Influence of water pressure dynamics and fluid flow on the streaming‐potential response for unsaturated conditions

Abstract

International audienceA B S T R A C T Streaming-potentials are produced by electrokinetic effects in relation to fluid flow and are used for geophysical prospecting. The aim of this study is to model streaming potential measurements for unsaturated conditions using an empirical approach. A conceptual model is applied to streaming potential measurements obtained from two drainage experiments in sand. The streaming potential data presented here show a non-monotonous behaviour with increasing water saturation, following a pattern that cannot be predicted by existing models. A model involving quasi-static and dynamic components is proposed to reproduce the streaming potential measurements. The dynamic component is based on the first time derivative of the driving pore pressure. The influence of this component is investigated with respect to fluid velocity, which is very different between the two experiments. The results demonstrate that the dynamic component is predominant at the onset of drainage in experiments with the slowest water flow. On the other hand, its influence appears to vanish with increasing drainage velocity. Our results suggest that fluid flow and water distribution at the pore scale have an important influence on the streaming potential response for unsat-urated conditions. We propose to explain this specific streaming potential response in terms of the behaviour of both rock/water interface and water/air interfaces created during desaturation processes. The water/air interfaces are negatively charged, as also observed in the case of water/rock interfaces. Both the surface area and the flow velocity across these interfaces are thought to contribute to the non-monotonous behaviour of the streaming potential coefficient as well as the variations in its amplitude. The non-monotonous behaviour of air/water interfaces created during the flow was highlighted as it was measured and modelled by studies published in the literature. The streaming potential coefficient can increase to about 10 to 40 when water saturation decreases. Such an increase is possible if the amount of water/air interfaces is increased in sufficient amount, which can be the case

A review on electrokinetically induced seismo-electrics, electro-seismics, and seismo-magnetics for Earth sciences

“…A non-monotonous behaviour is supported by the observations of Allègre et al (2010) and also by the observations of Revil et al (2007) and Revil and Cerepi (2004) as detailed in Allègre et al (2011). Recently Allègre et al (2015) showed that the interface between water and air should also be taken into account, since this interface is negatively charged, as the interface between the rock matrix and the water. Moreover during a drainage the amount of this interface does not decrease with decreasing water saturation, but first increases before decreasing, leading to a non-monotonic behaviour of the resulting SPC (Allègre et al, 2015).…”

“…Recently Allègre et al (2015) showed that the interface between water and air should also be taken into account, since this interface is negatively charged, as the interface between the rock matrix and the water. Moreover during a drainage the amount of this interface does not decrease with decreasing water saturation, but first increases before decreasing, leading to a non-monotonic behaviour of the resulting SPC (Allègre et al, 2015). The Table 1 summarises the ratios C s0 (S w )/C sat proposed by different authors.…”

An analytical solution to assess theSHseismoelectric response of the vadose zone

“…In eq. (9), we use the following relation, which was experimentally derived from laboratory measurements of the streaming potential coefficient in partially saturated sands and displays a non-monotonic dependence with water saturation (Allègre et al 2010(Allègre et al , 2012(Allègre et al , 2014(Allègre et al , 2015Fiorentino et al 2017):…”

Permeability dependence of streaming potential coefficient in porous media

“…Allegre et al . () studied the streaming potential under unsaturated conditions, although a large number of studies on the electrokinetics of saturated samples have been carried out. The results show that the SPC depends on the dynamics of the driving pressure and the fluid flow for unsaturated conditions.…”

“…This means that it could not explain the relationship between the SPC and the permeability in detail for a specific type of porous samples in a small range of permeability. Allegre et al (2015) studied the streaming potential under unsaturated conditions, although a large number of studies on the electrokinetics of saturated samples have been carried out. The results show that the SPC depends on the dynamics of the driving pressure and the fluid flow for unsaturated conditions.…”