Clarisse Bordes

Frequency-Dependent Streaming Potentials: A Review

The interpretation of seismoelectric observations involves the dynamic electrokinetic coupling, which is related to the streaming potential coefficient. We describe the different models of the frequency-dependent streaming potential, mainly Packards and Prides model. We compare the transition frequency separating low-frequency viscous flow and high-frequency inertial flow, for dynamic permeability and dynamic streaming potential. We show that the transition frequency, on a various collection of samples for which both formation factor and permeability are measured, is predicted to depend on the permeability as inversely proportional to the permeability. We review the experimental setups built to be able to perform dynamic measurements. And we present some measurements and calculations of the dynamic streaming potential.

Experimental quantification of the seismoelectric transfer function and its dependence on conductivity and saturation in loose sand

ABSTRACT Under certain circumstances, seismic propagation within porous media may be associated to the conversion of mechanical energy to electromagnetic energy, which is known as a seismo‐electromagnetic phenomenon. The propagation of fast compressional P‐waves is more specifically associated to the manifestations of a seismoelectric field linked to the fluid flows within the pores. The analysis of seismoelectric phenomena, which requires the combination of the theory of electrokinetics and Biots theory of poroelasticity, provides us with transfer function Symbol that links the coseismic seismoelectric field E to the seismic acceleration Symbol. To measure the transfer function, we have developed an experimental setup enabling seismoelectric laboratory observation in unconsolidated quartz sand within the kilohertz range. The investigation focused on the impact of fluid conductivity and water saturation over the coseismic seismoelectric field. During the experiment, special attention was given to the accuracy of electric field measurements. We concluded that, to obtain a reliable estimate of the electric field amplitude, the dipole from which the potential differences are measured should be of much smaller length than the wavelength of the propagating seismic field. Time‐lapse monitoring of the seismic velocities and seismoelectric transfer functions were performed during imbibition and drainage experiments. In all cases, the quantitative analysis of the seismoelectric transfer function Symbol was in good agreement with theoretical predictions. While investigating saturation variations from full to residual water saturation, we showed that the Symbol ratio undergoes a switch in polarity at a particular saturation Symbol, which also implies a sign change of the filtration, traducing a reversal of the relative fluid displacement with respect to the frame. This sign change at critical saturation Symbol stresses a particular behaviour of the poroelastic medium: the dropping of the coseismic electric field to zero traduces the absence of relative pore/fluid displacements representative of a Biot dynamically compatible medium. We concluded from our experimental study in loose sand that the measurements of the coseismic seismoelectric coupling may provide information on fluid distribution within the pores and that the reversal of the seismoelectric field may be used as an indicator of the dynamically compatible state of the medium. Symbol. No Caption available. Symbol. No Caption available. Symbol. No Caption available. Symbol. No Caption available. Symbol. No Caption available. Symbol. No Caption available.

Spectral Analysis of Seismoelectric Signal for Water Saturation Monitoring

The seismo-electromagnetic method consists of measuring electric and magnetic fields (Bordes et al. 2006, 2008) induced by seismic wave propagation from electrokinetic coupling which are sensitive to the pore fluid properties. Even if the role of water content was studied at low frequencies (electrofiltration), the effect of saturation on seismoelectric is poorly understood. In this study, we performed laboratory experiments in order to monitor seismoelectric signals as a function of water saturation in sand (99% silica). First analysis on direct P waves in time domain seems to show a frequency dependence of the seismic and seismoelectric signal (Senechal et al., 2010). In this further discussion, a spectral analysis by Continuous Wavelet Transform (CWT) is proposed to study frequency content variations occuring during imbibition and/or drainage of the sand.