3-D transport and acoustic properties of Fontainebleau sandstone during true-triaxial deformation experiments

• We investigate the effect of 3-D stress on deformation, transport and ultrasonic properties of sandstone rocks.
• 3-D independent permeability values measured along the three axes are sensitive to initial values of applied σ3 and σ2.
• AE hypocenter locations show the timely development of polymodal fault system under 3-D state of stress.
• Source mechanism inferred from AE events validates the formation of such fault systems.

Fontainebleau sandstone specimens have been tested in a true-triaxial geophysical imaging cell under hydrostatic (σ1=σ2=σ3), traditional triaxial (σ1=σ2>σ3) and true-triaxial (σ1> σ2>σ3) stress regimes. The research objectives are to recognize the effect of various stress ratios σ2/σ3 under varying magnitudes of σ2 and σ3 on 3-D stress–strain responses, 3-D transport, 3-D ultrasonic wave velocities, and acoustic emission properties. Comparison of the variation of 3-D directional permeability between the specimens shows that higher magnitude of σ3 and σ2 disrupts the 3-D interconnectivity of the pore spaces and intergranular cracks, irrespective of the magnitude of applied stress ratio σ2/σ3. The observed permeability anisotropy between the two specimens is a function of inherent oriented pores and cracks, stress ratio σ2/σ3 and the magnitude of applied σ3 and σ2 stresses. Temporal and spatial evolution of the AE hypocenter locations combined with the examination of micro-CT and thin section images shows the timely development of polymodal fault systems during peak to post-peak stresses and validates the development of the source mechanism inferred from the acoustic emission events.