Effects of gas pressure on failure and deviatoric stress on permeability of reservoir rocks: initial studies on a Vosges sandstone

Abstract

The geomechanical behaviour of a specific type of sandstone is investigated using tri-axial compression tests, a hydrostatic compression test to measure Biot’s coefficient, tri-axial tests with pressurised gas, and permeability tests under deviatoric stress. The tri-axial compression tests reveal a nonlinear stress–strain relationship at low confining pressures. Biot’s coefficient initially decreases with increasing confining pressure, and then gradually increases if the confining pressure is increased beyond a certain level. At constant confining pressure and deviatoric stress, the gas pressure loading path has a significant influence on the mechanical failure of the sandstone sample. If the gas pressure is increased by comparatively small increments, the critical pressure the sample can withstand is found to be close to the theoretical value, whereas if the gas pressure increments are comparatively large, the sample is found to fail at lower values of gas pressure. The injection and release of gas also accelerate the sample’s failure. At high confining pressures, there is a significant decrease in permeability with increasing deviatoric stress. However, at low confining pressures, the decrease in permeability is not as significant as that observed under high confining pressures. The reloading–unloading of deviatoric stress does not have a predictable effect on permeability.