Streaming potential measurements: 2. Relationship between electrical and hydraulic flow patterns from rock samples during deformation

International audienceStreaming potential and resistivity measurements have been performed on Fontainebleau sandstone and Villejust quartzite samples in a triaxial device during compaction, uniaxial compression, and rupture. Measurements on individual samples do not show any clear intrinsic dependence of the streaming potential coefficient with permeability. An apparent dependence of the streaming potential coefficient with permeability is, however, observed during deformation. The effect of surface conductivity is taken into account and is small compared with the observed changes in the streaming potential coefficient. The observed dependence is therefore interpreted in terms of a difference in the evolution of the electrical and hydraulic connectivity patterns during deformation. This effect causes the streaming potential coefficient, and consequently the inferred • potential, to be reduced by a geometrical factor R G representing the electrical efficiency of the hydraulic network. Estimates of the RG factor varying between 0.2 and 0.8 for electrolyte resistivity larger than 100 l • m are obtained by comparing the values of the • potential inferred from intact rock samples with the values obtained from crushed rock samples, where the geometrical effects are assumed to be negligible. The reduction of the streaming potential coefficient observed during compaction or uniaxial compression suggests that the tortuosity of the hydraulic network increases faster than the tortuosity of the electrical network. Before rupture, an increase in the streaming potential coefficient associated with the onset of dilatancy was observed for three samples of Fontainebleau sandstone and one sample of Villejust quartzite. The changes in streaming potential coefficient prior to failure range from 30% to 50%. During one experiment, an increase in the concentration of sulfate ions was also observed before failure. These experiments suggest that observable streaming potential and geochemical variations could occur before earthquakes. 1. Introduction This paper is the second part of a study devoted to an experimental investigation of the electrokinetic effect (EKE) in the laboratory. As discussed at length by Lorne et al. [this issue], this study is motivated by the fact that variations of electric potential observed in a variety of geophysical contexts (geothermal fields, volcanoes, and, in some instances, before earthquakes) remain poorly understood. In the companion paper we used crushed samples to investigate the properties of the electrical double layer (EDL), which is at the core of the EKE, and some empirical laws were obtained. To assess the EKE in natural systems, in the present study we investigate the relationship between fluid flow and electric potentials during deformation and rupture. The streaming potential coefficient, which is the ratio of the electric potential to the pressure gradient, is measured by recording the voltage across a sample through which an electrolyte is flowing. The streaming potential coefficient (also called the electrokinetic coupling coefficient) is proportional to the pressure gradient and a quantity called the s r potential, which characterizes the structure of the EDL