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Abstract In the traditional rock mechanics analysis in petroleum engineering，the orthotropic formation is always simplified to be transverse isotropic. However，this simplification is not always suitable in some special formations. The rock mechanics orthotropic experiment has been done with drilling core. The test results show that the anisotropy of mechanical property at different directions，in the plane perpendicular to wellbore axis is objective. Based on the mechanics of orthotropic materials，the new model for stress analysis of borehole surrounding rock has been established with superposition principle. The model includes the function of hydrostatic pressure of drilling mud，maximum and minimum horizontal in-situ stresses respectively. The computational results show that no matter how the horizontal stress is uniform or not，if the formation is orthotropic，the stress distribution of borehole surrounding rock will always be nonuniform. Moreover，the maximum stress of orthotropic formation is larger than that of isotropic formation. The orthotropic of formation mechanical property can make the borehole stress distribution worse. So this feature should be considered into the mechanical analysis of corresponding projects.
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Received: 19 March 2010
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2011, Vol. 30 
