Study On Mechanics Problems Of Borehole Stability In Anisotropic Formation

Most of the existing models for mechanics study of borehole stability borehole are based on isotropic rock,which makes the predicted fracture pressure and collapse pressure for maintaining borehole stability incapable of meeting the need of drilling safety.This paper considers rock elastic anisotropy and studies on mechanics problems of borehole stability based on homogenization linear elastic mechanics theory.The major research includes as follows: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 regarding transversely isotropic formation is proposed.Based on the theory of mechanics of anisotropic materials and the complex function solution of generalized plane strain elasticity problem,circular-hole boundary conditions of stress distribution,a calculation model of stress around the borehole in anisotropic formations is built up.The analytical expressions for the stress field are formulated which can reflect the influences of the formation anisotropy,the bedding plane inclination and dip direction,the direction and the ratio of the in-situ stresses the borehole deviation and azimuth angles.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.The coordinate transformation of in-situ stress and elastic coefficient is performed in the study on mechanics problems of borehole stability in anisotropic formation.A parametric study is carried out to investigate the effect of the elastic parameters,the bedding plane inclination and orientation,the ratios of horizontal in-situ stress,the borehole deviation and azimuth angles on the principal stress distribution.The formulation might provide a more realistic basis of theory for analyzing the stability of the wellbore and optimizing the well trajectory in anisotropic formation.