefficient of anisotropic material have been derived,and the matrix for coordinate transform and its calculation method are given.This paper proposed a wel bore stability model of weak planes rock by using the tensor transformation.This paper analysis the collapse pressure changes with different deviation and azimuth in different stress state by using the stability model of weak plane rock.The formulation might provide a more realistic basis of theory for analyzing the stability of the wel bore and optimizing the well trajectory in anisotropic formation.