Evolution of Gas Permeability of Rock Salt Under Different Loading Conditions and Implications on the Underground Hydrogen Storage in Salt Caverns

We performed a complete set of laboratory experiments on a rock salt specimen to study the complex evolution of gas permeability under different loading conditions. The porosity of the studied rock salt is very low (similar to 1%) and the initial permeability varies over 4.5 orders of magnitude. The Klinkenberg effect is only observed for the less permeable and damaged samples. The poroelastic coupling is almost negligible in our samples. Deviatoric loading under low confining pressure (1 MPa) induces a moderate increase in gas permeability from the dilatancy threshold due to microcracking. Measurement of ultrasonic wave velocities during uniaxial compression test showed an almost irreversible closure of pre-existing micro-cracks and the opening of axial micro-cracks that are perpendicular and parallel, respectively, to the uniaxial stress direction and allowed a precise determination of the dilatancy threshold. Under higher confining pressure (5 MPa), no increase in permeability was measured because the material becomes fully plastic which practically eliminates microcracking and thus dilatancy. Under hydrostatic loading, gas permeability decreases because of cracks closure and this decrease is irreversible due to the time-dependent self-healing process. Permeability increases slightly during dynamic mechanical and thermal fatigue due to microcracking, while it reduces during static fatigue (creep) thanks to the self-recovery process. All these results give strong confidence in the underground hydrogen storage in salt caverns which remains by far the safest solution because the different mechanisms (viscoplasticity with strain hardening, microcracking and cracks healing) involved in material deformation act in a competitive way to annihilate any significant permeability evolution.