Franck Agostini

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

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.

About adsorption effects on the poroelastic properties and gas permeability of COx claystone

Abstract Argillaceous rocks have long been selected as privileged host rock candidates for the nuclear waste disposal. For the sake of evaluating the sealing capacity of these natural barriers, one urgent issue is to study the poromechanical properties of them in the long-term gas migration process. As the key parameter of poroelasticity, Biot’s tensor of argillaceous rocks should be firstly studied. In this paper, a series of poromechanical tests was firstly carried out to measure Biot’s tensor components. ‘Non-logical’ values that Biot’s tensor components were larger than unity (1.0) were obtained in the tests. It was then suspected that the process of injecting pore pressure is not a purely poromechanical problem. In order to deeper study about these values, complementary poromechanical tests were handed on two samples with helium and argon or helium and carbon dioxide (CO2), respectively, to confirm the adsorption-swelling phenomenon. The results showed that Biot’s tensor components are obviously affected by the gases used, which is owing to their different adsorption capacities. To evaluate these swelling affect consequences, a series of permeability and triaxial compression tests was performed with different type of gases. The results obtained reveal that gas adsorption-swelling deeply effect the transport and (poro)mechanical properties of argillite.