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

Permeabilities lower than 10(-17) m2 cannot be measured using classica l methods. To overcome this problem, the pulse decay method may be app lied. 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 bee n constructed to measure ultra-low permeabilities of rocks under therm omechanical stresses; intrinsic permeabilities between 10(-15) and 10( -22) m2 can be measured. Several thermo-mechanical tests, including pe rmeability 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 th e undisturbed state of the formation. Nevertheless, the permeability c hanges observed during these tests seem to be directly connected to th e nature and structure of the material and are indicative of the struc tural modifications the material sustains under changes of temperature and stress. During isotropic deformation tests, the response of perme ability to pressure varies depending on the specific lithological char acteristics of the samples. Deformation tests under deviatoric stress with constant confining pressure and temperature show that, for all th e lithofacies, the permeability decreases continuously. This can be at tributed to creep deformation of the halite.