Electrical conductivity evolution of non-saturated carbonate rocks during deformation up to failure

We present electrical conductivity measurements (at a fixed frequency of 1 kHz) performed on three directions on limestone samples from the quarry of Meriel, during uniaxial tests of deformation up to failure. Samples were saturated from 100 to 80 per cent by drainage. The samples showed brittle fracture with Young's modulus in the range 10-13 MPa. Formation factor (sample resistivity divided by water resistivity) values range between 2 and 4. In saturated conditions the electrical measurements reflect the initial rock compaction, followed by dilatancy due to new axial cracks formation and finally crack coalescence, fracture localization and failure. The conductivity increase is related to the crack porosity Phi(c), which starts to increase at relatively low stress (31 per cent of strength). The magnitude of the electrical conductivity variation is 1-4 per cent of the initial value. We show that when saturation is decreased the conductivity increase occurs earlier during the deformation process, from 68 to 17 per cent of strength for 100 to 80 per cent of water saturation, respectively, so that the decrease in conductivity at low stress is less and less present. The induced relative rock conductivity variation in non-saturated and undrained conditions is the result of two competing effects: the relative porosity variation and the relative saturation variation during the deformation process. During compaction the electrical conductivity can show either a small decrease or a small increase; since the size of the partially saturated pores and cracks is reduced, the water occupies a larger percentage of the pore space, and then conductivity can be increased at this stage. We show a continuous increase of the conductivity both during the compaction and the dilatancy phases when the initial saturation is about 80-85 per cent. Finally a power law is shown between conductivity and stress, so that the relative electrical conductivity increase is larger as one goes along the compression process. Just before failure, at 90-95 per cent of strength, the rate increase in horizontal conductivity drops, so that the anisotropy between axial and radial conductivity is about 0.5-2 per cent. At failure a drastic increase of this anisotropy can be seen, up to 5-6 per cent (CME21, CME24 and CME13 samples).