Brittle–Ductile Transition of Oil Shale

The importance of oil shales as an economical substitute to conventional resource has increased many folds. The extraction of petroleum from these resources at a commercial level can be achieved with the help of wellbore and hydro-fracturing. The extremely high in-situ stresses result in the collapse of the wellbore during excavation. Therefore, the selection of material model for wellbore design will depend on the failure mechanism. At greater depth the failure of rocks changes from brittle to ductile, and hence the rheological model that applies to either the brittle or ductile region is not applicable in the semi-brittle region. Therefore, in the present study, an attempt is made to understand the brittle–ductile transition of anisotropic oil shale rock. The oil shale samples obtained from Assam are tested under the high confining pressure, ranging from 2 to 30 MPa, at different anisotropic angles. The effect of confinement and anisotropy on the strength is studied in detail. From the investigation, it is found that the material shows brittle behaviour at low confinement, whereas at high confinement it shows brittle–ductile transition. It is also observed that the anisotropy plays a critical role in defining the brittle–ductile transition zone. It is observed that the oil shale rock undergoes the transition from brittle to ductile at 30 MPa of confinement for 0 and 90° anisotropy angles. This suggests that he generalised design philosophy may not be effective at greater depth. Further, the transition for 30° anisotropy angle starts at 12.5 MPa of confinement pressure. Therefore, the designing of wellbore should be carried out by using nonlinear anisotropic strength criterion.