Evolution of dilatancy and permeability in rock salt during hydrostatic compaction and triaxial deformation

Combined gas permeability and P and S wave velocity measurements were carri ed out on rock salt samples from the Gorleben salt dome and the Morsleben s alt mine under hydrostatic and triaxial loading condions, mostly at room te mperature. Permeabilities in the as-received samples vary between 10(-16) a nd 2x10(-20) m(2). The initial permeability is primarily due to decompactio n induced by drilling core retrieval and sample preparation. Hydrostatic lo ading gives rise to a marked decrease of permeability and a coeval signific ant increase of P and S wave velocities due to progressive closure of grain boundary cracks, tending to approach the in situ matrix permeability (< 10 (-20) m(2)). The pore space sensitivity of P and S wave velocities is used to monitor the in situ state of the microstructure. Their reversals define the boundary in the state of stresses between dilatant and compactive domai ns (dilatancy boundary). Dilatancy during triaxial deformation of the compa cted rock salt samples is found to evolve stress dependent in various stage s. The crack initiation stress increases from <similar to>18 MPa differenti al stress at 10 MPa confining pressure to similar to 30 MPa at confining pr essures above similar to 70 MPa. Dilatancy is due to the opening of grain b oundary and (100) cleavage cracks and depends on the applied confining pres sure. The orientation of the open cracks is primarily controlled by the loa ding geometry system (compression, extension). As a consequence, permeabili ty increases dramatically with progressive dilatancy, followed by a period of plus/minus constant permeability during strain hardening up to 10% axial strain or even more. This suggests that the evolution of permeability is n ot only a function of dilatancy but also of microcrack linkage. Importantly , the anisotropic crack array within the samples causes a strong directiona l dependence of permeability.