Coupled triaxial testing of rock salt specimens

An apparatus is described that permits simultaneous measurement of several coupled geo-mechanical and geophysical parameters on intact specimens of rock salt saturated with brine and subjected to triaxial test conditions. Representative experimental results are presented for hydraulic permeability, electrical resistivity, and p-wave velocity which were determined while the test specimen equilibrated at a confining pressure of 6.2 MPa and a pore pressure of 2.75 MPa, during axial loading and deformation to a total axial strain of 5%, and subsequent to the deformation. The results are cross-correlated with the applied (differential) axial stress. The extremely low permeability of rock salt and corrosive nature of saturated brine required development of special sample preparation and jacketing techniques, as well as wetted apparatus parts made from Hastalloy, and the use of the transient pulse technique for permeability determinations. Measurement of permeability as low as 10 nanodarcy (10-20 m2) were successfully made. Measured pore pressure variations caused by either a pressure step increase or decrease showed excellent reproducibility and agreement with calculated curves. During the last 12 hours of equilibration under confining pressure, the permeability of the salt specimen decreased from slightly more than 1 microdarcy to about 500 nanodarcy; permeability increased to approximately 70 microdarcy after deformation to 5% strain