Poromechanical Properties of a Sandstone Under Different Stress States

Poromechanical properties of a sandstone from an underground gas storage site are investigated with the use of a neutral gas to control the pore pressure. The poroelastic theory of anisotropic medium is used to evaluate elastic properties and coupling coefficients under different stress states. In-situ CT observations have been made under uniaxial compression test and they are used to underline some effects due to the occurrence of damage and microcraks. Under hydrostatic loading, pores and micro-cracks are gradually compressed. The initial state of the sandstone is “slightly” transversely isotropic but in a first approach the Biot’s tensor reduces to a scalar. This coefficient decreases with the increase in confining pressure that can be attributed to the closure of micro-cracks. The effect of damage and cracking is investigated with two series of conventional triaxial tests conducted at different confining pressures, either with an increase in pore pressure (series 1) or with a decrease in pore pressure (series 2), to evaluate the coupling coefficients. The results, derived from these two test series, are consistent as regards the effects of cracking on the material behavior. There is an obvious damage due to the triaxial loading which induces a parallel decrease in the ratio E₃/(1 − ν₃) and in the modulus H₃. Axial compaction results in the continual increase in the H₁ modulus observed for every sample whatever the measurement technique was.