GAS-PERMEABILITY CHANGES OF ROCK-SALT SUBJECTED TO THERMOMECHANICAL STRESSES

Permeabilities lower than 10(-17) m2 cannot be measured using classical methods. To overcome this problem, the pulse decay method may be applied. In this method the evolution of a pressure pulse applied on the upstream side of the sample at t = 0 is recorded. An apparatus has been constructed to measure ultra-low permeabilities of rocks under thermomechanical stresses; intrinsic permeabilities between 10(-15) and 10(-22) m2 can be measured. Several thermo-mechanical tests, including permeability determinations have been carried out on samples from saline lithofacies of the Bresse basin (France). The initial permeabilities are more representative of the condition of the test sample than of the undisturbed state of the formation. Nevertheless, the permeability changes observed during these tests seem to be directly connected to the nature and structure of the material and are indicative of the structural modifications the material sustains under changes of temperature and stress. During isotropic deformation tests, the response of permeability to pressure varies depending on the specific lithological characteristics of the samples. Deformation tests under deviatoric stress with constant confining pressure and temperature show that, for all the lithofacies, the permeability decreases continuously. This can be attributed to creep deformation of the halite.