Geoelectrical signatures of reactive mixing

Uddipta Ghosh (1) , Aditya Bandopadhyay (1) , Damien Jougnot (2) , Tanguy Le Borgne (1) , Yves Méheust (1)

1 GR - Géosciences Rennes
2 UPMC - Université Pierre et Marie Curie - Paris 6

Characterizing the effects of fluid mixing on geochemical reactions in the subsurface is of paramount importance owing to their pivotal role in processes such as contaminant migration or aquifer remediation, to name a few [1]. Large velocity gradients in the porous media are expected to lead to enhanced diffusive mixing accompanied by augmented reaction rates [2]. Despite its importance, accurate monitoring of such processes still remains an open challenge, mainly due to the opacity of the medium and to the lack of access to it. However, in recent years, geophysical methods based on electrical conductivity and polarization have come up as a promising tool for mapping and monitoring such reactions in the subsurface. In this regard, one of the main challenges is to properly characterize the multiple sources of electrical signals and in particular isolate the influence of reactive mixing on the electrical conductivity from those of other sources [3]. In this work, we explore the coupling between fluid mixing, reaction and charge migration in porous media to evaluate the potential of obtaining a spatially-resolved measurement of local reaction rates in the subsurface from electrical measurements. To this end, we employ a lamellar description of the mixing interface [4] with novel semi-analytical upscaling techniques to quantify changes in electrical conductivity induced by chemical reactions across mixing fronts. The changes in electrical conductivity are strongly dependent on the concentration of ionic species as well as on the polarization of the pore (water) solution around the grains, which in turn are controlled by local reaction rates and, consequently, by the local velocity gradients. Hence, our results essentially suggest that local variations in the electrical conductivity may be quantitatively related to the mixing and reaction dynamics, and thus be used as a measurement tool to characterize these dynamics. References 1. M. Dentz, T. Le Borgne, A. Englert, and B. Bijeljic, J. Cont. Hyd., 120, 1-17, 2011. 2. T. Le Borgne, T. R. Ginn, and M. Dentz, Geophys. Res. Lett., 41(22), 7898-7906, 2014. 3. R Knight et al. Reviews of Geophysics, 48(4), 2010. 4. T. Le Borgne, M. Dentz, and E. Villermaux, J. Fluid Mech., 770, 458-498, 2015.

Domaines: Hydrologie

Uddipta Ghosh, Aditya Bandopadhyay, Damien Jougnot, Tanguy Le Borgne, Yves Méheust. Geoelectrical signatures of reactive mixing . American Geophysical Union Fall Meeting 2017, Dec 2017, New Orleans, United States. pp.H33A-1642. ⟨insu-01731831⟩