Numerical modeling of thick-walled hollow cylindrical samples: effect of status of applied stresses on deformation and shape of failure

In addition to high cost levels, shaft excavation in different sectors such as mining, oil and gas extraction have always been dealing with instability problems. Instability hazards can be minimized by controlling the deformation and failure of wellbore wall. The purpose of this paper is to study the deformation and failure mechanism of thick-walled hollow cylindrical specimens (for modeling of oil well) during the excavation in two different stress conditions. Laboratory tests were carried out on artificial specimens of gypsum and natural specimens of marl, sandstone and clayed limestone in order to determine the mechanical properties of the aforementioned rocks. The experiments in high stress conditions could not be carried out due to some limitations related to laboratory. Therefore, triaxial tests were done on thick-walled hollow cylindrical specimens of gypsum using modified Hoek cell. The results of tests were employed to develop and modify numerical models which were used for prediction of deformation and failure mechanisms in thick-walled hollow cylindrical natural specimens of marl, sandstone and clayed limestone in the following stress condition. It was revealed that the failure mechanism in the wall of thick-walled hollow cylindrical specimens for stress condition of is shear failure which occurs in two opposite points on the borehole wall. However, in the stress condition of the failure occurs in three points with random distribution. The maximum and minimum values of borehole pressure in the failure moment occurred for sandstone and clayed limestone respectively in both stress conditions. Also the outcome of numerical modeling revealed the maximum and minimum values of plastic strain to be in cases of clayed limestone and sandstone, respectively. Pressure within the borehole at the moment of failure could be used as initial pressure of drilling fluid.