Solute transport in porous media during drying: The chlorine isotopes point of view
Résumé
Drying-out phenomena during injection of dry gas (CO2, CH4, H2, …) in geological reservoirs can be damaging for industrial processes as it can alter the transport properties and the injectivity of a reservoir due to salt precipitation. The distribution of salt precipitate, which may result in clogging of the formation, depends on two competing transport processes. Brine capillary-driven flows, which provide a continuous supply of dissolved salt towards the drying zone, and molecular diffusion, which tends to rehomogenize dissolved salt concentrations. In this study, we experimentally investigated the possibility that chlorine isotopes could be used as a geochemical tool to constrain the interplay of salt precipitation and solute transports process in drying saline porous media. Drying experiments were carried out on Lavoux carbonate plugs initially saturated with a 100 g L− 1 NaCl brine and dried in an oven at controlled temperature (60 °C) with vapor evacuation. Cl-content and δ37Cl profiles along the length of the plugs were obtained for different evaporation stages (Sw = 1; Sw = 0.82; Sw = 0.68). The results show a clear Cl-ion redistribution during drying, with Cl-accumulation near the plug evaporative surface together with the growth of salt efflorescence on the surface. A “u”-shaped profile of δ37Cl values is observed in the drying porous media. After 30 h of drying, salt crystals as well as the upper and lower parts of the plug present higher isotope ratios than the initial brine, while the center of the plug presents lower ratios. These experimental results suggest, as predicted by previous experimental and theoretical studies, that 35Cl and 37Cl are transported without fractionation by upward capillary flow to the evaporative surface, where salt precipitation occurs with a small preference for the heavier isotope. The resulting concentration gradient drives backward diffusion. Because 35Cl moves faster than 37Cl by diffusion this leads to more negative δ37Cl towards the center of the plug. Cl-isotopes transport modeling results suggest that backward advection may also have occurred, as indicated by a relatively high effective diffusion coefficient and a low Cl-isotope fractionation factor. This study provides new insights regarding the use of Cl-isotopes to characterize drying of porous media.