Seismoelectric Coupling in a Poroelastic Material Containing Two Immiscible Fluid Phases

Abstract:

Recently, a new approach of seismoelectric imaging has been proposed in which seismic waves are focused in the subsurface to scan its heterogeneous nature and determine saturation fronts. Such type of imaging requires a complete modeling of the seismoelectric properties of porous media saturated by two fluid phases, one being the wetting phase for the solid grains and one being the non-wetting phase (for instance water and oil). We combine an extension of the poroelastic modeling to two-phase flow conditions with an extension of the electrokinetic theory based on the notion of an effective charge per unit pore volume that can be dragged by the flow of each fluid phase. These effective charge densities can be related directly to the permeability. We used finite element approach to simulate the seismoelectric signals in the partially saturated condition to formulate a transfer equation connecting the macroscopic electrical field to the acceleration of the fast P-wave and to study the influence of the water content on the amplitude of the co-seismic waves. The amplitude of the co-seismic wave is very sensitive to the water content with an increase in the amplitude of the electro-coseismic wave with water saturation. We also investigate the seismoelectric conversions occurring at the water table and the absence of conversion below the irreducible water saturation. We show that the conversion response at the water table can be identifiable only when the saturation contrasts between the vadose and saturated zones is sharp. Then a relatively dry vadose zone represents the best condition to identify the ground water level through seismoelectric measurements especially because in this case, the co-seismic signal is very small compared to the seismoelectric conversion response.