MODELING THE STRESS-STRAIN BEHAVIOR OF SATURATED ROCKS UNDERGOING TRIAXIAL DEFORMATION USING COMPLEX ELECTRICAL-CONDUCTIVITY MEASUREMENTS

Measurement of complex electrical conductivity as a function of freque ncy is an extremely sensitive probe for changes in pore and crack volu me, crack connectivity, and crack surface topography. Such measurement s have been made as a function of pore fluid chemistry, hydrostatic co nfining pressure, as well as uniaxial and triaxial deformation. This p aper will; (1) describe the effects of triaxial deformation on the com plex electrical conductivity of saturated porous rocks, (2) use the el ectrical data to model the mechanical stress-strain behaviour, and, (3 ) compare the modelled behaviour with the stress-strain behaviour meas ured during the deformation. Experimental conductivity data tracks how the rock undergoes compaction with progressive loss of crack volume, followed by dilatation due to new crack formation, growth of existing cracks, crack interlinkage, and finally failure, as axial strain is in creased. We have used the complex electrical data to produce a directi on-sensitive (anisotropic) crack damage parameter, and used it to calc ulate the effective Young's modulus by employing the models of Walsh a nd Bruner. Comparison of the synthetic stress-strain curves so produce d, with the experimentally derived stress-strain curves shows good agr eement, particularly for undrained tests. This modelling is an improve ment on similar curves produced using isotropic crack damage parameter s derived from acoustic emission data. The improvement is likely to be due to the directional sensitivity of the electrical conductivity mea surement, and its ability to discriminate between the formation of iso lated cracks, and those cracks that contribute to the inter-connected crack space i.e. those cracks upon which transport properties of the r ock such as electrical conductivity, and mechanical properties depend most critically during triaxial deformation.