Damage of saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements: Experimental results

Abstract

The frequency dependent complex electrical conductivity of brine saturated rocks is extremely sensitive to changes in the volume, connectivity, orientation, and surface topography of pores and cracks. We have made triaxial deformation experiments on sandstone specimens saturated with distilled water. Experiments were carried out for several values of confining pressure, and in both drained and undrained regimes. During the deformation the full complex (in-phase and out-of-phase) electrical parameter set was measured (i.e. conductivity, resistivity, permittivity etc.) for 50 frequencies from 20 Hz to 1 MHz. Only the data at 1 kHz will be discussed here. This data tracks how the rock undergoes crack closure, followed by dilatancy, crack linking, and finally failure, as axial strain is increased. The data indicates well how early the formation of new cracks begins, showing that the quasilinear portion of the stress-strain curve for triaxial deformation of saturated rocks does not represent truly elastic behaviour, but represents the combined effects of crack closure perpendicular to the strain axis and the formation of tensile cracks parallel to the strain axis. The electrical data has also been used to derive an electrical-equivalent change in porosity, and to examine the way that the cementation exponent and the tortuosity of the pore and crack network change during deformation.