Behavior of Anisotropic Rocks under Combined Stresses

Abstract

Abstract Torsional strength tests on slate and Green River shale were performed at various orientations of the cleavage or bedding planes and at various states of stress to study the strength-deformation behavior of anisotropic rocks. The strength characteristics of the rocks, as a function of both the state of stress and the orientation of anisotropy, were determined by twisting cylindrical specimens at a constant rate of 0.1 radian/min. under confining pressures of 5,000 to 30,000 psi, with pore pressure held constant at atmospheric pressure, and under axial loads of 0 to 10,000 pressure, and under axial loads of 0 to 10,000 psi. Samples having an orientation of the plane psi. Samples having an orientation of the plane of anisotropy relative to the cylindrical axis of 0, 30, 45, 60 and 90 degrees were used. In general, torsional failure strength of the rocks studied increased with mean pressure; the strength was a function of the initial state of stress and the orientation of the plane of anisotropy. Several failure criteria for anisotropic rocks were discussed and compared with experimental data. It was found that current failure criteria could be applied with some degree of success to experimentally generated data. The orientation of the maximum and minimum torsional strength for both rocks were found to be independent of the initial stress state studies. The failure angle was affected by anisotropy for cleavage or bedding plane over a limited range of orientations. The failure angle was also affected by the initial state of stress. The average modulus of rigidity increased with increasing confining pressure and decreased with axial compressive load. The cohesive strength and the coefficient of internal friction varied with the orientation, and the specific variance was a function of the type of anisotropy. Introduction The fundamental problem of rock mechanics in drilling is to determine the state of stress necessary to cause failure. Unfortunately, the failure mechanisms and strength characteristics of rock are not constant. They depend upon rock type, pressure, temperature, strain rate, foliation, type of loading and presence of pore fluid. Therefore, a better understanding of the aspects which govern rock failure may contribute to the solution of drilling and fracturing problems. Early works by von Karmon and Boker [cf. Nadai], Robertson, Griggs, Handin, Heard, Robinson and Serdengecti have provided considerable information about the effects of temperature, rate of loading and confining and pore pressures on the mechanical behavior of rocks. Inhomogeneity and anisotropy are inherent in the process of sedimentation.