Analysing the effect of bedding plane orientation on the wellbore failure

The paper presents a theory of the “plane of weakness” modelling applied to a deviated borehole that penetrated through laminated shale containing numerous parallel weakness planes. The obtained results show that the strength envelope for anisotropic rock is characterised by a U-shaped reduction in strength for failure along the weakness plane, confining pressure from 20 MPa to 80 MPa whithout any change in the failure mechanism of the shale. In terms of borehole failure, there is a risk zone within the well, whose inclination varies between 60o - 90o, and the azimuth changes from 100o - 165o.